A method of analyzing rectal surface area irradiated and rectal complications in prostate conformal radiotherapy

Quantifying cell migration distance as a contributing factor to the development of rectal toxicity after prostate radiotherapy

PURPOSE

Spatial information is usually neglected in mathematical models of radiation-induced toxicity. In the presence of inhomogeneous dose distributions produced by intensity modulated radiation therapy (IMRT) and volumetric arc therapy, this may be a limitation. We present a model to quantify the spatial characteristics of the dose distribution on the rectum through the quantification of the distribution of distances between dose points on the surface of the rectum in three-dimensions. The method allows us to evaluate the hypothesis that distances between lower and higher dose regions on the rectum influence radiation damage repair due to the migration of normal cells into damaged areas, and consequently, the development of radiation-induced toxicity in patients treated with radiation for prostate cancer.

METHODS

We present a method to compute distances between dose points on the surface of the rectum in three dimensions (3D) and to generate distance maps representing the distances between specific dose regions on the rectum. We introduce the concept of the distance dose surface histogram (DDSH), which is computed from the distance maps. The DDSH is a 2D histogram of rectum area on a grid defined by pairwise combinations of dose and distance. Each bin in the DDSH quantifies the area of the rectum exposed to a given dose and at a given distance from other another dose region on the rectum. By summing across the columns and rows of the DDSH, the authors can generate the dose surface histogram (DSH) and distance surface histogram (DiSH) for a particular dose region. The DiSH is a marginal histogram showing the distribution of distances for the dose points in a specific dose region from another region. We computed the DDSH, DiSH, and DSH for 33 patients treated with IMRT for prostate cancer, nine of whom developed late Grade 2 or higher late rectal toxicity.

RESULTS

We show how even though the total area of the rectum exposed to a given dose may be the same for different patients, the distribution of distances for points receiving that dose can be different depending on the shape and contiguity of the region(s) containing those dose points. We also show that area of the rectum in the region receiving more than 75 Gy and at a distance of 16 to 22 mm from the 50 Gy isodose line was significantly correlated to the development of toxicity (p = 0.004, two sided t-test). This suggests that examining the distance distribution of points in specific dose regions could provide valuable additional information in predicting the risk of a patient developing radiation-induced rectal toxicity.

CONCLUSIONS

We present a new method to quantify the spatial distribution of points in a given region relative to other regions on the rectum. The method provides a means to evaluate the hypothesis that distances between lower and higher dose regions on the rectum influence radiation damage repair due to the migration of normal cells into damaged areas, and may be a contributing factor to the development of radiation-induced toxicity in patients treated with radiation for prostate cancer.

ACR Appropriateness Criteria® definitive external beam irradiation in stage T1 and T2 prostate cancer

PURPOSE

: External beam radiation therapy is a standard of care treatment for men who present with clinically localized (T1-T2) prostate cancer. The purpose of this review was to provide clarification on the appropriateness criteria and management considerations for the treatment of prostate cancer with external beam radiation therapy.

METHODS

: A panel consisting of physicians with expertise on prostate cancer was assembled and provided with a number of clinical scenarios for consensus treatment and management guidelines. Prostate cancer patient vignettes were presented along with specific management recommendations based on an extensive review of the modern external beam radiotherapy literature. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed every 2 years by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer reviewed journals and the application of a well established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances, where evidence is lacking or not definitive, expert opinion may be used to recommend imaging or treatment.

RESULTS

: Modern external beam radiation therapy series demonstrate favorable biochemical control rates for patients with localized prostate cancer. Morbidity profiles are also favorable and it is clear that this is enhanced by modern techniques like 3-dimensional conformal radiation therapy and intensity-modulated radiation therapy. An active area of investigation is evaluating the use of hypofractionated dosing.

CONCLUSIONS

: Continued investigation to refine patient selection, external beam radiation technology application, and alternative dosing schedules should result in further improvements in biochemical outcome and decreased morbidity with external beam radiation treatment for localized prostate cancer.

Low-dose rate brachytherapy for men with localized prostate cancer

BACKGROUND

Localized prostate cancer is a slow growing tumor for many years for the majority of affected men. Low-dose rate brachytherapy (LDR-BT) is short-distance radiotherapy using low-energy radioactive sources. LDR-BT has been recommended for men with low risk localized prostate cancer.

OBJECTIVES

To assess the benefit and harm of LDR-BT compared to radical prostatectomy (RP), external beam radiotherapy (EBRT), and no primary therapy (NPT) in men with localized prostatic cancer.

SEARCH STRATEGY

The Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (from 1950), and EMBASE (from 1980) were searched in June 2010 as well as online trials registers and reference lists of reviews.

SELECTION CRITERIA

Randomized, controlled trials comparing LDR-BT versus RP, EBRT, and NPT in men with clinically localized prostate cancer.

DATA COLLECTION AND ANALYSIS

Data on study methods, participants, treatment regimens, observation period and outcomes were recorded by two reviewers independently.

MAIN RESULTS

We identified only one RCT (N = 200; mean follow up 68 months). This trial compared LDR-BT and RP. The risk of bias was deemed high. Primary outcomes (overall survival, cause-specific mortality, or metastatic-free survival) were not reported. Biochemical recurrence-free survival at 5 years follow up was not significantly different between LDR-BT (78/85 (91.8%)) and RP (81/89 (91.0%)); P = 0.875; relative risk 0.92 (95% CI: 0.35 to 2.42).For severe adverse events reported at 6 months follow up, results favored LDR-BT for urinary incontinence (LDR-BT 0/85 (0.0%) versus RP 16/89 (18.0%); P < 0.001; relative risk 0) and favored RP for urinary irritation (LDR-BT 68/85 (80.0%) versus RP 4/89 (4.5%); P < 0.001; relative risk 17.80, 95% CI 6.79 to 46.66). The occurrence of urinary stricture did not significantly differ between the treatment groups (LDR-BT 2/85 (2.4%) versus RP 6/89 (6.7%); P = 0.221; relative risk 0.35, 95% CI: 0.07 to 1.68). Long-term information was not available.We did not identify significant differences of mean scores between treatment groups for patient-reported outcomes function and bother as well as generic health-related quality of life.

AUTHORS' CONCLUSIONS

Low-dose rate brachytherapy did not reduce biochemical recurrence-free survival versus radical prostatectomy at 5 years. For short-term severe adverse events, low-dose rate brachytherapy was significantly more favorable for urinary incontinence, but radical prostatectomy was significantly more favorable for urinary irritation. Evidence is based on one RCT with high risk of bias.

Rectal dose constraints for intensity modulated radiation therapy of the prostate

Prostate dose escalation appears to have resulted in increased cancer control. Such escalation has been made possible by the ability to deliver more conformal treatment that spares normal tissue from the higher radiation doses. The supposition is that this has enabled higher doses, but without an increase in toxicity. The most disabling toxicity in prostate cancer radiotherapy is rectal. We evaluated the current status of conformal radiation and late rectal toxicity with the goal of determining whether reasonable rectal dose and volume constraints can be determined. Although the literature is inexact, we believe that some generalized constraints can be recommended and show that those recommendations are consistent with what is being used at experienced centers.

Radiobiological impact of reduced margins and treatment technique for prostate cancer in terms of tumor control probability (TCP) and normal tissue complication probability (NTCP)

Dose escalation in prostate radiotherapy is limited by normal tissue toxicities. The aim of this study was to assess the impact of margin size on tumor control and side effects for intensity-modulated radiation therapy (IMRT) and 3D conformal radiotherapy (3DCRT) treatment plans with increased dose. Eighteen patients with localized prostate cancer were enrolled. 3DCRT and IMRT plans were compared for a variety of margin sizes. A marker detectable on daily portal images was presupposed for narrow margins. Prescribed dose was 82 Gy within 41 fractions to the prostate clinical target volume (CTV). Tumor control probability (TCP) calculations based on the Poisson model including the linear quadratic approach were performed. Normal tissue complication probability (NTCP) was calculated for bladder, rectum and femoral heads according to the Lyman-Kutcher-Burman method. All plan types presented essentially identical TCP values and very low NTCP for bladder and femoral heads. Mean doses for these critical structures reached a minimum for IMRT with reduced margins. Two endpoints for rectal complications were analyzed. A marked decrease in NTCP for IMRT plans with narrow margins was seen for mild RTOG grade 2/3 as well as for proctitis/necrosis/stenosis/fistula, for which NTCP <7% was obtained. For equivalent TCP values, sparing of normal tissue was demonstrated with the narrow margin approach. The effect was more pronounced for IMRT than 3DCRT, with respect to NTCP for mild, as well as severe, rectal complications.

Dosimetric and anatomic indicators of late rectal toxicity after high-dose intensity modulated radiation therapy for prostate cancer

We seek to identify dosimetric and anatomic indicators of late rectal toxicity in prostate cancer patients treated with intensity modulated radiation therapy (IMRT). Data from 49 patients sampled from 698 patients treated for clinically localized prostate cancer at the Memorial Sloan-Kettering Cancer Center with IMRT to a dose of 81 Gy were analyzed. The end point of the study was late Grade 2 or worse rectal toxicity within 30 months of treatment. Dosimetric analysis was performed on the rectum surface in three dimensions and on two-dimensional dose maps obtained by flattening the rectum surface using a conformal mapping procedure. Several parameters including the percentage and absolute surface area of the rectum irradiated, mean dose as a function of location on the rectum, planning target volume (PTV) size and rectum size were analyzed for correlation to toxicity. Significance was set at p < 0.05 for a two-sided t-test. Correlation between absolute areas irradiated and toxicity was observed on both the rectum surface and flattened rectum. Patients with toxicity also received a significantly higher mean dose to the superior 25% of the rectum surface and 15% of the flattened rectum. PTV volume, PTV height, rectum surface area and average cross-sectional area were significantly larger in patients with toxicity. The conformal mapping procedure has potential utility for evaluating dose to the rectum and risk of toxicity. Late rectal toxicity was related to the irradiation of the upper part of the rectum and also to the absolute area irradiated, PTV size, and rectum size on the planning computed tomography (CT) scan.

Rectal bleeding, fecal incontinence, and high stool frequency after conformal radiotherapy for prostate cancer: normal tissue complication probability modeling

PURPOSE

To analyze whether inclusion of predisposing clinical features in the Lyman-Kutcher-Burman (LKB) normal tissue complication probability (NTCP) model improves the estimation of late gastrointestinal toxicity.

METHODS AND MATERIALS

This study includes 468 prostate cancer patients participating in a randomized trial comparing 68 with 78 Gy. We fitted the probability of developing late toxicity within 3 years (rectal bleeding, high stool frequency, and fecal incontinence) with the original, and a modified LKB model, in which a clinical feature (e.g., history of abdominal surgery) was taken into account by fitting subset specific TD50s. The ratio of these TD50s is the dose-modifying factor for that clinical feature. Dose distributions of anorectal (bleeding and frequency) and anal wall (fecal incontinence) were used.

RESULTS

The modified LKB model gave significantly better fits than the original LKB model. Patients with a history of abdominal surgery had a lower tolerance to radiation than did patients without previous surgery, with a dose-modifying factor of 1.1 for bleeding and of 2.5 for fecal incontinence. The dose-response curve for bleeding was approximately two times steeper than that for frequency and three times steeper than that for fecal incontinence.

CONCLUSIONS

Inclusion of predisposing clinical features significantly improved the estimation of the NTCP. For patients with a history of abdominal surgery, more severe dose constraints should therefore be used during treatment plan optimization.

Rectal wall sparing effect of three different endorectal balloons in 3D conformal and IMRT prostate radiotherapy

PURPOSE

To investigate the dosimetric consequences and rectal wall (Rwall) sparing effect of three different endorectal balloons (ERBs) for three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) for prostate cancer.

METHODS AND MATERIALS

In 20 patients, 4 planning computed tomography scans were made: 1 without ERB and 3 with ERB1, ERB2, or ERB3 inserted. Two different planning target volumes were defined: prostate only, and prostate plus seminal vesicles. The 3D-CRT and IMRT planning techniques were used, and the prescription dose was 78 Gy. In 284 treatment plans, the Rwall mean dose, the Rwall normal tissue complication probability, and the absolute Rwall volumes exposed to > or =50 Gy (V(50)) and > or =70 Gy (V(70)) were calculated. For spatial dose distribution analysis, inner rectal wall dose maps and dose surface histograms were generated.

RESULTS

Each ERB was tolerated well. In the case of 3D-CRT, each ERB showed a statistically significant reduction of all the measured parameters. ERB2 and ERB3 performed better than ERB1. In IMRT, a statistically significant reduction in the Rwall dose parameters could not be demonstrated for any of the ERBs. For 3D-CRT and IMRT, as a result of the rectal dilation, ranging from 8 to 20 cm in circumference, the ERBs resulted in a reduction of the relative inner Rwall surface exposed to intermediate and high doses.

CONCLUSIONS

In 3D-CRT, any ERB showed a significant rectal wall sparing effect. ERB2 and ERB3 were superior to ERB1. For both 3D-CRT and IMRT, a reduction of the relative inner Rwall surface exposed to intermediate and high doses was found, which may lead to reduced late rectal toxicity. Development of user- and patient-friendly ERBs is warranted to increase their acceptability.

Modelling the variation in rectal dose due to inter-fraction rectal wall deformation in external beam prostate treatments

Prostate radiotherapy inevitably deposits radiation dose in the rectal wall, and the dose delivered to prostate is limited by the expected rectal complications. Accurate evaluation of the rectal dose is non-trivial due to a number of factors. One of these is variation of the shape and position of the rectal wall (with respect to the clinical target volume (CTV)), which may differ daily from that taken during planning CT acquisition. This study uses data currently available in the literature on rectal wall motion to provide estimates of mean population rectal wall dose. The rectal wall geometry is characterized by a population mean radius of the rectum as well as inter-patient and inter-fraction standard deviations in rectum radius. The model is used to evaluate the range of inter-fraction and inter-patient rectal dose variations. The simulation of individual patients with full and empty rectum in the planning CT scan showed that large variations in rectal dose (>15 Gy) are possible. Mean calculated dose accounting for treatment and planning uncertainties in the rectal wall surface was calculated as well as the map of planning dose over/underpredictions. It was found that accuracy of planning dose is dependent on the CTV-PTV margin size with larger margins producing more accurate estimates. Over a patient population, the variation in rectal dose is reduced by increasing the number of pre-treatment CT scans.

Phase II multicenter randomized study of amifostine for prevention of acute radiation rectal toxicity: topical intrarectal versus subcutaneous application

PURPOSE

To investigate the cytoprotective effect of subcutaneous vs. intrarectal administration of amifostine against acute radiation toxicity.

METHODS AND MATERIALS

Patients were randomized to receive amifostine either intrarectally (Group A, n = 27) or a 500-mg flat dose subcutaneously (Group B, n = 26) before irradiation. Therapy was delivered using a four-field technique with three-dimensional conformal planning. In Group A, 1,500 mg of amifostine was administered intrarectally as an aqueous solution in 40 mL of enema. Two different toxicity scales were used: the European Organization for Research and Treatment of Cancer/Radiation Therapy Oncology Group (RTOG) rectal and urologic toxicity criteria and the Subjective-RectoSigmoid scale based on the endoscopic terminology of the World Organization for Digestive Endoscopy. Objective measurements with rectosigmoidoscopy were performed at baseline and 1-2 days after radiotherapy completion. The area under the curve for the time course of mucositis (RTOG criteria) during irradiation represented the mucositis index.

RESULTS

Intrarectal amifostine was feasible and well tolerated without any systemic or local side effects. According to the RTOG toxicity scale, Group A had superior results with a significantly lower incidence of Grades I-II rectal radiation morbidity (11% vs. 42%, p = 0.04) but inferior results concerning urinary toxicity (48% vs. 15%, p = 0.03). The mean rectal mucositis index and Subjective-RectoSigmoid score were significantly lower in Group A (0.44 vs. 2.45 [p = 0.015] and 3.9 vs. 6.0 [p = 0.01], respectively), and the mean urinary mucositis index was lower in Group B (2.39 vs. 0.34, p < 0.028).

CONCLUSIONS

Intrarectal administration of amifostine (1,500 mg) seemed to have a cytoprotective efficacy in acute radiation rectal mucositis but was inferior to subcutaneous administration in terms of urinary toxicity. Additional randomized studies are needed for definitive decisions concerning the cytoprotection of pelvic irradiated areas.

Comparative analyses of the dynamic properties of the bladder wall studied by repetitive pelvic CT scans of patients and cryo-sections of cadavers

In radiotherapy planning systems, delineation of hollow normal tissue organs, such as the bladder, is time-consuming. Automated delineation may presuppose two assumptions: (1) the bladder resembles a spherical shell and (2) the volume of bladder tissue is preserved regardless of the volume of urine (luminal volume) inside. The purpose of the present study was to test these assumptions. 22 CT scans from 7 patients were studied retrospectively. Transverse cross-sectional areas enclosed by the outer contour (A(out)) and inner contour of the bladder (A(in)) were recorded from the images. Hence, the transverse cross-sectional area of the wall, A(wall)=A(out)-A(in), and the volume of bladder tissue at various luminal volumes, could be calculated. To quantify the method uncertainty, the same procedure was applied on three spherical plastic phantoms. The results were also compared with data from the Visible Human Project's photographs of cadaver cryo-sections. Assumption no. 1 stated above, implies that A(wall) is constant regardless of the level of intersection of the sphere. The data from cryo-sections revealed a positive correlation for A(wall) and A(out), in contradiction to assumption no. 1 (p<0.001). The corresponding association derived from the repetitive CT scans of patients was also statistically significant (p<0.001) although linear regression revealed a less steep slope. A relationship was found between the volume of bladder tissue and luminal volume, hence contradicting assumption no. 2 (p<0.001). In conclusion the cross-sectional wall areas of the bladder, measured from patient CT scans, increase slightly with luminal cross-sectional areas in contradiction to expected values derived from a simplistic spherical shell model. In addition, the volume of bladder tissue is related to the luminal volume. Our results may be of practical value when developing automated delineation tools in radiotherapy planning systems.

Fitting late rectal bleeding data using different NTCP models: results from an Italian multi-centric study (AIROPROS0101)

BACKGROUND AND PURPOSE

Recent investigations demonstrated a significant correlation between rectal dose-volume patterns and late rectal toxicity. The reduction of the DVH to a value expressing the probability of complication would be suitable. To fit different normal tissue complication probability (NTCP) models to clinical outcome on late rectal bleeding after external beam radiotherapy (RT) for prostate cancer.

PATIENTS AND METHODS

Rectal dose-volume histograms of the rectum (DVH) and clinical records of 547 prostate cancer patients (pts) pooled from five institutions previously collected and analyzed were considered. All patients were treated in supine position with 3 or 4-field techniques: 123 patients received an ICRU dose between 64 and 70 Gy, 255 patients between 70 and 74 Gy and 169 patients between 74 and 79.2 Gy; 457/547 patients were treated with conformal RT and 203/547 underwent radical prostatectomy before RT. Minimum follow-up was 18 months. Patients were considered as bleeders if showing grade 2/3 late bleeding (slightly modified RTOG/EORTC scoring system) within 18 months after the end of RT. Four NTCP models were considered: (a) the Lyman model with DVH reduced to the equivalent uniform dose (LEUD, coincident with the classical Lyman-Kutcher-Burman, LKB, model), (b) logistic with DVH reduced to EUD (LOGEUD), (c) Poisson coupled to EUD reduction scheme and (d) relative seriality (RS). The parameters for the different models were fit to the patient data using a maximum likelihood analysis. The 68% confidence intervals (CI) of each parameter were also derived.

RESULTS

Forty six out of five hundred and forty seven patients experienced grade 2/3 late bleeding: 38/46 developed rectal bleeding within 18 months and were then considered as bleeders The risk of rectal bleeding can be well calculated with a 'smooth' function of EUD (with a seriality parameter n equal to 0.23 (CI 0.05), best fit result). Using LEUD the relationship between EUD and NTCP can be described with a TD50 of 81.9 Gy (CI 1.8 Gy) and a steepness parameter m of 0.19 (CI 0.01); when using LOGEUD, TD50 is 82.2 Gy and k is 7.85. Best fit parameters for RS are s=0.49, gamma=1.69, TD50=83.1 Gy. Qualitative as well as quantitative comparisons (chi-squared statistics, P=0.005) show that the models fit the observed complication rates very well. The results found in the overall population were substantially confirmed in the subgroup of radically treated patients (LEUD: n=0.24 m=0.14 TD50=75.8 Gy). If considering just the grade 3 bleeders (n=9) the best fit is found in correspondence of a n-value around 0.06, suggesting that for severe bleeding the rectum is more serial.

CONCLUSIONS

Different NTCP models fit quite accurately the considered clinical data. The results are consistent with a rectum 'less serial' than previously reported investigations when considering grade 2 bleeding while a more serial behaviour was found for severe bleeding. EUD may be considered as a robust and simple parameter correlated with the risk of late rectal bleeding.

Phase III pilot study of dose escalation using conformal radiotherapy in prostate cancer: PSA control and side effects

Radical radiotherapy is a standard form of management of localised prostate cancer. Conformal treatment planning spares adjacent normal tissues reducing treatment-related side effects and may permit safe dose escalation. We have tested the effects on tumour control and side effects of escalating radiotherapy dose and investigated the appropriate target volume margin. After an initial 3-6 month period of androgen suppression, 126 men were randomised and treated with radiotherapy using a 2 by 2 factorial trial design. The initial radiotherapy tumour target volume included the prostate and base of seminal vesicles (SV) or complete SV depending on SV involvement risk. Treatments were randomised to deliver a dose of 64 Gy with either a 1.0 or 1.5 cm margin around the tumour volume (1.0 and 1.5 cm margin groups) and also to treat either with or without a 10 Gy boost to the prostate alone with no additional margin (64 and 74 Gy groups). Tumour control was monitored by prostate-specific antigen (PSA) testing and clinical examination with additional tests as appropriate. Acute and late side effects of treatment were measured using the Radiation Treatment and Oncology Groups (RTOG) and LENT SOM systems. The results showed that freedom from PSA failure was higher in the 74 Gy group compared to the 64 Gy group, but this did not reach conventional levels of statistical significance with 5-year actuarial control rates of 71% (95% CI 58-81%) in the 74 Gy group vs 59% (95% CI 45-70%) in the 64 Gy group. There were 23 failures in the 74 Gy group and 33 in the 64 Gy group (Hazard ratio 0.64, 95% CI 0.38-1.10, P=0.10). No difference in disease control was seen between the 1.0 and 1.5 cm margin groups (5-year actuarial control rates 67%, 95% CI 53-77% vs 63%, 95% CI 50-74%) with 28 events in each group (Hazard ratio 0.97, 95% CI 0.50-1.86, P=0.94). Acute side effects were generally mild and 18 weeks after treatment, only four and five of the 126 men had persistent > or =Grade 1 bowel or bladder side effects, respectively. Statistically significant increases in acute bladder side effects were seen after treatment in the men receiving 74 Gy (P=0.006), and increases in both acute bowel side effects during treatment (P=0.05) and acute bladder sequelae (P=0.002) were recorded for men in the 1.5 cm margin group. While statistically significant, these differences were of short duration and of doubtful clinical importance. Late bowel side effects (RTOG> or =2) were seen more commonly in the 74 Gy and 1.5 cm margin groups (P=0.02 and P=0.05, respectively) in the first 2 years after randomisation. Similar results were found using the LENT SOM assessments. No significant differences in late bladder side effects were seen between the randomised groups using the RTOG scoring system. Using the LENT SOM instrument, a higher proportion of men treated in the 74 Gy group had Grade > or =3 urinary frequency at 6 and 12 months. Compared to baseline scores, bladder symptoms improved after 6 months or more follow-up in all groups. Sexual function deteriorated after treatment with the number of men reporting some sexual dysfunction (Grade> or =1) increasing from 38% at baseline to 66% at 6 months and 1 year and 81% by year 5. However, no consistent differences were seen between the randomised groups. In conclusion, dose escalation from 64 to 74 Gy using conformal radiotherapy may improve long-term PSA control, but a treatment margin of 1.5 cm is unnecessary and is associated with increased acute bowel and bladder reactions and more late rectal side effects. Data from this randomised pilot study informed the Data Monitoring Committee of the Medical Research Council RT 01 Trial and the two studies will be combined in subsequent meta-analysis.

Comparison of rectal dose-wall histogram versus dose-volume histogram for modeling the incidence of late rectal bleeding after radiotherapy

PURPOSE

To compare the fits of normal-tissue complication probability (NTCP) models based on rectal dose-wall histograms (DWHs) vs. dose-volume histograms (DVHs) when the two are used to analyze a common set of late rectal toxicity data.

METHODS AND MATERIALS

Data were analyzed from 128 prostate cancer patients treated with 3-dimensional conformal radiotherapy (3D-CRT) at The University of Texas M.D. Anderson Cancer Center (UTMDACC). The DVH for total rectal volume, including contents, was obtained for each patient from the treatment-planning system. A DWH was also computed, using the outer rectal contour plus an autogenerated inner contour that corresponds to an assumed 3-mm rectal wall thickness. The endpoint for analysis was Grade 2 or higher late rectal bleeding within 2 years of treatment; all patients had at least 2 years of follow-up. Four different NTCP models were fitted to the response data by using either the DVH or the DWH to describe the dose distribution to rectum or rectal wall, respectively. The 4 models considered were the Lyman model, the mean dose model, the parallel-architecture model, and a model based on the volume of a organ receiving more than a specified dose (the "cutoff-dose" model).

RESULTS

For each of the models, the fit to the late rectal bleeding data was slightly improved when the analysis was based on the rectal DWH instead of on the DVH. In addition, the results of the cutoff dose and parallel architecture models were consistent with one another for the DWH data but not for the DVH data. For the DWH data, both models predict a 50% or higher incidence of Grade 2 or worse late rectal bleeding within 2 years if 80% or more of the rectal wall is exposed to doses greater than 32 Gy. A 50% or higher incidence of rectal bleeding is also predicted if the mean dose to rectal wall exceeds 53.2 Gy.

CONCLUSIONS

A consistent, although modest, improvement occurs in the fits of NTCP models to the UTMDACC 2-year late rectal bleeding data when the fit is based on the rectal dose-wall histogram instead of on the dose-volume histogram for entire rectum, including contents.

A geometric model for evaluating the effects of inter-fraction rectal motion during prostate radiotherapy

A geometric model is presented which allows calculation of the dosimetric consequences of rectal motion in prostate radiotherapy. Variations in the position of the rectum are measured by repeat CT scanning during the courses of treatment of five patients. Dose distributions are calculated by applying the same conformal treatment plan to each imaged fraction and rectal dose-surface histograms produced. The 2D model allows isotropic expansion and contraction in the plane of each CT slice. By summing the dose to specific volume elements tracked by the model, composite dose distributions are produced that explicitly include measured inter-fraction motion for each patient. These are then used to estimate effective dose-surface histograms (DSHs) for the entire treatment. Results are presented showing the magnitudes of the measured target and rectal motion and showing the effects of this motion on the integral dose to the rectum. The possibility of using such information to calculate normal tissue complication probabilities (NTCP) is demonstrated and discussed.

Conformal radiotherapy of urinary bladder cancer

Recent advances in radiotherapy (RT) are founded on the enhanced tumour visualisation capabilities of new imaging modalities and the precise deposition of individualised radiation dose distributions made possible with the new systems for RT planning and delivery. These techniques have a large potential to also improve the results of RT of urinary bladder cancer. Major challenges to take full advantage of these advances in the management of bladder cancer are to control, and, as far as possible, reduce bladder motion, and to reliably account for the related intestine and rectum motion. If these obstacles are overcome, it should be possible in the near future to offer selected patients with muscle invading bladder cancer an organ-sparing, yet effective combined-modality treatment as an alternative to radical surgery.

Irradiation of the inguinal lymph nodes in patients of differing body habitus: A comparison of techniques and resulting normal tissue complication probabilities

The treatment of the inguinal lymph nodes with radiotherapy is strongly influenced by the body habitus of the patient. The effect of 7 radiotherapy techniques on femoral head doses was studied. Three female patients of differing body habitus (ectomorph, mesomorph, endomorph) were selected. Radiation fields included the pelvis and contiguous inguinal regions and were representative of fields used in the treatment of cancers of the lower pelvis. Seven treatment techniques were compared. In the ectomorph and mesomorph, normal tissue complication probability (NTCP) for the femoral heads was lowest with use of anteroposterior (AP) and modified posteroanterior (PA) field with inguinal electron field supplements (technique 1). In the endomorph, NTCP was lowest with use of AP and modified PA field without electron field supplements (technique 2) or a 4-field approach (technique 6). Technique 1 for ectomorphs and mesomorphs and techniques 2 and 6 for endomorphs were optimal techniques for providing relatively homogeneous dose distributions within the target area while minimizing the dose to the femoral heads.

Dose–volume response analyses of late rectal bleeding after radiotherapy for prostate cancer

PURPOSE

To compare the fits of various normal tissue complication probability (NTCP) models to a common set of late rectal toxicity data, with the aim of identifying the best model for predicting late rectal injury after irradiation.

METHODS AND MATERIALS

Late toxicity data from 128 prostate cancer patients treated on protocol with three-dimensional conformal radiotherapy at The University of Texas M.D. Anderson Cancer Center (UTMDACC) were analyzed. The dose-volume histogram for total rectal volume, including contents, was obtained for each patient, and the presence or absence of Grade 2 or worse rectal bleeding within 2 years of treatment was scored. Five different NTCP models were fitted to the data using maximum likelihood analysis: the Lyman model, the mean dose model, a parallel architecture model, and models based on either a cutoff dose or a cutoff volume.

RESULTS

All five of the NTCP models considered provided very similar fits to the UTMDACC rectal bleeding data. In particular, none of the more highly parameterized models (the four-parameter parallel model, three-parameter Lyman model, or three-parameter cutoff dose and volume models) provided a better fit than the simplest of the models, the two-parameter NTCP model describing rectal bleeding as a probit function of mean dose to rectum.

CONCLUSION

No dose-volume response model has yet been identified that provides a better description of the UTMDACC rectal toxicity data than the mean dose model. Because this model has relatively low predictive accuracy, the need to identify a better model remains.

On the use of margins for geometrical uncertainties around the rectum in radiotherapy planning

BACKGROUND AND PURPOSE

To derive planning organ at risk volume (PRV) margins for the rectum and to analyse the impact of such margins on rectum dose volume histograms (DVHs).

PATIENTS AND METHODS

Weekly repeat computer tomography (CT) scans of 19 bladder cancer patients acquired during a conformal radiotherapy course were registered with the corresponding planning CT scans. From these scans, the internal rectal motion was quantified, and the margins that had to be added to the rectum contour in the planning scan to encompass the observed span of rectum motion were determined. These margins were compared to the margins derived using a recent PRV margin recipe. To illustrate the impact of margins on rectum DVHs, the margins were applied in treatment plans of six prostate cancer patients.

RESULTS

Altogether 141 CT scans were analysed. On average 24% of the repeat scan rectum volume was displaced outside the planning scan contours, and wall movements of up to 30 mm were observed. Margins of 16 mm anterior and 11 mm posterior encompassed all rectal motion except for the two most displaced rectum walls in each of these directions, in 89% of the patients. Using a recently published statistics-based recipe, margins of 6 mm anterior and 5 mm posterior accounted for the systematic rectum variation, i.e. the average wall position, in 90% of the patients. Adding anterior margin only caused consistent increases (up to 20%) in the fraction of the volume inside the high-dose region (40-70 Gy) compared to the DVH of rectum only. When using both anterior and posterior margins only small shifts (<5%) in the volume fractions were observed.

CONCLUSIONS

Rectum PRV margins of 5-6 mm will encompass the systematic component of rectum motion, while margins up to 16 mm are required to also account for most of the random variation. Use of anterior margins only caused large shifts in the DVHs in the clinically significant dose range, while only minor shifts were seen when using both anterior and posterior margins.

Rectal dose-volume constraints in high-dose radiotherapy of localized prostate cancer

PURPOSE

To investigate the relationship between rectal bleeding and dosimetric-clinical parameters in patients receiving three-dimensional conformal radiotherapy (3D-CRT) for localized prostate cancer.

METHODS

In a retrospective national study (AIROPROS01-01, AIRO: Associazione Italiana Radioterapia Oncologica), planning/clinical data for 245 consecutive patients with stage T1-4N0-x prostate carcinoma who underwent 3D-CRT to 70-78 Gy (ICRU point) were pooled from four Italian institutions. The correlation between late rectal bleeding and rectal dose-volume data (the percentage of rectum receiving more than 50, 55, 60, 65, 70, and 75 Gy [V(50-70)]) and other dosimetric and clinical parameters were investigated in univariate (log-rank) and multivariate (Cox regression model) analyses. Median follow-up was 2 years.

RESULTS

Twenty-three patients were scored as late bleeders according to a modified RTOG definition (Grade 2: 16; Grade 3: 7); the actuarial 2-year rate was 9.2%. Excepting V75, all median and third quartile V(50-70) values were found to be significantly correlated with late bleeding at univariate analysis. The smallest p value was seen for V(50) below/above the third quartile value (66%). The V70 (cut-off value: 30%) was found to be also predictive for late bleeding. In the high-dose subgroup (74-78 Gy), Grade 3 bleeding was highly correlated with this constraint. The predictive value of both V(50) and V(70) was confirmed by multivariate analyses.

CONCLUSIONS

The present article provides evidence for correlation between rectal DVH parameters and late rectal bleeding in patients treated with curative intent with 3D-CRT. To keep the rate of moderate/severe rectal bleeding below 5-10%, it seems advisable to limit V(50) to 60-65%, V(60) to 45-50%, and V70 to 25-30%.

Finding dose–volume constraints to reduce late rectal toxicity following 3D-conformal radiotherapy (3D-CRT) of prostate cancer

BACKGROUND AND PURPOSE

The rectum is known to display a dose-volume effect following high-dose 3D-conformal radiotherapy (3D-CRT). The aim of the study is to search for significant dose-volume combinations with the specific treatment technique and patient set-up currently used in our institution.

PATIENTS AND METHODS

We retrospectively analyzed the dose-volume histograms (DVH) of 135 patients with stage T1b-T3b prostate cancer treated consecutively with 3D-CRT between 1996 and 2000 to a total dose of 76 Gy. The median follow-up was 28 months (range 12-62). All late rectal complications were scored using RTOG criteria. Time to late toxicity was assessed using the Kaplan-Meyer method. The association between variables at baseline and > or=2 rectal toxicity was tested using chi(2) test or Fisher's exact test. A multivariate analysis using logistic regression was performed.

RESULTS

Late rectal toxicity grade > or=2 was observed in 24 of the 135 patients (17.8%). A 'grey area' of increased risk has been identified. Average DVHs of the bleeding and non-bleeding patients were generated. The area under the percent volume DVH for the rectum of the bleeding patients was significantly higher than that of patients without late rectal toxicity. On multivariate analysis the correlation between the high risk DVHs and late rectal bleeding was confirmed.

CONCLUSIONS

The present analysis confirms the role of the rectal DVH as a tool to discriminate patients undergoing high-dose 3D-CRT into a low and a high risk of developing late rectal bleeding. Based on our own results and taking into account the data published in the literature, we have been able to establish new dose-volume constraints for treatment planning: if possible, the percentage of rectal volume exposed to 40, 50, 60, 72 and 76 Gy should be limited to 60, 50, 25, 15 and 5%, respectively.

Patient-Reported Toxicity Correlated to Dose–Volume Histograms of the Rectum in Radiotherapy of the Prostate

We studied 73 patients treated with 3-dimensional conformal radiotherapy for prostate cancer to determine whether there is a correlation between dose per volume to either the whole rectum, rectal wall, rectal surface, or anal canal and the development of late rectal complications measured with prostate-specific quality-of-life (QOL) questionnaires. Given doses were 66.6 to 72 Gy. The prostate cancer modules used were the UCLA-Prostate Cancer Index module (UCLA-PCI) (5 questions), the Functional Assessment of Cancer Therapy-Prostate module (FACT-P) (1 question), and the European Organization for Research and Treatment of Cancer prostate cancer module (EORTC QLQ-PR25) (4 questions). A Spearman correlation analysis between the total toxicity score and the dose-volume histograms (DVHs) was performed. All statistical tests were 2-sided. Sixty-five (89%) patients returned the questionnaire, and 18 (28%) underwent endoscopy for rectal bleeding. We found that only patients who had had an endoscopy showed a correlation between rectal toxicity and dose per volume, as compared with the other patients who showed none. Correlation between rectal toxicity and dose per volume for all 4 structures was stronger for higher doses. For 70 Gy, all contours, except the anal canal, showed a significant dose-volume correlation. Our results indicate that only in cases of pronounced rectal toxicity is there a dose-volume correlation, especially for doses of 70 Gy or more. DVHs of the whole rectum, wall, or surface, but not the anal canal, are all equivalent in predicting late rectal toxicity.

Rectal dose sparing with a balloon catheter and ultrasound localization in conformal radiation therapy for prostate cancer

BACKGROUND AND PURPOSE

To compare the rectal wall and bladder volume in the high dose region with or without the use of a balloon catheter with both three-dimensional (3D)-conformal and intensity modulated radiation therapy (CRT, IMRT) approaches in the treatment of prostate cancer.

MATERIAL AND METHODS

Five patients with a wide range of prostate volumes and treated with primary external beam radiation therapy for localized prostate cancer were selected for analysis. Pinnacle treatment plans were generated utilizing a 3D conformal six-field design and an IMRT seven coplanar-field plan with a novel, three-step optimization and with ultrasound localization. Separate plans were devised with a rectal balloon deflated or air inflated with and without inclusion of the seminal vesicles (SV) in the target volume. The prescription dose was 76Gy in 38 fractions of 2Gy each. Cumulative dose-volume histograms (DVHs) were analyzed for the planning target volume (PTV), rectal wall, and bladder with an inflated (60cc air) or deflated balloon with and without SV included. The volumes of rectal wall and bladder above 60, 65, and 70Gy with each treatment approach were evaluated.

RESULTS

Daily balloon placement was well-tolerated with good patient positional reproducibility. Inflation of the rectal balloon in all cases resulted in a significant decrease in the absolute volume of rectal wall receiving greater than 60, 65, or 70Gy. The rectal sparing ratio (RSR), consisting of a structure's high dose volume with the catheter inflated, divided by the volume with the catheter deflated, was calculated for each patient with and without seminal vesicle inclusion for 3D-CRT and IMRT. For 3D-CRT, RSRs with SV included were 0.59, 0.59, and 0.56 and with SV excluded were 0.60, 0.58, and 0.54 at doses of greater than 60, 65, and 70Gy, respectively. Similarly, for IMRT, the mean RSRs were 0.59, 0.59, and 0.63 including SV and 0.71, 0.66, and 0.67 excluding SV at these same dose levels, respectively. Averaged over all conditions, inflation of the rectal balloon resulted in a significant reduction in rectal volume receiving > or =65Gy to a mean ratio of 0.61 (P=0.01) or, in other words, a mean fractional high dose rectal sparing of 39%. There was a slight overall increase to 1.13 in the relative volume of bladder receiving at least 65Gy; however, this was not significant (P=0.6). Use of an endorectal balloon with a non-image-guided 3D-CRT plan produced about as much rectal dose sparing as a highly conformal, image-guided IMRT approach without a balloon. However, inclusion of a balloon with IMRT produced further rectal sparing still.

CONCLUSION

These results indicate that use of a rectal balloon with a 3D-CRT plan incorporating typical treatment margins will produce significant high dose rectal sparing that is comparable to that achieved by a highly conformal IMRT with ultrasound localization. Further sparing is achieved with the inclusion of a balloon catheter in an IMRT plan. Thus, in addition to a previously reported advantage of prostate immobilization, the use of a rectal displacement balloon during daily treatment results in high dose rectal wall sparing during both modestly and highly conformal radiotherapy. Such sparing could assist in controlling and limiting rectal toxicity during increasingly aggressive dose escalation.

Significant correlation between rectal DVH and late bleeding in patients treated after radical prostatectomy with conformal or conventional radiotherapy (66.6-70.2 Gy)

PURPOSE

Investigating the correlation between dosimetric/clinical parameters and late rectal bleeding in patients treated with adjuvant or salvage radiotherapy after radical prostatectomy.

METHODS AND MATERIALS

Data of 154 consecutive patients, including three-dimensional treatment planning and dose-volume histograms (DVHs) of the rectum (including filling), were retrospectively analyzed. Twenty-six of 154 patients presenting a (full) rectal volume >100 cc were excluded from the analysis. All patients considered for the analysis (n = 128) were treated at a nominal dose equal to 66.6-70.2 Gy (ICRU dose 68-72.5 Gy; median 70 Gy) with conformal (n = 76) or conventional (n = 52) four-field technique (1.8 Gy/fr). Clinical parameters such as diabetes mellitus, acute rectal bleeding, hypertension, age, and hormonal therapy were considered. Late rectal bleeding was scored using a modified Radiation Therapy Oncology Group scale, and patients experiencing >or=Grade 2 were considered bleeders. Median follow-up was 36 months (range 12-72). Mean and median rectal dose were considered, together with rectal volume and the % fraction of rectum receiving more than 50, 55, 60, and 65 Gy (V50, V55, V60, V65, respectively). Median and quartile values of all parameters were taken as cutoff for statistical analysis. Univariate (log-rank) and multivariate (Cox hazard model) analyses were performed.

RESULTS

Fourteen of 128 patients experienced >or=Grade 2 late bleeding (3-year actuarial incidence 10.5%). A significant correlation between a number of cutoff values and late rectal bleeding was found. In particular, a mean dose >or=54 Gy, V50 >or=63%, V55 >or=57%, and V60 >or=50% was highly predictive of late bleeding (p <or= 0.01). A rectal volume <60 cc and type of treatment (conventional vs. conformal) were also significantly predictive of late bleeding (p = 0.05). Concerning clinical variables, acute bleeding (p < 0.001) was significantly related to late bleeding, and a trend was found for hypertension (p = 0.11). After patients were grouped into those with V50 >or=63% and those with V50 <63% (DVH grouping), data were fitted with a Cox regression hazard model using DVH grouping, rectal volume, and the main clinical parameters as independent variables. Results of the analysis showed that DVH grouping (relative risk 3.3; p = 0.04) and acute bleeding (relative risk 7.1; p = 0.001) are independently predictive of late bleeding.

CONCLUSIONS

DVHs of the rectum are significantly correlated with late bleeding for patients irradiated at 66.6-70.2 Gy after radical prostatectomy.

Comparative analyses of the dynamic properties of the rectum studied by cryo-sections of human cadavers and pelvic CT scans of patients

Optimization of radiotherapy treatment plans based on dose-volume histograms relies on accurate organ delineation. Hollow organs, such as the rectum, are difficult and time-consuming to delineate owing to unclear visualization of the border between wall tissue and filling. Automated hollow organ delineation would be a valuable tool, but its development depends upon improved understanding of the dynamics of the rectum in response to filling. Two reasonable assumptions proposed in the literature are that (1) the rectal wall tissue along a constant length of the rectal cylinder is preserved over time and (2) the rectal wall tissue is distributed homogeneously along the cylinder. Therefore, variations in wall thickness can be explained by variable rectal filling. To investigate these assumptions, transversal cross-sectional areas enclosed by the outer contour (A(out)) and inner contour (A(in)) of the rectum were recorded from digital photographs of cadaver cryo-sections from the U.S. National Library of Medicine's Visible Human Project. In addition, A(out) and A(in) were recorded from 19 CT scans of 5 of our own patients. The transversal cross-sectional area of the wall of the rectum, A(wall)=A(out)-A(in), was calculated. The data derived both from cryo-sections and repetitive CT scans of patients, revealed that there was a significant correlation between A(wall) and A(out), in contradiction to assumption (1) stated above (male cryo-sections: p<0.001, female cryo-sections: p=0.03, repetitive CT scans p<0.001). Moreover, the mean A(wall) calculated from one CT scan differed significantly from the mean A(wall) from other CT scans and was correlated with the mean A(out), i.e. rectal filling (p<0.001). This finding was confirmed by careful analysis of another study (p=0.001) and opposes assumption (2). Hence, the amount of wall tissue within a constant length of rectum is not preserved over time, but increases with increased filling. This implies that the longitudinal length of the rectum decreases in response to distension of the organ.

Three-dimensional conformal therapy versus standard radiation therapy in localized carcinoma of prostate: an update

This study updates technical principles and results of 3-dimensional conformal radiation therapy (3D-CRT) in localized carcinoma of the prostate. Between January 1992 and December 1999, 312 patients were treated with 3D-CRT and 135 patients were treated with bilateral arcs standard radiation therapy (SRT) alone for clinical stage T1b-c or T2 histologically confirmed prostate cancer. None of these patients received hormonal therapy. Mean follow-up for patients in the 3D-CRT group was 3.2 years (range, 2-5.9 years) and for SRT patients, 4.7 years (range, 4-7 years). For 3D-CRT, 7 intersecting fields were used (cerrobend blocking or multileaf collimation) to deliver 68-74 Gy to the prostate. Standard radiation therapy consisted of bilateral 120 degree rotational arcs, with portals using 2-cm margins around the prostate to deliver 68-70 Gy to the prostate. The criterion for chemical disease-free survival was a postirradiation prostate-specific antigen (PSA) value following the American Society for Therapeutic Radiology and Oncology guidelines. Symptoms during treatment were quantitated weekly, and late effects were assessed every 4-6 months. Dose-volume histograms showed a two-thirds reduction with 3D-CRT in normal bladder or rectum receiving > or = 70 Gy with 3D-CRT. Higher 5-year chemical disease-free survival was observed with 3D-CRT (75%; for T1b-c and 79%; for T2 tumors) compared with SRT (61% and 65%, P = 0.01 and P = 0.12, respectively). There was no statistically significant difference in chemical disease-free survival in patients with Gleason score of < or = 4 (P = 0.85), but, with Gleason score of 5-7, the 5-year survival rates were 83% with 3D-CRT and 59% with SRT (P < or = 0.01). In 245 patients with pretreatment PSA of < or = 10 ng/mL treated with 3D-CRT, the chemical disease-free rate was 80% versus 72% in 98 patients treated with SRT (P = 0.21). In patients with PSA of 10.1-20 ng/mL, the chemical disease-free survival rate for 50 patients treated with 3D-CRT was 71% compared with 43% for 20 patients treated with SRT (P = 0.02). The corresponding values were 59% and 16%, respectively, for patients with PSA levels > 20 ng/mL (P = 0.09). On multivariate analysis, the most important prognostic factors for chemical failure were pretreatment PSA (P = 0.004), nadir PSA (P = 0.001), and 3D-CRT technique (P = 0.012). Moderate dysuria was reported by 2%-5% of patients treated with 3D-CRT in contrast to 6%-9% of patients treated with SRT. The incidence of moderate loose stools or diarrhea, usually after the fourth week of treatment, was 3%-5% in the 3D-CRT patients and 8%-19% in the SRT group. Late intestinal grade 2 morbidity (proctitis or rectal bleeding) was 1% in the 3D-CRT group in contrast to 7% in SRT patients. The 3D-CRT spares more normal tissues, yields higher chemical disease-free survival, and results in less treatment morbidity than SRT in treatment of stage T1-T2 prostate cancer. Follow-up at > or = 10 years is needed to confirm these observations.

Modeling volume effects of experimental brachytherapy in the rat rectum: uncovering the limitations of a radiobiologic concept

PURPOSE

To analyze the significance of volume effects in experimental brachytherapy, based on modeling normal tissue complication probability.

METHODS AND MATERIALS

Experimental brachytherapy in the rat rectum was based on an eight-step 2.5-mm step size source configuration for 192Ir, afterloaded into an unshielded polystyrene applicator. Volume effects were studied using a half-circumferential lead-shielded applicator and a shorter (two-step) source configuration. The main end point was rectal stenosis.

RESULTS

Rectal stenosis was always caused by a radiation ulcer. With the shielded configuration, single-dose ED50 (50% incidence of rectal stenosis) increased from 23 Gy to 36.5 Gy. Single-dose ED50 for the short configuration was 77.9 Gy. The data showed a reasonable fit to a three-parameter version of the biophysical model described by Jackson et al. (1995). This model assumes that organs consist of a large number of radiobiologically independent subunits and that radiation causes a complication if the fraction of the organ damaged is greater than its functional reserve. The fraction of the organ damaged is calculated summing over fractions of the organ damaged at each dose level. The calculated mean functional reserve (nu50) of the rat rectum, assuming a cumulative functional reserve distribution in the group of experimental rats, was 0.53.

CONCLUSIONS

The volume effect observed within small brachytherapy volumes agreed well with clinical experience of large tolerance doses in contact X-ray therapy. However, the nu50 value was comparable to the high functional reserve value reported for liver. Experimental volume effects probably reflect repair processes originating in the areas adjacent to small radiation fields of brachytherapy more than the radiobiologic characteristics of the cells in the irradiated volume.

Relationships between DVHs and late rectal bleeding after radiotherapy for prostate cancer: analysis of a large group of patients pooled from three institutions

BACKGROUND

Accurate modeling of late rectal reactions needs the collection of individual 3D dose-volume data (i.e. DVH) as well as clinical information of large cohorts of patients. The possibility of collecting a large number of patients with many different dose-volume combinations is very suitable for this purpose.

PURPOSE

The purpose of the study is to search for significant correlation between dose-volume histograms/dose statistics of the rectum and late rectum bleeding.

MATERIALS AND METHODS

Data from three institutions for 402 patients previously treated for prostate cancer with three to four field techniques, were retrospectively pooled and were collected with a number of clinical and physical parameters, including DVHs of the rectum (including filling). Patients with large air/fecal content in the rectum during planning computerized tomography (CT) scan were excluded from the analysis (n = 74). Out of 328 patients, 229 patients received an ICRU dose between 70 and 76Gy and the current analysis is referred to this subgroup of patients (median follow up: 30 months, range: 12-85 months). Out of these 229 patients, 189 patients were treated with conformal techniques. Rectum was contoured from the anal verge up to the sigmoid flessure by one observer for each institution. Dosimetric and contouring consistencies between the three institutions were previously investigated and the impact on DVHs was found to be quite modest for the purposes of the study. Median/quartile values of all parameters were considered as cut-off values for statistical analysis. We considered as bleeders those patients who experienced grades II-III late bleeding (modified RTOG scoring scale).

RESULTS

Twenty two of 229 patients experienced > or =grade II late bleeding (30 months actuarial incidence: 10.7%). Significant correlation between a number of parameters and late bleeding was found (log-rank test). With regard to DVH, all median and third quartile values for V50-V70 were found to be significantly associated with an increased risk of rectal bleeding, if excepting the median value of V70. Based on the results of univariate analysis, the patients were divided into two groups: 'high risk', with at least one value above quartiles in the range V50-V60 (V50: 70%, V55: 64%, V60: 55%); 'low risk', the remaining patients. The 30 months actuarial rates of bleeding were 19.2 and 5.9% for the 'high' and the 'low' risk group, respectively (P = 0.0003 log-rank test). A multivariate analysis (Cox regression model) including 'DVH grouping' and the main remaining variables (age, previous prostatectomy, diabetes, hypertension, adjuvant hormonal therapy, rectum volume and ICRU dose) showed that 'DVH grouping' is the most predictive parameter (P = 0.005) together with adjuvant hormonal therapy (P = 0.025) and ICRU dose (P = 0.06).

CONCLUSIONS

Our data confirm the role of the rectal DVH in separating groups of patients having prostate radiotherapy in low and high risk of developing late bleeding. Based on these results, V50 below 60-65% and V60 below 50-55% seem to be the robust cut-off values to keep the risk of developing late rectal bleeding reasonably low. However, due to the 'heterogeneity' of the considered population, the results found should be applied with caution in 'more homogeneous' groups of patients. The association of adjuvant hormone deprivation seems to be associated with an increased risk of rectal toxicity; the mechanism for this effect should be a focus of further research.

Radiobiological indices that consider volume: a review

Understanding and predicting the impact of any radiotherapy treatment is critical if patients are to receive treatment with a high likelihood of eliminating the tumour and low likelihood of complications. One of the major contributing factors in determining these effects is the volume treated. This review assesses the current use and accuracy of a series of models which consider volume, building on a previous review which investigated the impact of fractionation particularly with respect to the linear quadratic model. Volume is particularly important in assessing the overall effect with respect to destroying the clonogenic cells and preventing damage to the normal tissues. Dose volume histograms are one of the simplest and most useful forms of representing volume information, however it is difficult to correlate plans based only on DVHs. For this reason various reduction schemes have been introduced and tumour control probability and normal tissues complication probability models adjusted to use this information. Many of these models have proved quite useful in the clinic although they are limited by the available radiobiological data.

Rectum contouring variability in patients treated for prostate cancer: impact on rectum dose-volume histograms and normal tissue complication probability

BACKGROUND

Recent investigations showed some correlation between three-dimensional (3D) treatment planning dose-volume data (dose-volume histograms: DVH, dose statistics) and rectal toxicity for patients treated for prostate cancer. However, no data are available about the possible impact of inter-institute variability in contouring the rectum, so that the possibility of reliably using information from single-centre studies remains doubtful.

PURPOSE

Within a retrospective three-institutes study on correlation between dose-volume treatment planning data and rectum bleeding in patients treated for prostate cancer, an investigation about the impact of inter- and intra-observer variability in contouring the rectum was performed.

MATERIALS AND METHODS

Ten patients were considered for a dummy run exercise and three observers (one per Institute) contoured the rectum (including filling). An anatomically based definition of rectum extension was previously accepted by the three observers. Six of the ten patients were randomly chosen in the subgroup of patients (large spacing, LS) with a distance between computed tomography (CT) slices (outside the prostate region) equal to 10 mm; for the remaining four patients the distance between CT slices was 5 mm over the whole rectum volume (small spacing, SS). The original 3D treatment planning was recovered on the Cadplan treatment planning system for each patient and rectum dose statistics (mean, median and maximum rectum dose), volume, DVH and NTCP values were calculated for each observer. For DVH analysis, the values of V(50), V(55), V(60), V(65) and V(70) (defined as the % of rectum volume receiving at least 50, 55, 60, 65, 70 Gy) were considered. Normal tissue complication probabilities (NTCPs) were calculated for the original ICRU dose and for a 75.6 Gy ICRU dose (NTCP and NTCP(75.6), respectively). Intra-observer variability was investigated by asking the observers to redraw the same rectum contours 6 months later and comparing the two contouring sessions.

RESULTS

In general, a good agreement was found for most patients and, in particular, for all SS patients. The impact of inter-observer variability was quite significant on dose statistics and DVH in two of six LS patients. Looking at the patient population, some systematic deviations, even if quite small, were demonstrated between institute B and institute C (volume, P = 0.02) and between institute A and institute B (mean/median dose, V(50)-V(65), NTCP(75.6); P < 0.05). Four of six LS patients (0/4 in the SS group) presented a maximum difference among observers at the cranial and/or caudal limit of the rectum equal to 1 cm. For these patients, inter-observer variability was significantly higher than for the others (P < 0.03). When inter-observer variability was expressed in terms of standard deviations (SD), values around 2-3 Gy and 0.5 Gy for LS and SS patients, respectively, were found for mean/median dose; values around 3-4% and 0.5-2% for LS and SS patients, respectively, were found for V(50)-V(70). The average SD for NTCP and NTCP(75.6) were 0.4 and 0.6%, respectively (0.5 and 0.9% for LS patients; 0.2 and 0.3% for SS patients). Intra-observer variability was found to be lower than inter-observer variability even if the impact on dose statistics and DVH was visible.

CONCLUSIONS

Once a robust definition of rectum is assessed, inter- and intra-institute variability in contouring the rectum appear relatively modest. However, the results suggest that the number of LS patients in DVH correlation studies should be as low as possible; the low number of these patients in the multi-centric trial involving our institutions should not have significant impact on the results of the study.

A variable critical-volume model for normal tissue complication probability

Predicting late-term normal-tissue complication probability (NTCP) after radiotherapy is an important factor in the optimization of conformal radiotherapy. We propose a new NTCP model, based on the properties of the high dose region. The principal assumption of the new model is that a whole-organ complication will occur when the radiation damage to a normal organ volume (a portion of the total organ) exceeds a threshold value. The dose threshold for complications varies with the size of the volume (percent of the total organ). We hypothesize that a complication occurs if the complication threshold is exceeded for any organ volume. We used the average dose to a volume as a measure of radiation damage to that volume. Also, we used the power law to scale the average dose to various organ volumes to a whole-organ equivalent dose, and to identify the volume with the most harmful dose-size combination-the critical volume. We used a logistic distribution to calculate the probability that the patient will develop a complication, given the dose delivered to the critical volume. We used a maximum likelihood fit to estimate the model parameters for late-term rectal complications in a set of patients treated for prostate carcinoma with external photon beam radiotherapy (EBRT). Good correspondence was found between the experimental data and the model predictions.

Modeling the effects of inhomogeneous dose distributions in normal tissues

Conformal radiation therapy frequently produces inhomogeneous dose distributions in normal tissues near the target. Most mathematical models of normal tissue complication probabilities (NTCP) are based on uniform whole or partial organ irradiation, and the model parameters are chosen to obtain agreement with clinical outcomes in these simple situations. Frequently used NTCP models and methods for including inhomogeneous dose distributions in model calculations are outlined in this report. It has been found that the model adopted may qualitatively affect prediction of complications. Limitations placed on current models by the scarcity of reliable complications data and other approaches to using the calculated dose distribution to predict NTCP are discussed.

Rectal sequelae after conformal radiotherapy of prostate cancer: dose-volume histograms as predictive factors

PURPOSE

To identify clinically relevant parameters predictive of late rectal bleeding derived from cumulative dose-volume histograms (DVHs) of the rectum after conformal radiotherapy of prostate cancer.

MATERIALS AND METHODS

One hundred and nine patients treated with 3D conformal radiotherapy between 1/1994 and 1/1996 for localized prostate cancer (clinical stage T1-T3) were available for analysis. All patients received a total dose of 66 Gy/2 Gy per fraction (specified at the International Commission on Radiation Units and Measurements ICRU reference point). DVHs of the contoured rectum were analyzed by defining the absolute (aV) and relative (rV) rectum volume that received more than 30% (V30), 50% (V50), 70% (V70), 80% (V80), 90% (V90) and 100% (V100) of the prescribed dose. Additionally, a new aspect of DVH analysis was investigated by calculation of the area under the DVH-curve between several dose levels (area under the curve (AUC)-DVH). DVH-variables were correlated with radiation side effects evaluated in 3-6 months intervals and graded according to the EORTC/RTOG score. The median follow-up was 30 months (12-60 months).

RESULTS

Univariate and multivariate stepwise Cox-Regression analysis including age, PTV, rectum size, rV100, rV90, rV80, rV70, rV50 rV30 and aV30 to aV100 were calculated. Late rectal bleeding (EORTC/RTOG grade 2) was significantly correlated with the percentage of rectum volume receiving > or = 90% of the prescribed dose (rV90) (P = 0.007) and inversely correlated in a significant way with the size of contoured rectum (P = 0.006) in multivariate analysis. In our series, a proportion of the rectum volume > or = 57% were included in the 90%-isodose (rV90 > or = 57%) in one half of the patients, with an actuarial incidence of 31% of late rectal bleeding at 3 years. In the other half of the patients, when rV90 < 57%, the 3-year actuarial incidence was 11% (P < 0.03).

CONCLUSION

Our data demonstrate a dose-volume relationship at the reference dose of 60 Gy ( approximately 90% of the prescribed dose) with respect to late rectal toxicity. The rV90 seems to be the most useful and easily obtained parameter when comparing treatment plans to evaluate the risk of rectal morbidity.

Partially wedged beams improve radiotherapy treatment of urinary bladder cancer

BACKGROUND AND PURPOSE

Partially wedged beams (PWBs) having wedge in one part of the field only, can be shaped using dynamic jaw intensity modulation. The possible clinical benefit of PWBs was tested in treatment plans for muscle-infiltrating bladder cancer.

MATERIAL AND METHODS

Three-dimensional treatment plans for 25 bladder cancer patients were analyzed. The originally prescribed standard conformal four-field box technique, which includes the use of lateral ordinary wedge beams, was compared to a modified conformal treatment using customized lateral PWBs. In these modified treatment plans, only the anterior parts of the two lateral beams had a wedge. To analyze the potential clinical benefit of treatment with PWBs, treatment plans were scored and compared using both physical parameters and biological dose response models. One tumour control probability model and two normal tissue complication probability (NTCP) models were applied. Different parameters for normal tissue radiation tolerance presented in the literature were used.

RESULTS

By PWBs the dose homogeneity throughout the target volume was improved for all patients, reducing the average relative standard deviation of the target dose distribution from 2.3 to 1.8%. A consistent reduction in the maximum doses to surrounding normal tissue volumes was also found. The most notable improvement was demonstrated in the rectum where the volume receiving more than the prescribed tumour dose was halved. Treatment with PWBs would permit a target dose escalation of 2-6 Gy in several of the patients analyzed, without increasing the overall risk for complications. The number of patients suitable for dose escalation ranged from 3 to 15, depending on whether support from all or only one of the five applied NTCP model/parameter combinations were required in each case to recommend dose escalation.

CONCLUSION

PWBs represent a simple dose conformation tool that may allow radiation dose escalation in the treatment of muscle-infiltrating urinary bladder tumours.

A prospective comparison of three systems of patient immobilization for prostate radiotherapy

PURPOSE

The study compared the setup reliability of 3 patient immobilization systems, a rubber leg cushion, the alpha cradle, and the thermoplastic Hipfix device, in 77 patients with cT1-T3, N0, M0 prostate cancer receiving conformal radiotherapy.

METHODS AND MATERIALS

Port films were analyzed and compared to simulation films to estimate the setup errors in the three coordinate axes (anterior-posterior, cranial-caudal, medial-lateral). A total vector error was calculated from these shifts.

RESULTS

The Hipfix was found significantly superior to the other two devices in reducing mean setup errors in all axes (p < 0.005). The average field-positioning error with the Hipfix ranged from 1.9 mm to 2.6 mm for all axes, whereas the deviation for the other two systems ranged from 2.7 to 3. 4 mm. Errors greater than 10 mm were virtually eliminated with the Hipfix system. There was a reduction in the mean total vector error in the alpha cradle and Hipfix patient cohorts over time, reflecting improved efficacy as a result of experience.

CONCLUSION

There was a significant difference in the performance of each immobilization device. The Hipfix was consistently more reliable in reducing setup errors than the other devices.

A new three-dimensional dose distribution reduction scheme for tubular organs

In tubular structures, spatial aspects of the dose distribution may be important in determining the normal tissue response. Conventional dose-volume-histograms (DVHs) and dose-surface-histograms (DSHs) lack spatial information and may not be adequate to represent the three-dimensional (3D) dose data. A new 3D dose distribution data reduction scheme which preserves its longitudinal and circumferential character is presented. Dose distributions were generated at each axial level for esophagus or rectum in 123 patients with lung cancer or prostate cancer. Dose distribution histograms at each axial level were independently analyzed along the esophageal or rectal circumference to generate dose-circumference-histogram (DCH) sheets. Two types of plots were then generated from the DCH sheet. The first considered the percentage of the circumference at each axial level receiving various doses. The second considered the minimum dose delivered to any percentage of the circumference at each axial level. The DCH as a treatment planning tool can be easily implemented in a 3D planing system and is potentially useful for the study of the relationship between the complication risk and the longitudinal and circumferential dose distributions.

Three-dimensional conformal therapy or standard irradiation in localized carcinoma of prostate: preliminary results of a nonrandomized comparison

PURPOSE

We present preliminary results of a nonrandomized comparison of three-dimensional conformal radiation therapy (3D CRT) and standard radiation therapy (SRT) in localized carcinoma of the prostate in two groups of patients with comparable prognostic factors treated during the same period.

METHODS AND MATERIALS

Between January 1992 and December 1997, 146 patients were treated with 3D CRT and 131 with SRT alone for clinical stage T1c or T2 histologically confirmed carcinoma of the prostate. None of these patients received hormonal therapy. Mean follow-up for all patients is 3 years (range, 1-6 years). For 3D CRT, 7 intersecting fields were used (Cerrobend blocking or multileaf collimation) to deliver 68-73.8 Gy to the prostate; 3D dose distributions and dose-volume histograms (DVHs) of the planning target volume, bladder, and rectum were obtained. SRT consisted of bilateral 120 degrees rotational arcs, with portals with 2-cm margins around the prostate to deliver 68-70 Gy to the prostate. The criterion for chemical disease-free survival was a postirradiation prostate-specific antigen (PSA) (Tandem-R, Hybritech) value following the American Society for Therapeutic Radiology and Oncology guidelines. Symptoms during treatment were quantitated weekly, and late effects were assessed every 4-6 months.

RESULTS

DVHs showed a two-thirds reduction in normal bladder or rectum receiving 70 Gy or more with 3D CRT. Higher 5-year chemical disease-free survival was observed with 3D CRT (91% for T1c and 96% for T2 tumors) compared with SRT (53% and 58%, respectively). There was no statistically significant difference in chemical disease-free survival in patients with Gleason score of 4 or less (p = 0.83), but with Gleason score of 5-7, the 5-year survival rates were 96% with 3D CRT and 53% with SRT (p < or = 0.01). In 111 patients with pretreatment PSA of 10 ng/mL or less, treated with 3D CRT, the chemical disease-free rate was 96% vs. 65% in 94 patients treated with SRT (p < or = 0.01). In patients with PSA of 10. 1-20 ng/mL, the chemical disease-free survival rate for 26 patients treated with 3D CRT was 88% compared with 40% for 20 patients treated with SRT (p = 0.05). The corresponding values were 71% and 26%, respectively, for patients with PSA levels of greater than 20 ng/mL (p = 0.30). On multivariate analysis, the most important prognostic factors for chemical failure were pretreatment PSA (p = 0. 023), nadir PSA (p = 0.001), and 3D CRT technique (p = 0.033). Moderate dysuria and difficulty in urinating were reported by 2-5% of patients treated with 3D CRT in contrast to 6-9% of patients treated with SRT; moderate urinary frequency and nocturia were reported by 18-24% treated with 3D CRT and 18-27% of patients in the SRT group. The incidence of moderate loose stools/diarrhea, usually after the 4th week of treatment, was 3-5% in the 3D CRT patients and 8-19% in the SRT group. Late intestinal morbidity (proctitis, rectal bleeding) was very low (1.7%) in the 3D CRT group in contrast to the SRT patients (8%).

CONCLUSION

Three-dimensional CRT spares more normal tissues, yields higher chemical disease-free survival, and results in less treatment morbidity than SRT in treatment of Stage T1-T2 prostate cancer. Longer follow-up is needed to confirm these preliminary observations.

Conformal irradiation of concave-shaped PTVs in the treatment of prostate cancer by simple 1D intensity-modulated beams

BACKGROUND

In the case of concave-shaped PTVs including prostate (P) and seminal vesicles (SV), intensity-modulated radiation therapy (IMRT) should improve the therapeutic ratio of the treatment of prostate cancer.

PURPOSE

Comparing IMRT by simple 1D modulations with conventional 3D conformal therapy (i.e. non-IMRT) in the treatment of concave-shaped PTVs including P+SV.

MATERIALS AND METHODS

For five patients having a concave-shaped PTV (P+SV) previously treated at our Institute with conformal radiotherapy, conventional 3- and 4-fields conformal plans were compared with IMRT plans in terms of biological indices. IMRT plans were generated by using five equi-spaced beams with a partial shielding of the rectum obtainable with our single-absorber modulation technique (Fiorino C, Lev A, Fusca M, Cattaneo GM, Rudello F, Calandrino R. Dynamic beam modulation by using a single dynamic absorber. Phys. Med. Biol. 1995;40:221-240). The modulation was one-dimensional and the shape of the beams was at single minimum in correspondence with the 'core' of the rectum; the beam intensity in the minimum was set equal to 20 or 40% of the open beam intensity. All plans were simulated on the CADPLAN TPS using a pencil-beam based algorithm (with 18 MV X-rays). Tumour control probability (TCP) and normal tissue complication probabilities (NTCPs) (for rectum, bladder and femoral head) were calculated for all situations when varying the isocentre dose from 60 to 90 Gy. Dose distributions were corrected taking dose fractionation into account through the linear-quadratic model; for the TCP/NTCP estimations the Webb-Nahum and the Lyman-Kutcher models were respectively applied. Three different scores were considered: (a) increase of TCP while keeping rectum NTCP equal to 5% (TCP(5%)); (b) increase of the uncomplicated tumour control probability (P+); (c) increase of the biological-based scoring function (S+), developed by Mohan et al. (Mohan R, Mageras GS, Baldwin B, Clinically relevant optimization of 3D conformal treatments. Med. Phys. 1992;19:933-944). The impact of the uncertainty in the knowledge of the parameters of the biological models was investigated for TCP(5%).

RESULTS

(a) The average gain in TCP(5%) when considering IMRT against non-IMRT conformal plans was 7.3% (range 5.0-13.5%); (b) the average increase of P+ was 3.4% (range: 1. 0-8.5%); and (c) the average increase of S+ was 5.4% (range 2.9-12. 4%). The largest gain was found for one patient (patient 5) showing a significantly larger overlapping between PTV and rectum.

CONCLUSIONS

Simple 1D-IMRT may clearly improve the therapeutic ratio in the treatment of concave-shaped PTVs including P and SV. In the range of clinically suitable values, the impact of the uncertainty of the parameters n and sigma(alpha) does not significantly alter the main results concerning the gain in TCP(5%). The reported gain in terms of P+ and S+ should be considered with great caution because of the intrinsic uncertainties of the model's parameters and, for bladder, because the 'true' DVH (considering variations of the shape and dimension due to variable filling) may be very different from the DVH calculated on a single CT scan. Further investigations should consider inversely-optimised 1D and 2D-IMRT plan in order to compare them in terms of cost-benefit.

Bone marrow doses and leukaemia risk in radiotherapy of prostate cancer

BACKGROUND AND PURPOSE

As more and more patients with prostate cancer are cured and survive with only minor chronic morbidity, other potentially treatment related morbidity, in particular second cancers, becomes an urgent problem which may influence decisions on treatment strategy and treatment plan optimisation. Epidemiological data suggest a radiotherapy associated risk of AML in prostate cancer patients of approximately 0.1% in 10 years. The aim of the study was to determine the range of bone marrow doses from different treatment plans and in different patients in order to develop criteria for optimisation of treatment plans in conformal radiotherapy of prostate cancer to further minimise the small risk of secondary leukaemia.

MATERIALS AND METHODS

Doses to the pelvic bone marrow were calculated for eight different plans used in radiotherapy of prostate cancer to determine the variability of bone marrow doses in radiotherapy of prostate cancer. Computer tomography (CT) slices of the entire pelvic region of an Alderson phantom were acquired and transferred to the TPS. Critical bone marrow structures were outlined in each slice. Different treatment plans were evaluated on this phantom and dose-volume histograms (DVH) for the pelvic bone marrow were obtained. Similarly, the DVH for the bone marrow of 14 patients who received conformal radiotherapy for prostate cancer was determined.

RESULTS

Mean total bone marrow doses ranged from 3.4 to 5.6 Gy in the phantom study. Approximately 99% of the mean dose to the total bone marrow comes from the dose to bone marrow located in the pelvic bones and lumbar vertebrae. Mean bone marrow doses of 14 patients given the same conformal radiotherapy plan ranged from 3.5 to 7.7 Gy.

CONCLUSIONS

No correlation was found between the rectum normal tissue complication probability (NTCP) and the mean bone marrow dose. This means that in the process of treatment planning, exposure to both critical organs, the rectum as well as the bone marrow, should be minimised independently to arrive at the optimal treatment plan.

Normal tissue complication probabilities correlated with late effects in the rectum after prostate conformal radiotherapy

PURPOSE

Radiation therapy of deep-sited tumours will always result in normal tissue doses to some extent. The aim of this study was to calculate different risk estimates of late effects in the rectum for a group of cancer prostate patients treated with conformal radiation therapy (CRT) and correlate these estimates with the occurrences of late effects. Since the rectum is a hollow organ, several ways of generating dose-volume distributions over the organ are possible, and we wanted to investigate two of them.

METHODS AND MATERIALS

A mathematical model, known as the Lyman-Kutcher model, conventionally used to estimate normal tissue complication probabilities (NTCP) associated with radiation therapy, was applied to a material of 52 cancer prostate patients. The patients were treated with a four field box technique, with the rectum as organ at risk. Dose-volume histograms (DVH) were generated for the whole rectum (including the cavity) and of the rectum wall. One to two years after the treatment, the patients completed a questionnaire concerning bowel (rectum) related morbidity quantifying the extent of late effects.

RESULTS

A correlation analysis using Spearman's rank correlation coefficient, for NTCP values calculated from the DVHs and the patients' scores, gave correlation coefficients which were not statistically significant at the p<0.01 level. The correlation coefficients based on histograms of the whole rectum were larger than those derived from histograms of the rectum wall. Also, simpler descriptive measures as Dmax, of the whole rectum, correlated better to observed late toxicity than Dmax derived from histograms of the rectum wall. Correlation coefficients from "high-dose" measures were larger than those calculated from the NTCP values. Accordingly, as the volume parameter of the Lyman-Kutcher model was reduced, raising the impact of small high-dose volumes on the NTCP values, the correlation between observed effects and NTCP values became significant at p<0.01 level.

CONCLUSIONS

1) High-dose levels corresponding to small volume fractions of the cumulative dose-volume histograms were best correlated with the occurrences of late effects in the rectum as measured with questionnaires. This is compatible with a more serial organisation of the rectal tissue architecture than previously reported. 2) Reducing the Lyman-Kutcher model's volume parameter, thus allowing small high-dose regions to determine the NTCP, improved the correlation, but not beyond that of high-dose levels corresponding to small volume fractions of the cumulative dose-volume histograms.

Murine strain differences in the volume effect and incidence of radiation-induced colorectal obstruction

PURPOSE

Interindividual variation in the level of normal tissue damage after radiotherapy has been clinically observed. Murine models have suggested that there may be a genetic component to the variation in susceptibility of different radiation-induced normal tissue complications. Currently, there are no experimental data available describing interstrain differences in the "volume effect" for irradiated normal tissues, such as the colorectum. The aims of this study are to determine if there are strain differences in: 1. the incidence of colorectal obstruction; and 2. the volume effect, after irradiation of the colorectum using two mouse strains that are known to vary in their susceptibility for developing pulmonary fibrosis.

METHODS AND MATERIALS

Various lengths (5.2 to 22.9 mm) of the colorectum of male C57B1/6 and C3Hf/Kam mice were irradiated with a single dose (30 Gy) of 137Cs gamma rays. Also, various doses (20 to 35 Gy) were given to a single length (22.9 mm) of colorectum. The incidence of obstruction was determined as a function of length and dose at 6 months after irradiation. The Threshold Probability model was fit to the length-response data.

RESULTS

C57B1/6 mice developed colorectal obstruction at significantly higher incidence than C3Hf/Kam mice at all lengths after a single dose of 30 Gy. In addition, the data showed a strain difference in the threshold length of colorectum that had to be irradiated before obstructions were observed.

CONCLUSION

Strain differences in the incidence of radiation-induced colorectal obstruction were observed, consistent with previous studies that showed a strain difference in radiation-induced pulmonary fibrosis. The presence of a threshold length of colorectum that was different for the two strains is consistent with the concept that there may be a critical threshold amount of colorectal tissue that can tolerate a high dose without complication, and that the dimensions of the threshold may vary among individuals.

Analysis of the dose-surface histogram and dose-wall histogram for the rectum and bladder

Dose distribution throughout a hollow organ is represented by either a dose-surface histogram (DSH) or a dose-wall histogram (DWH). A spherical shell model for the bladder and a cylindrical shell model for the rectum were introduced for quantifying the difference between the DWH and DSH. The difference was given by subtraction of the percent volume of the wall from the percent area of the surface for any specific dose level. Taking the dose-grid size and contour-delineation uncertainties into account, the DSH and DWH calculation errors were estimated by simplified formulas. The DSHs and DWHs for the rectum and bladder in patients undergoing four-field-prostate treatment and gynecological intracavitary brachytherapy were computed with a refined numerical algorithm. Results of the analytic models and the numerical calculations demonstrated that the difference between the DWH and DSH was small (about 5%) for a fully filled bladder or rectum but large (about 10%) for an empty rectum or a contracted bladder. The error of DSH was about 3%, which is smaller than that of DWH.