Sylviane Aubin

A phantom study of an in vivo dosimetry system using plastic scintillation detectors for real-time verification of 192Ir HDR brachytherapy

PURPOSE The goal of the present work was to evaluate the accuracy of a plastic scintillation detector (PSD) system to perform in-phantom dosimetry during 192Ir high dose rate (HDR) brachytherapy treatments. METHODS A PSD system capable of stem effect removal was built. A red-green-blue photodiode connected to a dual-channel electrometer was used to detect the scintillation light emitted from a green scintillation component and transmitted along a plastic optical fiber. A clinically relevant prostate treatment plan was built using the HDR brachytherapy treatment planning system. An in-house fabricated template was used for accurate positioning of the catheters, and treatment delivery was performed in a water phantom. Eleven catheters were inserted and used for dose delivery from 192Ir radioactive source, while two others were used to mimic dosimetry at the rectum wall and in the urethra using a PSD. The measured dose and dose rate data were compared to the expected values from the planning system. The importance of removing stem effects from in vivo dosimetry using a PSD during 192Ir HDR brachytherapy treatments was assessed. Applications for dwell position error detection and temporal verification of the treatment delivery were also investigated. RESULTS In-phantom dosimetry measurements of the treatment plan led to a ratio to the expected dose of 1.003 +/- 0.004 with the PSD at different positions in the urethra and 1.043 +/- 0.003 with the PSD inserted in the rectum. Verification for the urethra of dose delivered within each catheter and at specific dwell positions led to average measured to expected ratios of 1.015 +/- 0.019 and 1.014 +/- 0.020, respectively. These values at the rectum wall were 1.059 +/- 0.045 within each catheter and 1.025 +/- 0.028 for specific dwell positions. The ability to detect positioning errors of the source depended of the tolerance on the difference to the expected value. A 5-mm displacement of the source was detected by the PSD system from 78% to 100% of the time depending on the acceptable range value. The implementation of a stem effect removal technique was shown to be necessary, particularly when calculating doses at specific dwell positions, and allowed decreasing the number of false-error detections-the detection of an error when it should not be the case--from 19 to 1 for a 5% threshold out of 43 measurements. The use of the PSD system to perform temporal verification of elapsed time by the source in each catheter--generally on the order of minutes--was shown to be in agreement within a couple of seconds with the treatment plan CONCLUSIONS We showed that the PSD system used in this study, which was capable of stem effect removal, can perform accurate dosimetry during 192Ir HDR brachytherapy treatment in a water phantom. The system presented here shows some clear advantages over previously proposed dosimetry systems for HDR brachytherapy, and it has the potential for various online verifications of treatment delivery quality.

Robustness and precision of an automatic marker detection algorithm for online prostate daily targeting using a standard V‐EPID

An algorithm for the daily localization of the prostate using implanted markers and a standard video-based electronic portal imaging device (V-EPID) has been tested. Prior to planning, three gold markers were implanted in the prostate of seven patients. The clinical images were acquired with a BeamViewPlus 2.1 V-EPID for each field during the normal course radiotherapy treatment and are used off-line to determine the ability of the automatic marker detection algorithm to adequately and consistently detect the markers. Clinical images were obtained with various dose levels from ranging 2.5 to 75 MU. The algorithm is based on marker attenuation characterization in the portal image and spatial distribution. A total of 1182 clinical images were taken. The results show an average efficiency of 93% for the markers detected individually and 85% for the group of markers. This algorithm accomplishes the detection and validation in 0.20-0.40 s. When the center of mass of the group of implanted markers is used, then all displacements can be corrected to within 1.0 mm in 84% of the cases and within 1.5 mm in 97% of cases. The standard video-based EPID tested provides excellent marker detection capability even with low dose levels. The V-EPID can be used successfully with radiopaque markers and the automatic detection algorithm to track and correct the daily setup deviations due to organ motions.

Dosimetric impact of the variation of the prostate volume and shape between pretreatment planning and treatment procedure

PURPOSE The goal of this study is to evaluate the dosimetric impact on a pretreatment planning of prostatic volume and shape variations occurring between the moment of the volume study (preplanning) and just before a transperineal permanent seed implant procedure. Such variations could be an obvious source of misplacement of the seeds relative to the prostate gland and organs at risk. Other sources of dosimetric uncertainties, such as misplacement due to the procedure itself or edema, are eliminated by looking at these variations before the implant procedure. METHODS AND MATERIALS For 35 clinical cases, prostate contours were taken at preplanning time as well as in the operating room (OR) minutes before the procedure. Comparison of shape and volume between the two sets was made. The impact on V100 was evaluated by placing the seeds in their planned positions in the new volume (clinical situation) and also by performing a new plan with the second set of contours to simulate an intraoperative approach. RESULTS The volume taken in the OR remained unchanged compared to the pretreatment planning volume in only 37% of the cases. While on average the dose coverage loss from pretreatment planning due to a combination of variations of volume and shape was small at 5.7%, a V100 degradation of up to 20.9% was observed in extreme cases. Even in cases in which no changes in volume were observed, changes in shape occurred and strongly affected implant dosimetry. CONCLUSIONS Variations of volume and shape between pretreatment planning and the implant procedure can have a strong impact on the dosimetry if the planning and the implant procedure are not performed on the same day. This is an argument in favor of performing implant dosimetry in the OR.

Dose Escalation to the Dominant Intraprostatic Lesion Defined by Sextant Biopsy in a Permanent Prostate I-125 Implant: A Prospective Comparative Toxicity Analysis

PURPOSE Using real-time intraoperative inverse-planned permanent seed prostate implant (RTIOP/PSI), multiple core biopsy maps, and three-dimensional ultrasound guidance, we planned a boost volume (BV) within the prostate to which hyperdosage was delivered selectively. The aim of this study was to investigate the potential negative effects of such a procedure. METHODS AND MATERIALS Patients treated with RTIOP/PSI for localized prostate cancer with topographic biopsy results received an intraprostatic boost (boost group [BG]). They were compared with patients treated with a standard plan (reference group [RG]). Plans were generated using a simulated annealing inverse planning algorithm. Prospectively recorded urinary, rectal, and sexual toxicities and dosimetric parameters were compared between groups. RESULTS The study included 120 patients treated with boost technique who were compared with 70 patients treated with a standard plan. Boost technique did not significantly change the number of seeds (55.1/RG vs. 53.6/BG). The intraoperative prostate V150 was slightly higher in BG (75.2/RG vs. 77.2/BG, p = 0.039). Urethra V100, urethra D90, and rectal D50 were significantly lower in the BG. No significant differences were seen in acute or late urinary, rectal, or sexual toxicities. CONCLUSIONS Because there were no differences between the groups in acute and late toxicities, we believe that BV can be planned and delivered to the dominant intraprostatic lesion without increasing toxicity. It is too soon to say whether a boost technique will ultimately increase local control.

Inverse-planned gynecologic high-dose-rate interstitial brachytherapy: Clinical outcomes and dose–volume histogram analysis

PURPOSE To present clinical outcomes and dose-volume histogram parameters of three-dimensional image-based high-dose-rate interstitial brachytherapy (HDR-ISBT) in patients with primary or recurrent gynecologic cancer unsuitable for intracavitary brachytherapy (ICB). METHODS AND MATERIALS Records of 43 women treated between 2001 and 2009 with iridium-192 gynecologic HDR-ISBT boost, using a Syed-Neblett template and inverse planning simulated annealing dose optimization, were reviewed. Median HDR-ISBT dose was 30Gy, delivered in 4-6Gy/fraction. Dose-volume histogram parameters recommended by the Groupe Européen de Curiethérapie-European Society for Therapeutic Radiology and Oncology for image-based ICB were analyzed. Total doses were normalized to 2Gy fractions (biologically equivalent dose in 2Gy fractions). Local control (LC) and survival were calculated using Kaplan-Meier method. Toxicities were defined according to Common Terminology Criteria for Adverse Events v3.0. RESULTS There were 34 primary malignancies (cervix=12, vagina=15, Bartholins gland=5, and vulva=2) and 9 recurrences. International Federation of Gynecology and Obstetrics stage distribution for primary cancers was I=2, II=13, III=15, and IV=4. Median followup was 19.3 months (range, 0-92.2). Two-year LC was 87% for primary cancers, and 45% for recurrent cancers, respectively (p=0.0175). Median V(100), D(90), and D(100) for clinical target volume were 97.6%, 90.2, and 68.7Gy(10), respectively. Median bladder and rectal D(2)(cc) were 76.6 and 79.5Gy(3), respectively. Median urethral D(10) was 80.6Gy(3). Twelve patients experienced Grades 3 and 4 late morbidity, but toxicities were transient. Only 2 patients had persistent severe toxicities. A trend toward increased risk for vaginal necrosis was observed with a clinical target volume >84cc. CONCLUSIONS HDR-ISBT may achieve good LC in gynecologic cancer unsuitable for ICB, especially in primary malignancies with a 2-year LC rate higher than 85%. Delivery of such high doses has potential advantages but may predispose to adverse effects, reversible in most cases.

Comparison of dose and catheter optimization algorithms in prostate high-dose-rate brachytherapy

PURPOSE The purpose of this work was to compare the hybrid inverse treatment planning optimization (HIPO), inverse dose-volume histogram-based optimization (DVHO), and fast simulated annealing stochastic algorithm (IPSA). The catheter optimization algorithm HIPO was also compared with the Centroidal Voronoi Tessellation (CVT) algorithm. METHODS AND MATERIALS In this study, eight high-dose-rate prostate cases were randomly selected from an anonymized bank of patients. Oncentra Prostate v4.1 was used to run DVHO and the HIPO catheter optimization (HIPO_cat), whereas Oncentra Brachy v4.3 was used for the remaining. For fixed catheter configurations, DVHO plans were compared with IPSA and HIPO. For catheter positions optimization, CVT and HIPO_cat algorithms were compared with standard clinical template plans. CVT catheters were further restrained to the template grid (CVT_grid) and compared with HIPO_cat. RESULTS For dose optimization, IPSA and HIPO were not different from each other. The urethra D10 and the computation time were found significantly better with IPSA and HIPO compared with DVHO (p < 0.0001). All other dosimetric indices were not statistically different from each others (p > 0.05). For catheter placement, CVT plans were better, whereas HIPO_cat plans were significantly worse (p < 0.05) than standard clinical plans. CVT_grid plans were similar to clinical plans and fulfilling American Brachytherapy Society guidelines down to 12 catheters, whereas HIPO_cat plans do not for all catheter numbers. The CVT algorithm run time was significantly faster than HIPO_cat (p < 0.0001). CONCLUSIONS Dose optimization engines IPSA, DVHO, and HIPO give similar dosimetric results. The CVT approach was found to be better than HIPO_cat and was able to reduce the number of catheters significantly.

Commissioning and evaluation of an extended SSD photon model for PINNACLE3: an application to total body irradiation.

Total body irradiations (TBIs) are unusual radiation therapy techniques used to treat specific hematological diseases. Most TBI techniques use extended source to patient distances [source-to-skin distance (SSD)] to provide lateral or anteroposterior irradiations. Those techniques differ from one institution to the other since they need to be customized to accommodate for local material constraints. However, with those unusual techniques come additional challenges for dose calculation. The purpose of this study was to obtain an accurate (better than 4%) dose calculation model for extended source-to-skin distance (eSSD) treatment techniques, which will be used for TBI planning. The studied dynamic TBI technique has special aspects (eSSD, beam spoiler, large field, and out of field dose contribution) that need to be considered in dose calculation. The first part of this study presents an eSSD beam model commissioning in PINNACLE3 and its validation. The second part looks at the comparison between two dose calculation algorithms, the 3D pencil beam and the superposition-convolution algorithms implemented in THERAPLAN PLUS and PINNACLE3 , respectively. A regular linac beam was commissioned in each treatment planning system and an additional dedicated TBI beam model was implemented in PINNACLE3 . The comparison results indicate that the quality of the TBI treatment greatly depends on the treatment planning system and its beam commissioning. The superposition-convolution algorithm (PINNACLE3 ) provides a better dose calculation tool for TBI than the 3D pencil beam algorithm (THERAPLAN PLUS) with a maximum mean error of 2.2% on a dynamic treatment. The TBI specific beam model of PINNACLE3 (ESSP-P3) also improves the dose calculation. The maximum difference between calculations and measurements (depth doses and beam profiles) was 2% except for extreme cases (build-up region and depth of 20cm) where the error was higher. Output factor determination and the dose contribution outside the primary beam weaknesses were found in PINNACLE3 . Methods are proposed to overcome these limitations. With the correction method applied, the TBI specific beam model allows a maximum mean error of -0.68% on a dynamic treatment. Accurate TBI dose computation necessitates a good dose calculation algorithm combined with a realistic beam model. Inappropriate dose calculation could lead to an important over- or underdose estimation. No perfect algorithm and beam model were found, but methods are proposed to overcome some of the limitations. Those methods are simple and can be used for other eSSD treatment types.

Attenuator design for organs at risk in total body irradiation using a translation technique.

Total body irradiation (TBI) is an efficient part of the treatment for malignant hematological diseases. Dynamic TBI techniques provide great advantages (e.g., dose homogeneity, patient comfort) while overcoming treatment room space restrictions. However, with dynamic techniques come additional organs at risk (OAR) protection challenges. In most dynamic TBI techniques, lead attenuators are used to diminish the dose received by the OARs. The purpose of this study was to characterize the dose deposition under various shapes of attenuators in static and dynamic treatments. This characterization allows for the development of a correction method to improve attenuator design in dynamic treatments. The dose deposition under attenuators at different depths in dynamic treatment was compared with the static situation based on two definitions: the coverage areas and the penumbra regions. The coverage area decreases with depth in dynamic treatment while it is stable for the static situation. The penumbra increases with depth in both treatment modes, but the increasing rate is higher in the dynamic situation. Since the attenuator coverage is deficient in the dynamic treatment mode, a correction method was developed to modify the attenuator design in order to improve the OAR protection. The correction method is divided in two steps. The first step is based on the use of elongation charts, which provide appropriate attenuator coverage and acceptable penumbra for a specific depth. The second point is a correction method for the thoracic inclination, which can introduce an orientation problem in both static and dynamic treatments. This two steps correction method is simple to use and personalized to each patients anatomy. It can easily be adapted to any dynamic TBI techniques.

3D heterogeneous dose distributions for total body irradiation patients

One major objective of total body irradiation (TBI) treatments is to deliver a uniform dose in the entire body of the patient. Looking at 3D dose distributions for constant speed (CstSpeed) and variable speed (VarSpeed) translating couch TBI treatments, dose uniformity and the effect of body heterogeneities were evaluated. This study was based on retrospective dose calculations of 10 patients treated with a translating couch TBI technique. Dose distributions for CstSpeed and VarSpeed TBI treatments have been computed with Pinnacle 3 treatment planning system in homogeneous (Homo) and heterogeneous (Hetero) dose calculation modes. A specific beam model was implemented in Pinnacle 3 to allow an accurate dose calculation adapted for TBI special aspects. Better dose coverages were obtained with Homo/VarSpeed treatments compared to Homo/CstSpeed cases including smaller overdosage areas. Large differences between CstSpeed and VarSpeed dose calculations were observed in the brain, spleen, arms, legs, and lateral parts of the abdomen (differences between V100% mean values up to 57.5%). Results also showed that dose distributions for patients treated with CstSpeed TBI greatly depend on the patient morphology, especially for pediatric and overweight cases. Looking at heterogeneous dose calculations, underdosages (2%–5%) were found in high‐density regions (e.g., bones), while overdosages (5%–15%) were found in low‐density regions (e.g., lungs). Overall, Homo/CstSpeed and Hetero/VarSpeed dose distributions showed more hot spots than Homo/VarSpeed and were greatly dependent on patient anatomy. CstSpeed TBI treatments allow a simple optimization process but lead to less dose uniformity due to the patient anatomy. VarSpeed TBI treatments require more complex dose optimization, but lead to a better dose uniformity independent of the patient morphology. Finally, this study showed that heterogeneities should be considered in dose calculations in order to obtain a better optimization and, therefore, to improve dose uniformity. PACS number: 87.55.D

Sci—Thurs AM: YIS—02: Optimizing Number and Position of Catheters within Inverse Planning Simulated Annealing (IPSA) for Prostate and Breast High Dose Rate Brachytherapy

Introduction: In clinical high dose rate (HDR) brachytherapy for prostate and breast, catheters are generally implanted using a template, without considering precise tumor size and shape. In this work, we present a method to optimize the number and position of catheters before the implantation stage. Methods: A research version of IPSA was modified to gradually remove uniformly distributed catheters on target volume, until the desired number of catheters is reached. Criterion for removal is based on the fraction of total treatment time attributable to each catheter. We have applied this method to a clinical prostate case implanted with 18 catheters and a breast implant of 21 catheters. For prostate case, some chosen catheters were fixed to take into account the low, but essential treatment time needed near the urethra. Results: For the studied prostate case, the cost function value is reduced for optimizations with 18 catheters down to 15, compared with the clinical plan. Bladder and urethra receive lower dose for all plans and rectum V75 is independent of the number of catheters. For the breast case, a plan with 19 catheters leads to a similar cost function value than the clinical case and optimized plans lead to a better skin protection, down to 13 catheters. Conclusion: We have devised a simple and efficient method to optimize the locations and number of catheters which could be extended to all types of interstitial HDR brachytherapy. The results indicate that it is possible to obtain clinically optimal treatment plans with fewer catheters.