Dosimetry of intracavitary applications in carcinoma of the cervix: rectal dose analysis

Evaluation of Volumetric Doses of Organs at Risk in Carcinoma Cervix Patients with HDR Intracavitary Brachytherapy and Comparison of CT-based and Conventional Plans

Background:

Brachytherapy treatment planning in cervix carcinoma patients using two dimensional (2D) orthogonal images provides only point dose estimates while CT-based planning provides volumetric dose assessment helping in understanding the correlation between morbidity and the dose to organs at risk (OARs) and treatment volume.

Objective:

Aim of present study is to compare International Commission on Radiation Units and Measurements Report 38 (ICRU 38) reference point doses to OARs with volumetric doses using 2D images and CT images in patients with cervical cancer.

Material and Methods:

In this prospective study, 20 patients with cervical cancer stages (IIB-IIIB) were planned for a brachytherapy dose of 7Gy per fraction for three fractions using 2D image-based treatment plan and CT-based plan. ICRU 38 points for bladder and rectum were identified on both 2D image-based plan and CT-based plan and doses (D_ICRU) at these points were compared to the minimum dose to 2cc volume (D_2cc) of bladder and rectum receiving the highest dose.

Results:

D_2cc bladder dose was 1.60 (±0.67) times more than D_ICRUb bladder dose whereas D_2cc rectum dose was 1.13±0.40 times D_ICRUr. Significant difference was found between D_ICRUb and D_2cc dose for bladder (p=.0.016) while no significant difference was seen between D_ICRUr and D_2cc dose for rectum (p=0.964).

Conclusion:

The study suggests that ICRU 38 point doses are not the true representation of maximum doses to OARs. CT-based treatment planning is more a reliable tool for OAR dose assessment than the conventional 2D radiograph-based plan.

CT based 3-dimensional treatment planning of intracavitary brachytherapy for cancer of the cervix: comparison between dose-volume histograms and ICRU point doses to the rectum and bladder

BACKGROUND

CT based brachytherapy allows 3-dimensional (3D) assessment of organs at risk (OAR) doses with dose volume histograms (DVHs). The purpose of this study was to compare computed tomography (CT) based volumetric calculations and International Commission on Radiation Units and Measurements (ICRU) reference-point estimates of radiation doses to the bladder and rectum in patients with carcinoma of the cervix treated with high-dose-rate (HDR) intracavitary brachytherapy (ICBT).

MATERIALS AND METHODS

Between March 2011 and May 2012, 20 patients were treated with 55 fractions of brachytherapy using tandem and ovoids and underwent post-implant CT scans. The external beam radiotherapy (EBRT) dose was 48.6 Gy in 27 fractions. HDR brachytherapy was delivered to a dose of 21 Gy in three fractions. The ICRU bladder and rectum point doses along with 4 additional rectal points were recorded. The maximum dose (DMax) to rectum was the highest recorded dose at one of these five points. Using the HDR plus 2.6 brachytherapy treatment planning system, the bladder and rectum were retrospectively contoured on the 55 CT datasets. The DVHs for rectum and bladder were calculated and the minimum doses to the highest irradiated 2cc area of rectum and bladder were recorded (D2cc) for all individual fractions. The mean D2cc of rectum was compared to the means of ICRU rectal point and rectal DMax using the Student's t-test. The mean D2cc of bladder was compared with the mean ICRU bladder point using the same statistical test .The total dose, combining EBRT and HDR brachytherapy, were biologically normalized to the conventional 2 Gy/fraction using the linear-quadratic model. (α/β value of 10 Gy for target, 3 Gy for organs at risk).

RESULTS

The total prescribed dose was 77.5 Gy α/β10. The mean dose to the rectum was 4.58 ± 1.22 Gy for D 2cc, 3.76 ± 0.65 Gy at D ICRU and 4.75 ± 1.01 Gy at DMax. The mean rectal D 2cc dose differed significantly from the mean dose calculated at the ICRU reference point (p<0.005); the mean difference was 0.82 Gy (0.48 -1.19 Gy). The mean EQD2 was 68.52 ± 7.24 Gy α/β3 for D 2cc, 61.71 ± 2.77 Gy α/β3 at D ICRU and 69.24 ± 6.02 Gy α/β3 at DMax. The mean ratio of D 2cc rectum to D ICRU rectum was 1.25 and the mean ratio of D 2cc rectum to DMax rectum was 0.98 for all individual fractions. The mean dose to the bladder was 6.00 ± 1.90 Gy for D 2cc and 5.10 ± 2.03 Gy at D ICRU. However, the mean D 2cc dose did not differ significantly from the mean dose calculated at the ICRU reference point (p=0.307); the mean difference was 0.90 Gy (0.49-1.25 Gy). The mean EQD2 was 81.85 ± 13.03 Gy α/β3 for D 2cc and 74.11 ± 19.39 Gy α/β3 at D ICRU. The mean ratio of D 2cc bladder to D ICRU bladder was 1.24. In the majority of applications, the maximum dose point was not the ICRU point. On average, the rectum received 77% and bladder received 92% of the prescribed dose.

CONCLUSIONS

OARs doses assessed by DVH criteria were higher than ICRU point doses. Our data suggest that the estimated dose to the ICRU bladder point may be a reasonable surrogate for the D 2cc and rectal DMax for D 2cc. However, the dose to the ICRU rectal point does not appear to be a reasonable surrogate for the D 2cc.

Severe gastrointestinal complications in the era of image-guided high-dose-rate intracavitary brachytherapy for cervical cancer

PURPOSE

The purposes of this analysis are to report a modern series of severe gastrointestinal toxic effects after definitive chemoradiotherapy in the treatment of locally advanced cervical cancer at our institution and to review the existing literature on factors that contribute to toxic effects and preventive strategies and management.

METHODS

Our institution's cervical cancer cohort was evaluated for patients with late grade 3 to 4 gastrointestinal toxic effects who were retrospectively reviewed for clinical or dosimetric parameters that could have contributed to late toxic effects. A review of the published literature was performed to identify factors associated with late toxic effects, prophylactic agents, and corrective therapy.

FINDINGS

Five of 85 patients were identified as having late grade 3 to 4 gastrointestinal toxic effects with a median follow-up of 13.3 months. Two of 5 patients developed late grade 3 toxic effects, and 3 of 5 developed late grade 4 toxic effects. Three of the 5 patients reviewed ultimately required permanent colostomies. Cumulative median dose (in equivalent dose in 2-Gy fractions) of clinical target volume to the hottest 90% was 107.2 Gy, rectal dose to the hottest 2 cc (D2cc) was 81.7 Gy, sigmoid D2cc was 61.7 Gy, and bladder D2cc was 79.5 Gy. No patient had evidence of disease recurrence in the pelvis. One patient developed oligometastatic disease in the suprarenal gland and was successfully salvaged with adrenalectomy.

IMPLICATIONS

Despite its risk of toxic effects, intracavitary brachytherapy remains a critical component of the treatment of locally advanced cervical cancer. Even with modern radiotherapy planning and delivery techniques, extra attention is warranted to continue to strive for optimal outcomes.

Radiochromic film dosimetry of rectal inhomogeneity and applicator attenuation in high dose rate brachytherapy of uterine cervix

Heterogeneities existing in the patient during treatment are neglected, as the treated subject is considered homogeneous in most of the commercially‐available treatment planning systems (TPSs) used for high dose rate (HDR) brachytherapy. The choice of a suitable dosimeter for experimental dosimetry near the HDR source is crucial, mainly due to existence of steep dose gradients. The present work aimed to assess the effect of rectal air heterogeneity and applicator attenuation in the HDR Ir‐192 brachytherapy treatment of carcinoma uterine cervix by utilizing GAFCHROMIC EBT2 film dosimetry. The dose to rectal walls under the condition of rectal air heterogeneity was measured experimentally using EBT2 film in a rectal phantom, and the measurements were validated by the Monte Carlo (MC) simulations. The applicator attenuation was measured by EBT2 film for a commonly used stainless steel uterine tube in a homogeneous water equivalent phantom. The measured doses were compared with the TPS calculated values. In case of the air cavity, the measured dose at the closest rectal surface was 12.8% less than the TPS calculated value due to lack of back scattering, whereas at the farthest rectal surface, it was higher by 24.5% due to no attenuation. The magnitude of attenuation due to the metal applicator was measured as high as 2% when compared with the TPS calculation. The dose reduction at the nearest rectal surface due to the effect of rectal air has indicated a clinically favorable dose distribution within the rectum, whereas the shielding effect posed by the metallic applicator was found to be less significant. Mutual agreement of the measured doses with the MC calculated dose values confirmed the suitability of EBT2 film for clinical dosimetry in HDR brachytherapy.

PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.bg, 87.55.Qr

Dosimetric evaluation of rectum and bladder using image-based CT planning and orthogonal radiographs with ICRU 38 recommendations in intracavitary brachytherapy

The purpose is to compare CT-based dosimetry with International Commission on Radiation Units and Measurements (ICRU 38) bladder and rectum reference points in patients of carcinoma of uterine cervix treated with intracavitary brachytherapy (ICA). Twenty-two consecutive patients were evaluated. Orthogonal radiographs and CT images were acquired and transferred to PLATO planning system. Bladder and rectal reference points were identified according to ICRU 38 recommendations. Dosimetry was carried out based on Manchester system. Patient treatment was done using ¹⁹²Iridium high dose rate (HDR) remote after-loading machine based on the conventional radiograph-based dosimetry. ICRU rectal and bladder point doses from the radiograph plans were compared with D₂, dose received by 2 cm³ of the organ receiving maximum dose from CT plan. V₂, volume of organ receiving dose more than the ICRU reference point, was evaluated. The mean (±standard deviation) volume of rectum and bladder was 60 (±28) cm³ and 138 (±41) cm³ respectively. The mean reference volume in radiograph and CT plan was 105 (±7) cm³ and 107 (±7) cm³ respectively. It was found that 6 (±4) cm³ of rectum and 16 (±10) cm³ of bladder received dose more than the prescription dose. V₂ of rectum and bladder was 7 (±1.7) cm³ and 20.8 (±6) cm³ respectively. Mean D₂ of rectum and bladder was found to be 1.11 (±0.2) and 1.56 (±0.6) times the mean ICRU reference points respectively. This dosimteric study suggests that comparison of orthogonal X-ray-based and CT-based HDR ICA planning is feasible. ICRU rectal point dose correlates well with maximum rectal dose, while ICRU bladder point underestimates the maximum bladder dose.

Dose-volume histogram parameters and late side effects in magnetic resonance image-guided adaptive cervical cancer brachytherapy

PURPOSE

To evaluate the predictive value of dose-volume histogram (DVH) parameters for late side effects of the rectum, sigmoid colon, and bladder in image-guided brachytherapy for cervix cancer patients.

METHODS AND MATERIALS

A total of 141 patients received external-beam radiotherapy and image-guided brachytherapy with or without chemotherapy. The DVH parameters for the most exposed 2, 1, and 0.1 cm(3) (D(2cc), D(1cc), and D(0.1cc)) of the rectum, sigmoid, and bladder, as well as International Commission on Radiation Units and Measurements point doses (D(ICRU)) were computed. Total doses were converted to equivalent doses in 2 Gy by applying the linear-quadratic model (α/β = 3 Gy). Late side effects were prospectively assessed using the Late Effects in Normal Tissues-Subjective, Objective, Management and Analytic score. The following patient groups were defined: Group 1: no side effects (Grade 0); Group 2: side effects (Grade 1-4); Group 3: minor side effects (Grade 0-1); and Group 4: major side effects (Grade 2-4).

RESULTS

The median follow-up was 51 months. The overall 5-year actuarial side effect rates were 12% for rectum, 3% for sigmoid, and 23% for bladder. The mean total D(2cc) were 65 ± 12 Gy for rectum, 62 ± 12 Gy for sigmoid, and 95 ± 22 Gy for bladder. For rectum, statistically significant differences were observed between Groups 1 and 2 in all DVH parameters and D(ICRU). Between Groups 3 and 4, no difference was observed for D(0.1cc). For sigmoid, significant differences were observed for D(2cc) and D(1cc), but not for D(0.1cc) in all groups. For bladder, significant differences were observed for all DVH parameters only comparing Groups 3 and 4. No differences were observed for D(ICRU).

CONCLUSIONS

The parameters D(2cc) and D(1cc) have a good predictive value for rectal toxicity. For sigmoid, no prediction could be postulated because of limited data. In bladder, DVH parameters were predictive only for major toxicity.

Variability of marker-based rectal dose evaluation in HDR cervical brachytherapy

In film-based intracavitary brachytherapy for cervical cancer, position of the rectal markers may not accurately represent the anterior rectal wall. This study was aimed at analyzing the variability of rectal dose estimation as a result of interfractional variation of marker placement. A cohort of five patients treated with multiple-fraction tandem and ovoid high-dose-rate (HDR) brachytherapy was studied. The cervical os point and the orientation of the applicators were matched among all fractional plans for each patient. Rectal points obtained from all fractions were then input into each clinical treated plan. New fractional rectal doses were obtained and a new cumulative rectal dose for each patient was calculated. The maximum interfractional variation of distances between rectal dose points and the closest source positions was 1.1 cm. The corresponding maximum variability of fractional rectal dose was 65.5%. The percentage difference in cumulative rectal dose estimation for each patient was 5.4%, 19.6%, 34.6%, 23.4%, and 13.9%, respectively. In conclusion, care should be taken when using rectal markers as reference points for estimating rectal dose in HDR cervical brachytherapy. The best estimate of true rectal dose for each fraction should be determined by the most anterior point among all fractions.

Late Rectal Complications Evaluated by Computed Tomography–Based Dose Calculations in Patients With Cervical Carcinoma Undergoing High-Dose-Rate Brachytherapy

PURPOSE

To investigate the efficacy of dose calculations at the computed tomography (CT)-based rectal point (CTRP) as a predictive factor for late rectal complications in patients with cervical carcinoma who were treated with a combination of high-dose-rate intracavitary brachytherapy and external beam radiotherapy.

METHODS AND MATERIALS

Ninety-two patients with uterine cervical carcinoma undergoing definitive radiotherapy alone were retrospectively analyzed. The median follow-up time for all patients was 32 months (range, 13-60 months). The cumulative biologically effective dose (BED) was calculated at the rectal reference point as defined by the International Commission on Radiation Units and Measurements Report 38 (BED(RP)) and at the CTRP (BED(CTRP)). Late rectal complications were recorded according to the Radiation Therapy Oncology Group grading system.

RESULTS

The late rectal complications were distributed as follows: Grade 0, 68 patients (74%); Grade 1, 20 patients (22%); Grade 2, 4 patients (4%). Univariate analysis showed that BED(RP), BED(CTRP), RP dose/point A dose ratio, and CTRP dose/point A dose ratio were significantly correlated with late rectal complications (p < 0.05). On multivariate analysis, patients with a rectal BED(CTRP) >/=140 Gy(3) presented with significantly greater frequency of rectal complications (p = 0.031).

CONCLUSIONS

The present results suggest that BED(CTRP) is a useful predictive factor for late rectal complications.

Image-based three-dimensional treatment planning of intracavitary brachytherapy for cancer of the cervix: Dose-volume histograms of the bladder, rectum, sigmoid colon, and small bowel

PURPOSE

The purpose of this study was to evaluate dose-volume histograms (DVHs) of bladder, rectum, sigmoid colon, and small bowel using image-based three-dimensional treatment planning for intracavitary brachytherapy.

METHODS AND MATERIALS

Between 2001 and 2003, 22 patients with cancer of the cervix (1 IB1, 5 IB2, 11 IIB, 5 IIIB) were treated with computerized tomography (CT)-compatible high-dose-rate intracavitary applicators and underwent postimplant pelvic CT scans with the applicator in place. The volumes of organs at risk were digitized. For radiography-based planning, International Commission on Radiation Units and Measurements (ICRU) bladder and rectum point doses were calculated. For the CT-based planning, the DVHs were computed for the bladder, rectum, sigmoid colon, and small bowel. To compare doses to organs at risk, the minimum dose in 2.0cm(3) volume receiving the highest dose (D(2)) was determined from DVHs. These D(2) doses were compared with radiography-based ICRU point doses.

RESULTS

The mean ICRU bladder point dose (401cGy) was markedly underestimated compared to the mean bladder D(2) dose (484cGy). However, the mean ICRU rectal point dose (412cGy) did not differ significantly from the mean rectal D(2) dose (373cGy). The most frequent organ receiving the highest D(2) dose was the sigmoid colon in 9 of 22 patients (41%) followed by the rectum in 7 of 22 patients (32%) and small bowel in 6 of 22 patients (27%).

CONCLUSIONS

From CT-based three-dimensional (3-D) evaluation, the ICRU bladder point dose was substantially lower than bladder D(2) dose. Special attention should also be given to the areas of proximal rectum and sigmoid colon due to more frequent high D(2) dose in these areas.

3D inverse treatment planning for the tandem and ovoid applicator in cervical cancer

PURPOSE

Three-dimensional treatment planning systems and inverse planning optimization for brachytherapy are becoming commercially available. Guidelines for target delineation and dose constrictions have not been established using this new software. In this study we describe a method of target delineation for the tandem and ovoids applicator. We then compare inverse planning dose distributions with the traditional methods of prescribing dose.

METHODS AND MATERIALS

Target and organ-at-risk volumes were defined using systematic guidelines on 15 patients treated in our department with high-dose-rate brachytherapy for cervical cancer using tandem and ovoids. High-dose-rate distributions were created according to three different dose optimization protocols: inverse planning simulated annealing (IPSA), point A, and point A with a normalization of 2 cc of the bladder receiving 80% of the dose (bladder-sparing method). An uniform cost function for dose constraints was applied to all IPSA generated plans, and no manual optimization was allowed for any planning method.

RESULTS

Guidelines for target and structure-at-risk volumes, as well as dose constraint cost functions, were established. Dose-volume histogram analysis showed that the IPSA algorithm indicated no difference in tumor coverage compared with point A optimization while decreasing dose to the bladder and rectum. The IPSA algorithm provided better target volume coverage compared with bladder-sparing method with equivalent doses to the bladder and rectum.

CONCLUSION

This study uses a systematic approach for delineating target and organ-at-risk volumes and a uniform cost function for generating IPSA plans for cervical cancer using tandem and ovoids. Compared with conventional dose prescription methods, IPSA provides a consistent method of optimization that maintains or improves target coverage while decreasing dose to normal structures. Image-guided brachytherapy and inverse planning improve brachytherapy dosimetry.

Unique role of proximal rectal dose in late rectal complications for patients with cervical cancer undergoing high-dose-rate intracavitary brachytherapy

PURPOSE

To investigate the correlation of the radiation dose to the upper rectum, proximal to the International Commission of Radiation Units and Measurements (ICRU) rectal point, with late rectal complications in patients treated with external beam radiotherapy (EBRT) and high-dose-rate (HDR) intracavitary brachytherapy (ICRT) for carcinoma of the uterine cervix.

METHODS AND MATERIALS

Between June 1997 and February 2001, 75 patients with cervical carcinoma completed definitive or preoperative RT and were retrospectively reviewed. Of the 75 patients, 62 with complete dosimetric data and a minimal follow-up of at least 1 year were included in this analysis. Of the 62 patients, 36 (58%) also received concurrent chemotherapy, mainly with cisplatin during EBRT. EBRT consisted of a mean of 50.1 +/- 1.3 Gy of 18-MV photons to the pelvis. A parametrial boost was given to 55 patients. Central shielding was used after 40-45 Gy of pelvic RT. HDR ICRT followed EBRT, with a median dose of 5 Gy/fraction given twice weekly for a median of four fractions. The mean dose to point A from HDR ICRT was 23.9 +/- 3.0 Gy. In addition to the placement of a rectal tube with a lead wire during ICRT, 30-40 mL of contrast medium was instilled into the rectum to demonstrate the anterior rectal wall up to the rectosigmoid junction. Late rectal complications were recorded according to the Radiation Therapy Oncology Group grading system. The maximal rectal dose taken along the rectum from the anal verge to the rectosigmoid junction and the ICRU rectal dose were calculated. Statistical tests were used for the correlation of Grade 2 or greater rectal complications with patient-related variables and dosimetric factors. Correlations among the point A dose, ICRU rectal dose, and maximal proximal rectal dose were analyzed.

RESULTS

Fourteen patients (23%) developed Grade 2 or greater rectal complications. Patient-related factors, definitive or preoperative RT, and the use of concurrent chemotherapy were not associated with the occurrence of rectal complications. The maximal rectal dose during ICRT was at the proximal rectum rather than at the ICRU rectal point in 55 (89%) of 62 patients. Patients with Grade 2 or greater rectal complications had received a significantly greater total maximal proximal rectal dose from ICRT (25.6 Gy vs. 19.2 Gy, p = 0.019) and had a greater maximal proximal rectal dose/point A dose ratio (1.025 vs. 0.813, p = 0.024). In contrast, patients with and without rectal complications had a similar dose at point A (25.0 Gy vs.23.6 Gy, p = 0.107). The differences in the ICRU rectal dose (17.8 Gy vs.15.4 Gy, p = 0.065) and the ICRU rectal dose/point A dose ratio (0.71 vs. 0.66, p = 0.210) did not reach statistical significance. Patients with >62 Gy of a direct dose sum from EBRT and ICRT to the proximal rectum (12 of 29 vs. 2 of 33, p = 0.001) and >110 Gy of a total maximal proximal rectal biologic effective dose (13 of 40 vs. 1 of 22, p = 0.012) presented with a significantly increased frequency of Grade 2 or greater rectal complications. The correlations between the maximal proximal rectal dose and the ICRU rectal dose were less satisfactory (Pearson coefficient 0.375). Moreover, 11 of the 14 patients with rectal complications had colonoscopic findings of radiation colitis at the proximal rectum, the area with the maximal rectal dose.

CONCLUSION

Eighty-nine percent of our patients had a maximal rectal dose from ICRT at the proximal rectum instead of the ICRU rectal point. The difference between patients with and without late rectal complications was more prominent for the proximal rectal dose than for the ICRU rectal dose. It is important and useful to contrast the whole rectal wall up to the rectosigmoid junction and to calculate the dose at the proximal rectum for patients undergoing HDR ICRT.

Bladder and rectum dose defined from MRI based treatment planning for cervix cancer brachytherapy: comparison of dose-volume histograms for organ contours and organ wall, comparison with ICRU rectum and bladder reference point

PURPOSE

To analyze the correlation between dose-volume histograms based on organ contour and organ wall delineation for bladder and rectum, and to compare the doses to these organs with the absorbed doses at the ICRU bladder and rectum reference points.

MATERIAL AND METHODS

Individual MRI based brachytherapy treatment planning was performed in 15 patients as part of a prospective comparative trial. The external contours and the organ walls were delineated for the bladder and rectum in order to compute the corresponding dose-volume histograms. The minimum dose in 2 cm(3), 5 cm(3) and 10 cm(3) volumes receiving the highest dose were referred to as [D2], [D5] and [D10] and compared with the absorbed dose at the ICRU rectum and bladder reference point.

RESULTS

The bladder (bext) and rectal (rext) doses derived from external contours and computed for volumes of 2 cm(3) [D2], provided a good estimate for the doses computed for the organ walls (bw and rw) only (mean ratio [D2](bext)/[D2](bw)=1.1+/-0.2 and [D2](rext)/[D2](rw)=1.2+/-0.1, respectively). This correspondence was no longer true when larger volumes were considered (5 and 10 cm(3)). The dose at the ICRU rectum reference point did overestimate the dose computed for 2 cm(3) of the rectum wall (mean ratio: 1.5+/-0.4). In contrast, the dose at the ICRU bladder reference point did-in the case of inappropriate topographic location of the balloon-underestimate the dose computed for 2 cm(3) of the bladder wall (overall mean ratio: 0.9+/-0.4).

CONCLUSION

For clinical applications, when volumes smaller than 5 cm(3) are considered, the dose-volume histograms computed from external organ contours for the bladder and rectum can be used instead of dose-volume histograms computed for the organ walls only. External organ contours are indeed easier to obtain. The dose at the ICRU rectum reference point provides a good estimate of the rectal dose computed for volumes smaller than 2 cm(3) [D2] only for a midline position of the rectum. The ICRU bladder reference point provides a good estimate of the dose computed for the bladder wall [D2] only in cases of appropriate balloon position.

The impact of sectional imaging on dose escalation in endocavitary HDR-brachytherapy of cervical cancer: results of a prospective comparative trial

PURPOSE

The purpose of this comparative prospective study was to assess the effect of CT and MR based individualisation and adaptation on the dose distribution in the target volume and organs at risk compared to a radiography based procedure.

MATERIAL AND METHODS

In 15 patients MR scans, in 10 patients additional axial CT-scans with compatible tube-ring applicator in situ were performed and digitally transferred to the PLATO(R) planning system. Considering clinical examination and MR-scan before radiotherapy individual 3-D dose distribution was calculated and adapted based on (1) two orthogonal radiographs; (2) isodoses superimposed on the CT images; and (3) isodoses superimposed on the MR images. Adaptation was strictly limited by the dose level at 2 cm(3) bladder or rectum volume (D(2)) to allow comparison of CT and MR plans. All three individualised dose distributions were superimposed on the MR images and cumulative dose-volume histograms were calculated for comparison.

RESULTS

3-D individualisation based on sectional imaging enabled higher dose to the target volume (isodose enclosing 95% of the CTV=CTV(95)) compared to individualised treatment plans based on orthogonal radiographs by a mean factor of 1.2 (1-1.7). The dose to bladder and/or rectum wall was at the same time not increased beyond the prescribed tolerance level (71% of the prescribed target dose). In a subgroup of 10 patients MRI based treatment plans were superior to CT based treatment plans allowing for a higher dose (138% vs. 124%).

CONCLUSION

Sectional imaging based treatment planning, in particular using MR, was superior to radiography allowing for a clinical meaningful dose escalation without increasing the dose to bladder and rectum beyond the tolerance level.

Rectal dosimetry in intracavitary applications of cervix carcinoma: Comparison of two methods

BACKGROUND: Brachytherapy of cervix carcinoma often results in high doses to surrounding structures, such as rectum and bladder. Therefore, these organs should be closely monitored. Purpose of this work was to evaluate rectal marker made in our institution for rectal dose measurements by comparing it with the method recommended in ICRU (International Commission on Radiation Units and Measurements) Report 38. METHODS: In this work rectal dosimetry was performed by two different methods. In one, rectal marker made in Institute of Oncology Sremska Kamenica was used, while in the other method recommended in ICRU Report 38 dose on ICRU rectal point was measured A total of 34 applications using Microselectron HDR and its standard applicator set were performed in a prospective way. The prescribed dose was 7.6 Gy to point A for each application. Rectal doses were calculated by Nucletron Plato Treatment Planning System. RESULTS: Differences found between the means of ICRU point R and rectal marker points Rref and Rmax were significant (P<0.002 and P<0.00002). The same result was obtained for Rref and Rmax pair (P<0.003). CONCLUSION: Maximal doses obtained using rectal marker were in most cases high- er than those obtained by ICRU method. It conforms well to several CT-based dosimetry studies where rectum dose was found to be higher from that obtained by ICRU method.

Survey of the use of the ICRU 38 in recording and reporting cervical cancer brachytherapy

BACKGROUND

A survey on the practice of reporting intracavitary cervix cancer brachytherapy (LDR and HDR) in clinical practice (CP) and in literature (LIT) was performed on the occasion of a workshop, 'ICRU 38: The Basis for a Revision', which took place at the Annual GEC ESTRO meeting in Naples in 1998.

MATERIALS AND METHODS

The answers (n=85) to a specific questionnaire which had been sent to all ESTRO members (n=1600), were evaluated. In parallel, a systematic survey on the literature reporting cervix cancer brachytherapy since 1985 was performed using the MEDLINE database. The main recommendations for reporting as given in the ICRU 38 were addressed for both surveys: technique; total reference air kerma (TRAK); dose specification to the target volume '60 Gy reference volume', to organs at risk 'ICRU rectum and bladder point' and other reference points and time-dose pattern. In addition, some other items were investigated such as mg h, Point A, B, and in vivo dosimetry in bladder and rectum.

RESULTS

Issues related to technique (source, machine and applicator type) and to time-dose pattern are reported in the majority of patients in CP and LIT. The same applies for the following parameters: Point A is indicated in 76% (LDR) to 89% (HDR) in CP, in 60% (LDR) to 96% (HDR) in LIT. Rectum and bladder ICRU points are recorded in 55% (HDR) to 90% (LDR) and 58% (HDR) to 84% (LDR), respectively, in CP. On the other hand, TRAK is given in 14% (HDR) to 43% (LDR) in CP, in 0% (HDR) to 10% (LDR) in LIT. '60 Gy reference volume' is recorded in 18% (HDR) to 51% (LDR) in CP, in 0% (HDR) to 17% (LDR) in LIT. Rectum and bladder ICRU points are reported in 18% (LDR) to 28% (HDR) and 14% (HDR) to 29% (LDR), respectively, in LIT. Other reference points and in vivo dosimetry measurements are given in a low percentage. Dose rate and overall treatment time is reported in 10-44%.

CONCLUSION

Recording and reporting in CP and in LIT meets the recommendations as given in ICRU 38 to different degrees. Specific items such as TRAK and the 'Reference volume' have only limited penetration into CP and LIT, which applies in particular to centers using HDR brachytherapy. The discrepancies between CP and LIT may be due to the well-known delay between change in CP and its translation into LIT. In order to arrive at a more common language for the better exchange of clinical results, it seems to be necessary to adapt some terms and recommendations. In particular, comprehensive concepts are needed for reporting dose to points and volumes in the target and in critical organs, according to the new potential from imaging and computer technology and from modern radiobiological insights, bridging the gap between LDR and HDR brachytherapy.

Comparison of radiography- and computed tomography-based treatment planning in cervix cancer in brachytherapy with specific attention to some quality assurance aspects

INTRODUCTION

A modern approach in treatment planning for cervix carcinoma is based on a series of computed tomography (CT) sections and 3D dose computation. When these techniques were not yet available, dose evaluation was based on orthogonal radiographs. The CT based planning provides information on target and organ volumes and dose-volume histograms. The radiography based planning provides only dimensions and doses at selected points. The aim of the presented study is to correlate the information obtained with the two approaches for high dose-rate (HDR) brachytherapy of cervix carcinoma.

METHODS

For the study 28 patients with 35 applications receiving HDR treatment with Ir-192 were investigated. The planning system PLATO (Nucletron) was used. The different aspects of available data, results and inaccuracies regarding quality assurance were looked at.

RESULTS

From the CT based planning, the volume, location and dose-volume histograms were calculated for the CTV, rectum and bladder. From the radiography-based planning, the dose to point A (prescription), point B, rectum and bladder ICRU reference points [14], points related to the bony structures could be evaluated as well as volumes receiving different dose levels. These two sets of information were compared and following mean values derived. For a dose prescription of 7 Gy at point A, as an average, 83% (44 cm(3)) of the clinical target volume (CTV) receives at least 7 Gy. The mean dose at the rectum ICRU reference point is 4.3 Gy, and 12% (9 cm(3)) of the rectum is encompassed by the 4.3 Gy isodose. The mean dose at the bladder ICRU reference point is 5.8 Gy, and 8% (16 cm(3)) of the bladder is encompassed by the 5.8 Gy isodose. The maximum dose to the rectum is 1.5 times higher than the dose at the ICRU reference point, and for the bladder 1.4 times higher. Uncertainties caused by the reconstruction of the applicator and merging of isodoses could be evaluated.

DISCUSSION

The subdivision of different approaches and the transfer from point doses to volumes in treatment planning is possible and practical for the treatment of cervix carcinoma in brachytherapy.

Rectal doses in intracavitary brachytherapy of gynecological malignancies: comparison of two dosimetric methods

PURPOSE

To compare calculated rectal doses obtained by two dosimetric methods in intracavitary brachytherapy of gynecological malignancies.

MATERIALS AND METHODS

This analysis included 124 intracavitary applications performed in 102 patients with cervical or endometrial cancer. The pelvic dose distribution based on orthogonal intracavitary placement films was calculated with the computer planning system. In each application the rectal dose was defined in the specific rectal point determined by both the use of a wire marker inserted into the rectum (R1) and by packing the vagina with radio-opaque gauze - the method recommended by the ICRU Report 38 (R2). The comparison included R1 and R2 doses as well as the respective radiobiological equivalent doses determined by the linear-quadratic model (r1 and r2).

RESULTS

In 83% of applications the absolute value of R1 was lower than R2. The mean difference between R1 and R2 was 3.7 Gy (95% CI 3.03-4.41 Gy) and between r1 and r2 7.2 Gy (95% CI 5.77-8.56 Gy). These differences were significant (P<0.001 for both comparisons). The difference between the doses was not influenced by the type of applicator and remained significant even when a systemic+/-10% error of method was assumed.

CONCLUSION

The rectal point dose determined with the use of rectal wire marker may be underestimated, therefore this method should be discouraged in gynecological brachytherapy.

Biologically effective doses in medium dose rate brachytherapy of cancer of the cervix

The amount of dose reduction on changing from low dose rate (LDR) brachytherapy to medium dose rate (MDR) or high dose rate (HDR) afterloading has been the subject of much debate. The magnitude of reduction depends, together with other possible factors, on two radiobiological parameters: the alpha/beta ratio and the half-time of repair of the relevant tissues. In an attempt to extract these radiobiological parameters for the late rectal complications observed in our previously published clinical results four different schedules using MDR and one using LDR are analyzed. The percentage incidence of complications was a function of increasing biologically effective dose (BED), but would yield nonsense scattergrams if plotted against raw total dose. In addition, for three other published MDR series, three LDR series, and two HDR series, the incidence of rectal complications is plotted against BED to examine the predictive potential of using BED as the surrogate of total dose. Our own results were published in 1996, consisting of 102 patients treated at the LDR of 0.44 Gy/hr and 88 patients treated by four different schedules using an MDR of 1.6-1.7 Gy/hr. Follow-up is at least 3 years in all schedules. The linear quadratic formula including the "g" dose rate factor was used to analyze them, assuming exponential repair of the repairable beta term. First, multivariate and profile likelihood analyses were carried out to obtain estimates of alpha/beta and T1/2 for rectal late responding tissues. Then graphs of incidence of rectal complications vs. BED were constructed, assuming alpha/beta = 3 Gy and T1/2 = 1.5 hr, values which had not been contradicted by the multivariate analysis. Graphs were drawn both for "all grades including mild reactions" (grades 1 + 2 + 3) and for "serious" complications (grade 3 in our system). In addition, other published cervical brachytherapy series were reviewed, with calculation of their BEDs if not published by the authors. It was necessary to review and compare their grading systems, so that "mild and moderate" (grades 1 and 2) could be contrasted with "serious" (grades 3 and 4 or 5 in various systems). Comparisons were made with other published results, including three LDR, three MDR, and two HDR series spanning from 1982 to 1997. The BEDs at which the incidence of rectal complications rose above the arbitrary level of 10% were compared for all three ranges of dose rate. The multivariate analysis gave estimates of alpha/beta and T1/2 which were not significantly different from 3 Gy and 1.5 hr, respectively, so these values were used to compute the BEDs for the subsequent comparisons. It was found that the graphs of incidence of rectal complications for "all grades including mild" agreed rather better between all series than might have been expected, within a provisional (10%) threshold BED of range 100-123 Gy3 (60-74 Gy given as 2 Gy fractionated external beam or as LDR). The dose-response curves diverged above these values, as expected until common grading systems such as SOMA/ LENT become more widely used. For "serious" complications the 10% incidence occurred at a median BED of 140 Gy3 (84 Gy given as 2 Gy fractionated external beam or as LDR), range 124-155 Gy3. The use of BED (or extrapolated response dose), assuming alpha/beta = 3 Gy and T1/2 = 1.5 hr, instead of total dose, enabled incidence of late rectal complications in cervical brachytherapy with LDR, MDR, and HDR to be plotted in a reasonably consistent way. This does not mean that those parameter values have been definitively determined, but they appear to be provisional values that may be of use in comparing the expected effects of new schedules until better values are obtained from greater use of common grading systems.