Reference dose rates for single- and double-plane 192Ir implants

Lifetime Ultraviolet Radiation Exposure and DNA Methylation in Blood Leukocytes: The Norwegian Women and Cancer Study

Ultraviolet radiation (UVR) exposure is a leading cause of skin cancers and an ubiquitous environmental exposure. However, the molecular mechanisms relating UVR exposure to melanoma is not fully understood. We aimed to investigate if lifetime UVR exposure could be robustly associated to DNA methylation (DNAm). We assessed DNAm in whole blood in three data sets (n = 183, 191, and 125) from the Norwegian Woman and Cancer cohort, using Illumina platforms. We studied genome-wide DNAm, targeted analyses of CpG sites indicated in the literature, global methylation, and accelerated aging. Lifetime history of UVR exposure (residential ambient UVR, sunburns, sunbathing vacations and indoor tanning) was collected by questionnaires. We used one data set for discovery and the other two for replication. One CpG site showed a genome-wide significant association to cumulative UVR exposure (cg01884057) (pnominal = 3.96e-08), but was not replicated in any of the two replication sets (pnominal ≥ 0.42). Two CpG sites (cg05860019, cg00033666) showed suggestive associations with the other UVR exposures. We performed extensive analyses of the association between long-term UVR exposure and DNAm. There was no indication of a robust effect of past UVR exposure on DNAm.

Dosimetric and clinical outcome in image-based high-dose-rate interstitial brachytherapy for anal cancer

PURPOSE

To evaluate dosimetric and clinical outcome in patients of anal cancer treated with image-based interstitial high-dose-rate brachytherapy following chemoradiation.

METHODS AND MATERIALS

Sixteen patients with anal cancer were treated with chemoradiation followed by brachytherapy boost with image-based high-dose-rate interstitial brachytherapy from January 2007 to June 2011. Two brachytherapy dose schedules were used: 21 Gy in seven fractions and 18 Gy in six fractions depending on response to chemoradiation. CT scan was done after placement of needles for confirmation of placement and treatment planning. Target volume was contoured on CT scans. Volumetric quality indices and dose parameters were calculated.

RESULTS

The mean clinical target volume was 17.7 ± 4.98 cm(3), and the median overall tumor size was 4.2cm (3.4-5cm). The mean values of coverage index, dose homogeneity index, overdose volume index, dose non-uniformity ratio, and conformal index were 0.94, 0.83, 0.21, 0.37, and 0.88, respectively. With a median followup of 41 months (range, 20-67.2 months), preservation of the anal sphincter was achieved in 14 patients. The 1- and 2-year local control rates were 93.8% and 87.5%, respectively. Treatment was well tolerated and none of the patients developed Grade 3 or higher late toxicity.

CONCLUSIONS

The combination of external beam radiotherapy with interstitial brachytherapy increases the dose to the tumor volume and limits the volume of irradiated normal tissue, thereby decreasing late toxicity. The use of image-based treatment planning provides better dose conformality with reduced toxicity and helps to prevent a geographic miss.

Quality assessment of interstitial implants in high- dose- rate brachytherapy after lumpectomy in patients of early stage breast cancer

To assess the quality of high dose rate (H,D,R.) interstitial implants in breast cancer by using different volumetric indices and correlating them with skin and subcutaneous tissue toxicity. Out of 15 patients who were selected for interstitial implants after undergoing breast conservation surgery, five were treated radically with 34 Gy in 10 fractions in 5 days @ 3.4 Gy # twice daily and 10 patients recieved boost dose of 12 Gy in 4 fractions @ 3 Gy /# twice daily. The median follow up was 15 months. During each follow up assessment of late skin and subcutaneous tissue toxicity as per RTOG criteria was done . Various dosimetric indices were analysed. Dose Volume Histogram for dose per unit volume of skin for 10cc,5cc,2cc,1cc,0.1cc and 0.01cc was calculated. Best estimates and correlation of toxicity was revealed by assessment of Dose Nonuniformity Ratio(DNR) which also correlated well with geometry defining indices like Uniformity Index (UI).Volumetric assessment of skin dose for less than 2 cc correlated most with toxicity. DNR and UI can help us to assess and correlate late skin and subcutaneous tissue toxicity and thus serve useful to determine the quality of implant.

Qualitative dosimetric and radiobiological evaluation of high - dose - rate interstitial brachytherapy implants

Radiation quality indices (QI), tumor control probability (TCP), and normal tissue complication probability(NTCP) were evaluated for ideal single and double plane HDR interstitial implants. In the analysis, geometrically-optimized at volume (GOV) treatment plans were generated for different values of inter-source-spacing (ISS) within the catheter, inter-catheter-spacing (ICS), and inter-plane-spacing (IPS) for single - and double - plane implants. The dose volume histograms (DVH) were generated for each plan, and the coverage volumes of 100%, 150%, and 200% were obtained to calculate QIs, TCP, and NTCP. Formulae for biologically effective equivalent uniform dose (BEEUD), for tumor and normal tissues, were derived to calculate TCP and NTCP. Optimal values of QIs, except external volume index (EI), and TCP were obtained at ISS = 1.0 cm, and ICS = 1.0 cm, for single-plane implants, and ISS = 1.0 cm, ICS = 1.0 cm, and IPS = 0.75 to 1.25 cm, for double - plane implants. From this study, it is assessed that ISS = 1.0 cm, ICS = 1.0 cm, for single - plane implant and IPS between 0.75 cm to 1.25 cm provide better dose conformity and uniformity.

Image fusion analysis of volumetric changes after interstitial low-dose-rate iodine-125 irradiation of supratentorial low-grade gliomas

The aim of this study was to compare the volumes of tumor necrosis, reactive zone and edema with the three-dimensional dose distributions after brachytherapy treatments of gliomas. The investigation was performed an average of 14.2 months after low-dose-rate (125)I interstitial irradiation of 25 inoperable low-grade gliomas. The prescribed dose was 50-60 Gy to the tumor surface. Dose planning and image fusion were performed with the BrainLab-Target 1.19 software. In the CT/ MRI images, the "triple ring" (tumor necrosis, reactive ring and edema) developing after the interstitial irradiation of the brain tumors was examined. The images with the triple ring were fused with the planning images, and the isodose curves were superimposed on them. The volumes of the three regions were measured. The average dose at the necrosis border was determined from the isodose distribution. For quantitative assessment of the dose distributions, the dose nonuniformity ratio (DNR), homogeneity index (HI), coverage index (CI) and conformal index (COIN) were calculated. The relative volumes of the different parts of the triple ring after the interstitial irradiation compared to the reference dose volume were the following: necrosis, 40.9%, reactive zone, 47.1%, and edema, 367%. The tumor necrosis developed at 79.1 Gy on average. The average DNR, HI, CI and COIN were 0.45, 0.24, 0.94 and 0.57, respectively. The image fusion analysis of the volume of tumor necrosis, reactive ring and edema caused by interstitial irradiation and their correlation with the dose distribution provide valuable information for patient follow-up, treatment options, and effects and side effects of radio therapy.

Brachytherapy technology and physics practice since 1950: a half-century of progress

The 50-year tenure of Physics in Medicine and Biology has coincided with some of the most important developments in radiological science, including the introduction of artificial radioactivity, computers and 3D imaging into medicine. These events have profoundly influenced the development of brachytherapy. Although it is not the dominant radiotherapy modality, it continues to play an important role in cancer therapy, more than a century after its introduction. This paper reviews the impact of three broad categories of innovation introduced since 1950 from the North American perspective: the introduction of artificial radioactivity, computer- and image-based treatment planning, and basic single-source dosimetry.

In vivo assessment of the tolerance dose of small liver volumes after single-fraction HDR irradiation

PURPOSE

To prospectively assess a dose-response relationship for small volumes of liver parenchyma after single-fraction irradiation.

METHODS AND MATERIALS

Twenty-five liver metastases were treated by computed tomography (CT)-guided interstitial brachytherapy. Magnetic resonance imaging was performed 1 day before and 3 days and 6, 12, and 24 weeks after therapy. MR sequences included T1-w gradient echo (GRE) enhanced by hepatocyte-targeted gadobenate dimeglumine. All MRI data sets were merged with 3D dosimetry data and evaluated by two radiologists. The reviewers indicated the border of hyperintensity on T2-w images (edema) or hypointensity on T1-w images (loss of hepatocyte function). Based on the total 3D data, a dose-volume histogram was calculated. We estimated the threshold dose for either edema or function loss as the D(90), i.e., the dose achieved in at least 90% of the pseudolesion volume.

RESULTS

Between 3 days and 6 weeks, the extension of the edema increased significantly from the 12.9 Gy isosurface to 9.9 Gy (standard deviation [SD], 3.3 and 2.6). No significant change was detected between 6 and 12 weeks. After 24 weeks, the edematous tissue had shrunk significantly to 14.7 Gy (SD, 4.2). Three days postbrachytherapy, the D(90) for hepatocyte function loss reached the 14.9 Gy isosurface (SD, 3.9). At 6 weeks, the respective zone had increased significantly to 9.9 Gy (SD, 2.3). After 12 and 24 weeks, the dysfunction volume had decreased significantly to the 11.9 Gy and 15.2 Gy isosurface, respectively (SD, 3 and 4.1).

CONCLUSIONS

The 95% interval from 7.6 to 12.2 Gy found as the minimal hepatocyte tolerance after 6 weeks accounts for the radiobiologic variations found in CT-guided brachytherapy, including heterogeneous dose rates by variable catheter arrays.

CT-Guided Interstitial Single-Fraction Brachytherapy of Lung Tumors

PURPOSES

To assess the safety of CT-guided brachytherapy of lung malignancies and to evaluate the initial therapeutic response.

PATIENTS AND METHODS

Fifteen patients with 30 lung malignancies were included in this prospective phase I trial (metastases, 28; non-small cell lung cancers, 2). Pre-interventionally two patients had a vital capacity of < 80% (39% and 63%). These two patients, and one other, had FEV1 values of < 80% predicted (17%, 48%, and 64%). Tumors with a maximum diameter of 4 cm were treated with a single brachytherapy catheter that was positioned under CT-fluoroscopy. In two tumors with tumor diameters of 5.5 and 6.5 cm, two applicators were used. In one patient with an 11-cm irregularly shaped tumor, nine catheters were inserted. Treatment planning for 192Ir brachytherapy was performed using three-dimensional CT data that were acquired after percutaneous applicator positioning. All procedures were performed under local anesthesia. A follow-up CT was performed 6 weeks later and every 3 months pos-tintervention.

RESULTS

The mean diameter of the 30 lung tumors was 2 cm (range, 0.6 to 11 cm; median diameter, 1.5 cm). The minimal dose within the tumor margin was 20 Gy in all 30 tumors treated. Except for nausea in one patient and focal hemorrhage detected on CT in two patients, no acute adverse events were recorded. One patient developed an abscess at the previous tumor location 9 months after treatment, which proved to be a local tumor recurrence. The median follow-up period was 5+ months with a local tumor control of 97%.

CONCLUSION

The novel technique of CT-guided interstitial brachytherapy was safe for the treatment of lung tumors and yielded a very low complication rate. The initial data on therapeutic response are promising.

Impact of target volume coverage with Radiation Therapy Oncology Group (RTOG) 98-05 guidelines for transrectal ultrasound guided permanent Iodine-125 prostate implants

PURPOSE

Despite the wide use of permanent prostate implants for the treatment of early stage prostate cancer, there is no consensus for optimal pre-implant planning guidelines that results in maximal post-implant target coverage. The purpose of this study was to compare post-implant target volume coverage and dosimetry between patients treated before and after Radiation Therapy Oncology Group (RTOG) 98-05 guidelines were adopted using several dosimetric endpoints.

MATERIALS AND METHODS

Ten consecutively treated patients before the adoption of the RTOG 98-05 planning guidelines were compared with ten consecutively treated patients after implementation of the guidelines. Pre-implant planning for patients treated pre-RTOG was based on the clinical target volume (CTV) defined by the pre-implant TRUS definition of the prostate. The CTV was expanded in each dimension according to RTOG 98-05 and defined as the planning target volume. The evaluation target volume was defined as the post-implant computed tomography definition of the prostate based on RTOG 98-05 protocol recommendations. Implant quality indicators included V(100), V(90), V(100), and Coverage Index (CI).

RESULTS

The pre-RTOG median V(100), V(90), D(90), and CI values were 82.8, 88.9%, 126.5 Gy, and 17.1, respectively. The median post-RTOG V(100), V(90), D(90), and CI values were 96.0, 97.8%, 169.2 Gy, and 4.0, respectively. These differences were all statistically significant.

CONCLUSIONS

Implementation of the RTOG 98-05 implant planning guidelines has increased coverage of the prostate by the prescription isodose lines compared with our previous technique, as indicated by post-implant dosimetry indices such as V(100), V(90), D(90). The CI was also improved significantly with the protocol guidelines. Our data confirms the validity of the RTOG 98-05 implant guidelines for pre-implant planning as it relates to enlargement of the CTV to ensure adequate margin between the CTV and the prescription isodose lines.

One-dimensional phased array with mechanical motion for conformal ultrasound hyperthermia

This paper investigates the feasibility of conformal heating for external ultrasound hyperthermia by using a phased array transducer with mechanical motion. In this system, a one-dimensional phased array is arranged on a shaft and moves along the shaft, while dynamically focusing on the planning target volume (PTV) with numerous focal spots. To prevent overheating in the intervening tissue between the skin and the PTV, the shaft and the phased array are rotated together to enlarge the acoustical window. With the purpose of conformal heating, the power deposition of the PTV is constructed by combinations of the focal spots and an iterative gradient descent method is then used to determine an optimal set of power weightings for the focal spots. Different tumour shapes are evaluated and the simulation results demonstrate that the volume percentage of the PTV with temperatures higher than 43 degrees C is over 95%. The overheating volume outside the PTV is less than 25% of the PTV. This method provides good conformal heating for external ultrasound hyperthermia. The concept of combining electrical focusing and mechanical motion has the advantages of both enlarging the acoustic window and providing dynamic focusing ability, which is essential for successful conformal heating.

Conformality and homogeneity of dose distributions in interstitial implants at idealized target volumes: a comparison between the Paris and dose-point optimized systems

BACKGROUND AND PURPOSE

The use of high dose rate stepping source in interstitial brachytherapy provides more possibility to conform the dose distribution to the target volume compared to the classical systems. The purpose of this study was to evaluate implants made according to the Paris, the stepping source and the conformal dosimetry system with respect to dose homogeneity and conformality, and to compare these systems using volumetric parameters.

MATERIALS AND METHODS

Single-plane and double-plane implants with catheters arranged in square and triangle pattern were used in the analysis. Twenty-seven idealized planning target volumes (PTV) were generated. They formed slabs with rectangular or trapezoidal cross-section. The lengths were 3, 5 and 7 cm, the widths and heights were determined according to the Paris system for catheter separation of 1, 1.5 and 2 cm. The dose specification was selected such that the coverage index was 0.95 for each implant. Optimal active lengths were determined according to the best conformality at the optimized implants. From the dose-volume histogram (DVH) the following indices were calculated for every implant: conformal (COIN), external volume (EI), relative dose homogeneity (HI) and overdose volume (OI). Furthermore, the mean central dose (MCD) and minimum target dose (MTD) was also determined. The dosimetry systems were compared through the mean values of these parameters and the volumetric indices were analyzed according to the geometry of the PTV.

RESULTS

For the optimized systems the optimal active length was 0.5-1.0 cm shorter than the target volume length, depending on the catheter separation and geometry of the PTV. For the Paris, the stepping source and the conformal dosimetry system, the mean COIN was 0.66, 0.82 and 0.82; the mean HI was 0.71, 0.68 and 0.68; the mean EI was 0.44, 0.17 and 0.17; the mean OI was 0.11, 0.13 and 0.12, respectively. The statistical analysis showed that the Paris system differed from the optimized systems significantly. For the Paris, the stepping source and the conformal dosimetry system, the mean reference isodose was 85, 90 and 95%, the MCD was 100, 100 and 109%, the MTD was 67, 71 and 73%, respectively. Regarding geometry of the PTV, the most conformal and homogeneous dose distributions occurred when the catheter separation was small, the target volume was long and its shape was a thick rectangular slab.

CONCLUSIONS

Positioning the catheters according to the rules of the Paris system, but applying optimization on dose points placed either between the catheters in the whole target volume or on the surface of the target volume, and selecting the reference isodose by DVH, can provide highly conformal dose distribution to the target volume, with only a slightly worsened dose homogeneity compared to the Paris system.

Dose homogeneity as a function of source activity in optimized I-125 prostate implant treatment plans

PURPOSE

In conventional treatment planning for permanent I-125 prostate implants, it has been suggested that lower seed activities result in more homogeneous dose distributions and also less overdose of the critical structures. We sought to determine if this hypothesis holds by analyzing treatment plans constructed using an automated optimized approach.

METHODS AND MATERIALS

We studied treatment plans for 10 patients using mixed-integer programming and the branch-and-bound method. Two mixed-integer models (that yielded somewhat different treatment plans) were developed: a "basic" model and a "dose homogeneity" model. For each resulting treatment plan, we examined dose homogeneity (by evaluating the dose non-uniformity ratio [DNR] and the full-width half-maximum [FWHM] of the differential dose-volume histogram [DVH]) as a function of three different source activities (0.35 mCi, 0.44 mCi, and 0.66 mCi). In addition, target coverage and critical structure dose distributions were evaluated. Plans using multiple source activities were also evaluated for resulting dose inhomogeneities.

RESULTS

The homogeneity model results in a more homogeneous dose distribution than the basic model. DNR is lowered by an average of 42% (standard deviation [SD] = 19%), 39% (SD = 21%), and 33% (SD = 21%) for the 0.35 mCi, 0.44 mCi, and 0.66 mCi seeds, respectively, when the homogeneity model is employed over the basic model. Corresponding average decreases in the FWHM of the DVH for 0.35 mCi, 0.44 mCi, and 0.66 mCi, respectively, are 29 Gy (SD = 28 Gy), 24 Gy (SD = 22 Gy), and 27 Gy (SD = 13 Gy). Seeds of 0.35 mCi and 0.44 mCi result in the lowest DNR and narrower FWHM of the DVH relative to 0.66 mCi seeds. In general, the 0.44 mCi seeds produce greater target coverage and require fewer seeds and needles than the 0.35 mCi seeds. Although 0.66 mCi seeds result in the greatest target coverage, they yield highest critical structure doses. They also yield solutions requiring the least number of seeds and needles. However, the dose distributions from 0.66 mCi seeds are highly inhomogeneous. Multiple source activities in the same treatment plan produce dose distributions that are comparable in homogeneity to 0.44 mCi seed implants.

CONCLUSIONS

Even when an optimization model that seeks to minimize dose inhomogeneity is employed, all factors involved in seed implants make 0.44 mCi the best activity choice in comparison with 0.35 mCi and 0.66 mCi.

Biologic treatment planning for high-dose-rate brachytherapy

PURPOSE

Interstitial brachytherapy treatment plans are conventionally optimized with respect to total target dose and dose homogeneity, which does not account for the biologic effects of dose rate. In an HDR implant, with a stepping source, the dose rate dramatically changes during the course of treatment, depending on location, as the source moves from dwell position to dwell position. These widely varying dose rates, together with the related sequencing of the dwell positions, may impart different biologic effects at points receiving the same total dose. This study applies radiobiologic principles to account for the potential biologic impact of dose delivery at varying dose rates within an HDR implant.

METHODS AND MATERIALS

The model under study uses a generalized version of the linear-quadratic (LQ) cell kill formula to calculate the surviving fraction of cells subjected to HDR irradiation. Using a planar interstitial HDR implant with the dwell times optimized to produce a homogeneous dose distribution along a reference plane parallel to the implant plane, surviving fractions were compared at selected reference points subjected to the same total dose. Biologic effect homogeneity was compared to dose homogeneity by plotting the effects at the reference points. The effects were examined with LQ parameters alpha, beta, and sublethal repair time T(1) varied over a range typical of human cells.

RESULTS

In a region in which dose is relatively uniform, surviving fraction for some values of the model parameters are found to vary by as much as an order of magnitude due to differences in the HDR irradiation profiles at different dose points. This effect is more pronounced for shorter repair times and smaller alpha/beta ratios, and increases with increasing total irradiation time.

CONCLUSION

Conventional HDR treatment planning currently considers dose distribution as the primary indicator of clinical effect. Our results demonstrate that plans optimized to maximize homogeneity within a target volume may not reflect the effect of the sequential nature of HDR dose delivery on cell kill. Biologic effect modeling may improve our understanding and ability to predict the adverse effects of our treatment, such as fat necrosis and fibrosis. Accounting for irradiation history and repair kinetics in the evaluation of HDR brachytherapy plans may add an important new dimension to our planning capabilities.

A method to improve the dose distribution of interstitial breast implants using geometrically optimized stepping source techniques and dose normalization

BACKGROUND AND PURPOSE

The standard linear source breast implant of our institution was compared with alternative linear source implant geometries and a stepping source implant, to evaluate the possibility of minimizing the treated volume. Normalization to a higher isodose than the conventional 85% of the mean central dose (MCD) was investigated for the stepping source implant to reduce the thickness of the treated volume and to increase dose uniformity. The purpose of this study was to develop an implant geometry yielding a high conformity and a more uniform dose distribution over the target volume.

MATERIALS AND METHODS

The dose distributions of four implant geometries were compared for a planning target volume (PTV) of 48 cm(3). Implants #1 (standard) and #2 had linear sources arranged in a triangular pattern of equal lengths and lengths adapted to the shape of the PTV. Implants #3 and #4 were squared pattern arranged implants with linear sources and a stepping source with geometric optimized dwell times. The active lengths were adapted to the shape of the PTV. Using implant #4 for PTVs of different volumes, the reference dose (RD) was normalized to 85 and 91% of the MCD.

RESULTS

Comparing implants #2, #3, and #4 with #1, the treated volume (V(100)) encompassed by the reference isodose was reduced by 22, 35, and 37%, respectively. The volumes receiving a dose of at least 125% (V(125)) of the reference dose was reduced by 16, 30, and 30%, respectively. The conformation number increased being 0.30, 0.39, 0.47, and 0.48 for implants #1, #2, #3, and #4, respectively. The average reduction of V(125) when the dose was normalized to 91% compared with 85% of the MCD was 18%.

CONCLUSIONS

A conformal treatment to a PTV could be best achieved with a geometrically optimized stepping source plan with needles arranged in a squared pattern. Reduction of high dose volumes within the implant was obtained by normalizing the RD to 91% instead of 85% of the MCD.

Interstitial high-dose-rate brachytherapy boost: the feasibility and cosmetic outcome of a fractionated outpatient delivery scheme

PURPOSE

To evaluate the feasibility, potential toxicity, and cosmetic outcome of fractionated interstitial high dose rate (HDR) brachytherapy boost for the management of patients with breast cancer at increased risk for local recurrence.

METHODS AND MATERIALS

From 1994 to 1996, 18 women with early stage breast cancer underwent conventionally fractionated whole breast radiotherapy (50-50.4 Gy) followed by interstitial HDR brachytherapy boost. All were considered to be at high risk for local failure. Seventeen had pathologically confirmed final surgical margins of less than 2 mm or focally positive. Brachytherapy catheter placement and treatment delivery were conducted on an outpatient basis. Preplanning was used to determine optimal catheter positions to enhance dose homogeneity of dose delivery. The total HDR boost dose was 15 Gy delivered in 6 fractions of 2.5 Gy over 3 days. Local control, survival, late toxicities (LENT-SOMA), and cosmetic outcome were recorded in follow-up. In addition, factors potentially influencing cosmesis were analyzed by logistic regression analysis.

RESULTS

The minimum follow-up is 40 months with a median 50 months. Sixteen patients were alive without disease at last follow-up. There have been no in-breast failures observed. One patient died with brain metastases, and another died of unrelated causes without evidence of disease. Grade 1-2 late toxicities included 39% with hyperpigmentation, 56% with detectable fibrosis, 28% with occasional discomfort, and 11% with visible telangiectasias. Grade 3 toxicity was reported in one patient as persistent discomfort. Sixty-seven percent of patients were considered to have experienced good/excellent cosmetic outcomes. Factors with a direct relationship to adverse cosmetic outcome were extent of surgical defect (p = 0.00001), primary excision volume (p = 0.017), and total excision volume (p = 0.015).

CONCLUSIONS

For high risk patients who may benefit from increased doses, interstitial HDR brachytherapy provides a convenient outpatient method for boosting the lumpectomy cavity following conventional whole breast irradiation without overdosing normal tissues. The fractionation scheme of 15 Gy in 6 fractions over 3 days is well tolerated. The volume of tissue removed from the breast at lumpectomy appears to dominate cosmetic outcome in this group of patients.

Evaluation of the dose uniformity for double-plane high dose rate interstitial breast implants with the use of dose reference points and dose non-uniformity ratio

BACKGROUND AND PURPOSE

To investigate the influence of dwell time optimizations on dose uniformity characterized by dose values in dose points and dose non-uniformity ratio (DNR) and to analyze which implant parameters have influence on the DNR.

MATERIALS AND METHODS

Double-plane breast implants with catheters arranged in triangular pattern were used for the calculations. At a typical breast implant, dose values in dose reference points inside the target volume and volumes enclosed by given isodose surfaces were calculated and compared for non-optimized and optimized implants. The same 6-cm treatment length was used for the comparisons. Using different optimizations plots of dose non-uniformity ratio as a function of catheter separation, source step size, number of catheters, length of active sections were drawn and the minimum DNR values were determined.

RESULTS

Optimization resulted in less variation in dose values over dose points through the whole volume and in the central plane only compared to the non-optimized case. At implant configurations consisting of seven catheters with 15-mm separation, 5-mm source step size and various active lengths adapted according to the type of optimization, the no optimization, geometrical (volume mode) and dose point (on dose points and geometry) optimization resulted in similar treatment volumes, but an increased high dose volume was observed due to the optimization. The dose non-uniformity ratio always had the minimum at average dose over dose normalization points, defined in the midpoints between the catheters through the implant volume. The minimum value of DNR depended on catheter separation, source step size, active length and number of catheters. The optimization had only a small influence on DNR.

CONCLUSIONS

In addition to the reference points in the central plane only, dose points positioned in the whole implant volume can be used for evaluating the dose uniformity of interstitial implants. The dose optimization increases not only the dose uniformity within the implant but also the high dose volume. The optimization on dose points and geometry provides the most uniform dose distribution. The dose non-uniformity ratio can be minimized by selecting the isodose line of the midpoints between the catheters in the whole volume for the dose prescription, but the dose coverage may not be adequate. For a clinically acceptable plan, a compromise should be made between dose non-uniformity and coverage.

Optimization of planar high-dose-rate implants

PURPOSE

Brachytherapy has long been used to deliver localized radiation to the breast and other cancer sites. For interstitial implants, proper source positioning is critical in obtaining satisfactory dose distributions. The present work examines techniques for optimizing source guide placement in high-dose-rate (HDR) biplanar implants, and examines the effects of suboptimal catheter placement.

METHODS AND MATERIALS

Control of individual dwell times in HDR implants allows a high degree of dose uniformity in planes parallel to the implant planes. Biplanar HDR implants can be considered optimized when the dose at the implant center is equal to the dose at the symmetric target boundaries. It is shown that this optimal dose uniformity is achieved when the interplanar separation is related to the target thickness T through the direct proportionality, s = T/square root2. To quantify the significance of source positioning, the average dose and a related quantity, equivalent uniform dose (EUD), were calculated inside the treatment volume for two conditions of suboptimal catheter geometry. In one case, the interplanar spacing was varied from 1 cm up to the target thickness T, while a second study examined the effects of off-center placement of the implant planes.

RESULTS

Both the average dose and EUD were minimized when the interplanar spacing satisfied the relationship s = T/square root2. EUD, however, was significantly smaller than the average dose, indicating a reduced relative cell killing in the high dose regions near the dwell points. It was also noted that in contrast to the average dose, the EUD is a relatively weak function of catheter misplacement, suggesting that the biological consequences of suboptimal implant geometry may be less significant than is indicated by the increase in average dose.

CONCLUSION

A concise formula can be used to determine the interplanar separation needed for optimal dose uniformity in Manchester-type implants. Deviations from optimal source geometry result in an increase in the average dose inside the treatment volume, but the weaker dependence of the EUD suggests that the surviving fraction of cells may not be not strongly affected by suboptimal source geometry.

Combined use of transverse and scout computed tomography scans to localize radioactive seeds in an interstitial brachytherapy implant

Various techniques have been developed to localize radioactive sources in brachytherapy implants. The most common methods include the orthogonal film method, the stereo-shift film method, and recently, direct localization from a series of contiguous CT transverse images. The major advantage of the CT method is that it provides the seed locations relative to anatomic structures. However, it is often the case that accurate identification and localization of the sources become difficult because of partial source artifacts in more than one transverse cut and other artifacts on CT images. A new algorithm has been developed to combine the advantages of using a pair of orthogonal scout views with the advantages of using a stack of transverse cuts. In the new algorithm, a common reference point is used to correlate CT transverse images and two orthogonal scout CT scans (AP and lateral). The radioactive sources are localized on CT transverse images. At the same time, the sources are displayed automatically on the two CT scout scans. In this way, the individual sources can be clearly distinguished and ambiguities arising from partial source artifacts are resolved immediately. Because of the finite slice thickness of transverse cuts, the longitudinal coordinates are more accurately obtained from the scout views. Therefore, the longitudinal coordinates of seeds localized on the transverse cuts are adjusted so that they match the position of the seeds on scout views. The algorithm has been tested on clinical cases and has proved to be a time saving and accurate method.

Correlation of medical dosimetry quality indicators to the local tumor control in patients with prostate cancer treated with iodine-125 interstitial implants

The treatment of prostate cancer by 125I interstitial implants has been extensively studied with mixed results by one institution or another. A recent study from Hahnemann [Int. J. Radiat. Oncol., Biol., Phys. 21,955-960 (1991)] reported results that were extremely poor compared to those reported in an earlier study at Yale [Int. J. Radiat. Oncol., Biol., Phys. 14, 1153-1157 (1988)] or those in an Eastern Virginia Study [Cancer 63, 2415-2420 (1989)]; differences in 5-yr survival rates being more than a factor of 2. Such large discrepancies from institution to institution led us to a reexamination of the dosimetry. This study analyzed quantitatively three-dimensional dosimetric parameters of 110 prostate cancer patients treated with 125I interstitial implants. The study searched for "cutoff" values in each parameter that divided the patients into two groups with statistically significant differences in the local recurrence-free survival rates. A comparison of the three-dimensional isodose surfaces of patients with favorable values in all of the parameters to those patients with all unfavorable parameters show how these characteristics translated into poor dose coverage and much inhomogeneity within the implant even for cases that met the traditional criteria for adequacy (160 Gy to the tumor volume). Patients in the favorable group had 10-yr survival rates higher by a factor of up to 2 compared to those in the unfavorable group. The strong correlation of three-dimensional volume-dose parameters to the local control rate observed in this study further emphasizes how important it is to assess the three-dimensional dosimetric adequacy of interstitial implants before deciding on their clinical efficacy. If implants are performed with appropriate attention to dosimetry parameters, excellent clinical results are obtained. On the other hand, if dosimetry parameters are not correct, the implant results can be poor.

Dosimetric properties of a novel brachytherapy balloon applicator for the treatment of malignant brain-tumor resection-cavity margins

PURPOSE

This paper characterizes the dosimetric properties of a novel balloon brachytherapy applicator for the treatment of the tissue surrounding the resection cavity of a malignant brain tumor.

METHODS AND MATERIALS

The applicator consists of an inflatable silicone balloon reservoir attached to a positionable catheter that is intraoperatively implanted into the resection cavity and postoperatively filled with a liquid radionuclide solution. A simple dosimetric model, valid in homogeneous media and based on results from Monte Carlo photon-transport simulations, was used to determine the dosimetric characteristics of spherical geometry balloons filled with photon-emitting radionuclide solutions. Fractional depth-dose (FDD) profiles, along with activity densities, and total activities needed to achieve specified dose rates were studied as a function of photon energy and source-containment geometry. Dose-volume histograms (DVHs) were calculated to compare idealized balloon-applicator treatments to conventional 125I seed volume implants.

RESULTS

For achievable activity densities and total activities, classical low dose rate (LDR) treatments of residual disease at distances of up to 1 cm from the resection cavity wall are possible with balloon applicators having radii between 0.5 cm and 2.5 cm. The dose penetration of these applicators increases approximately linearly with balloon radius. The FDD profile can be made significantly more or less penetrating by combining selection of radionuclide with source-geometry manipulation. Comparisons with 125I seed-implant DVHs show that the applicator can provide a more conformal therapy with no target tissue underdosing, less target tissue overdosing, and no healthy tissue "hot spots;" however, more healthy tissue volume receives a dose of the prescribed dosage or less.

CONCLUSIONS

This device, when filled with 125I solution, is suitable for classical LDR treatments and may be preferable to 125I interstitial-seed implants in several respects. Manipulation of the dosimetric properties of the device can improve its characteristics for brain tumor treatment and may make it suitable for boosting the lumpectomy margins in conservative breast cancer treatment.

A conformal index (COIN) to evaluate implant quality and dose specification in brachytherapy

PURPOSE

To propose a new index (COIN) that can be easily understood and computed to assess high dose rate (HDR) brachytherapy interstitial implant quality and dose specification and is an improvement on existing indexes.

METHODS AND MATERIALS

The COIN index is based on an extension of dose-volume histograms and employs an analogous concept to that of cost-benefit analysis, which has already been applied to quality-of-life assessments for two alternative treatment protocols. The COIN index calculation methodology is shown for two cases: with and without critical structures. An analysis is given of dose distributions for two planning treatment volumes (PTV) of simple geometrical shape, applying both the rules of the Paris system and that of the "Offenbach" system. 40 patients who have received interstitial implants form the clinical material. With current HDR brachytherapy technology both for dose delivery, using remote afterloaders, and for three-dimensional (3D) treatment planning, it is now possible to relatively easily plan conformal brachytherapy treatments that would have been impossible with manual afterloading techniques and two-dimensional (2D) treatment planning.

RESULTS

Examples of the use of the COIN index are presented for experimental and clinical data.

CONCLUSIONS

The results show that COIN is a useful and practical index to improve the quality of treatment of interstitial brachytherapy implants. Further work will be undertaken with a larger population of implanted cancer patients and a subdivision of the results by treatment site.

Dose volume assessment of high dose rate 192Ir endobronchial implants

PURPOSE

To study the dose distributions of high dose rate (HDR) endobronchial implants using the dose nonuniformity ratio (DNR) and three volumetric irradiation indices.

METHODS AND MATERIALS

Multiple implants were configured by allowing a single HDR 192Ir source to step through a length of 6 cm along an endobronchial catheter. Dwell times were computed to deliver a dose of 5 Gy to points 1 cm away from the catheter axis. Five sets of source configurations, each with different dwell position spacings from 0.5 to 3.0 cm, were evaluated. Three-dimensional (3D) dose distributions were then generated for each source configuration. Differential and cumulative dose-volume curves were generated to quantify the degree of target volume coverage, dose nonuniformity within the target volume, and irradiation of tissues outside the target volume. Evaluation of the implants were made using the DNR and three volumetric irradiation indices.

RESULTS

The observed isodose distributions were not able to satisfy all the dose constraints. The ability to optimally satisfy the dose constraints depended on the choice of dwell position spacing and the specification of the dose constraint points. The DNR and irradiation indices suggest that small dwell position spacing does not result in a more homogeneous dose distribution for the implant. This study supports the existence of a relationship between the dwell position spacing and the distance from the catheter axis to the reference dose or dose constraint points. Better dose homogeneity for an implant can be obtained if the spacing of the dwell positions are about twice the distance from the catheter axis to the reference dose or dose constraint points.

Implantation guidelines for 169 Yb seed interstitial treatments

The adequacy of an interstitial implant carried out with a new radioactive source, the 169 Yb seed model X1267, has been examined by computing volumetric indices based on dose-volume histograms. The comparison of these indices with the ones computed for 125I seed implantations shows that the use of ytterbium seeds presents an improvement of the dose homogeneity in interstitial implants. This is due to the significant build-up associated with 169 Yb photons that reduces the rapid dose fall-off with the distance from the source. Moreover, relative to 192Ir, the lower photon energy gives 169 Yb the advantage in clinical use of reduced radiation exposure (i) to health care workers, (ii) to relatives of treated patients and (iii) to healthy neighbouring tissues of the patients if appropriate thin shielding is used.

Dose uniformity in a planar interstitial implant system

PURPOSE

This work makes use of a volume-ratio technique to examine dose uniformity in a planar interstitial implant system based entirely on geometrical constraints. The rationale for determining an upper limit for acceptable dose variation is examined and discussed. Variation of ribbon spacing and interplanar separation is evaluated in terms of its effect on dose homogeneity.

METHODS AND MATERIALS

Volume-dose curves were generated for a range of planar implant dimensions. The volume inside the target region and enclosed between the reference isodose and a higher isodose surface was calculated as a measure of dose uniformity. Studies of homogeneity, target coverage, and external tissue irradiation were carried out to evaluate the importance of flexible interplanar spacing in optimizing implants. New dose tables were generated to accommodate the frequent clinical need to minimize the number of catheter insertions.

RESULTS

Implants carried out in accordance with specified geometric constraints were found also to provide optimal dose homogeneity as determined using the volume ratio method with a flexible high dose limit. For two-plane implants, the interplanar spacing should be determined specifically in each case to ensure accurate target coverage. Calculations for specific cases showed that the tissue volume treated to unnecessarily high dose levels can be reduced by a large factor by careful positioning of the implant planes. A smaller ribbon and seed spacing will, in general, lead to better dose uniformity when this is evaluated in terms of the volumes treated to very high dose levels.

CONCLUSIONS

Our studies showed that implants carried out using simple and useful geometric guidelines will also provide an acceptably uniform dose distribution. For double plane implants, the separation of the implant planes should be optimized for each target thickness.

Optimization of interstitial volume implants

For interstitial applications of high dose rate (HDR) afterloading brachytherapy, generally a single stepping iridium-192 source is used, enabling optimization of the dose distribution by optimization of the relative time (dwell time) that the source remains at a certain position (dwell position). We analysed the effects of geometric optimization in a regular volume implant, with strictly parallel catheters, and in an irregular volume implant, such as an implant for tumours of the base of the tongue characterized by a non-parallel geometry and varying catheter separations. In both examples the reference dose is specified at 85% of the mean central dose (as is done in the Paris system for dose specification) in the non-optimized as well as the optimized plan. The irradiated volume, the dose uniformity, and the choice of the reference dose of optimized and non-optimized dose distributions were compared. This was done by isodose plots for representative planes, volume dose histograms (distributed, contiguous, and natural), and dose non-uniformity ratios (DNRs). For the regular implant, optimization results in a 28% increase in the treated volume with a similar increase in the overdosed volumes. In order to keep the treated volume comparable with the non-optimized dose distribution, 90-95% of the mean central dose should be chosen as a reference dose or the range of active dwell positions should be shortened in case of optimization. In the case of the irregular volume implant at the base of the tongue, the method for dose specification should be kept unchanged after geometric optimization as the volume enclosed by the reference isodose does not increase. It is clear from the volume-dose histograms that there is a reduction of the overdosed volume due to optimization. This is accompanied by an increase in the uniformity index and a decrease of the DNR. In conclusion, geometric optimization appears to be an effective tool to improve the dose distribution of interstitial volume implants. Contiguous and natural volume dose histograms appear, apart from planar dose plots, valuable methods for evaluating the dose distribution of an implant.

Concept of dose nonuniformity in interstitial brachytherapy

PURPOSE

Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose uniformity ratio.

METHODS AND MATERIALS

Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume.

RESULTS

For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones.

CONCLUSION

The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

Commissioning and quality assurance of treatment planning computers

The process of radiation therapy is complex and involves many steps. At each step, comprehensive quality assurance procedures are required to ensure the safe and accurate delivery of a prescribed radiation dose. This report deals with a comprehensive commissioning and ongoing quality assurance program specifically for treatment planning computers. Detailed guidelines are provided under the following topics: (a) computer program and system documentation and user training, (b) sources of uncertainties and suggested tolerances, (c) initial system checks, (d) repeated system checks, (e) quality assurance through manual procedures, and in vivo dosimetry, and (f) some additional considerations including administration and manpower requirements. In the context of commercial computerized treatment planning systems, uncertainty estimates and achievable criteria of acceptability are presented for: (a) external photon beams, (b) electron beams, (c) brachytherapy, and (d) treatment machine setting calculations. Although these criteria of acceptability appear large, they approach the limit achievable with most of today's treatment planning systems. However, developers of new or improved dose calculation algorithms should strive for the goal recommended by the International Commission of Radiation Units and Measurements of 2% in relative dose accuracy in low dose gradients or 2 mm spatial accuracy in regions with high dose gradients. For brachytherapy, the aim should be 3% accuracy in dose at distances of 0.5 cm or more at any point for any radiation source. Details are provided for initial commissioning tests and follow-up reproducibility tests. The final quality assurance for each patient is to perform an independent manual check of at least one point in the dose distributions, as well as the machine setting calculation. As a check of the overall treatment planning process, in vivo dosimetry should be performed on a select number of patients.

Evaluation of the substitution of Ir-192 seed ribbons for wires in Paris system using dose nonuniformity ratio

The substitution of Ir-192 seed ribbons for wires in the Paris system of interstitial implants was re-evaluated using the dose nonuniformity ratio. The dose nonuniformity ratio, which is based on volumetric data, measures the dose nonuniformity of the implant quantitatively. The lower the dose nonuniformity ratio value, the smaller the dose nonuniformity, and the better is the dose homogeneity for the implant. Implants configured in a single-plane and double-planes in the form of squares or triangles using Ir-192 wires or seed ribbons were considered. The difference between the particular reference dose rate that yielded a minimum in the dose nonuniformity ratio curve for wire implants and seed implants is about 4 cGy/hr. The difference between the minimum dose nonuniformity ratio value representing the optimal dose homogeneity of the implants is about 5%. The dose homogeneity may be considered better for implants configured using seed ribbons.

Quantitative assessment of interstitial implants

Quantitative assessment of interstitial implants is proposed using volume versus dose curves and four well-defined dosimetric parameters. The volume versus dose curves, both differential and cumulative, provide quantitative data on the volumes of tissues irradiated to different doses. They also offer a qualitative assessment of the variations in dose delivery. The dose nonuniformity ratio (DNR) quantitatively determines the degree of dose nonuniformity specific to the implant configuration. The dose rate at which the DNR shows a minimum value, if selected as the treatment dose rate, gives an optimized dose distribution. The three volumetric irradiation indices are formulated with respect to a well-defined target volume. They offer quantitative data on the extent to which the implant delivers the prescribed dose to the target volume. These dosimetric parameters determine the degree of coverage of the target volume, dose homogeneity within the target volume, and irradiation of tissues outside the target volume. The method of quantitative assessment is demonstrated using, as examples, an ideal Ir-192 double-plane implant and an actual clinical Ir-192 double-plane breast implant.

Dosimetric considerations of stereotactic brain implants

Dose distributions of stereotactic brain implants performed by four institutions were analyzed. In these implants 192Ir or 125I sources were used. The analyses involved an evaluation of the isodose distributions in two orthogonal planes, the dose gradient outside, and the dose homogeneity within the target volume. Quantitative evaluation of the dose homogeneity was performed using three volumetric irradiation indices. The dose homogeneity was observed to improve as the number of catheters increased. However, the number of catheters used is influenced by neurosurgical considerations. Thus, it is necessary to make a compromise between dose homogeneity and the maximum number of catheters to be used. The dose gradient, a centimeter outside the target volume, was found to depend on the geometry of the implant and at distances beyond, it was found to depend on the type of radioisotopes used.