Devan Krishnamurthy

Dosimetric Impact of Interfraction Catheter Movement in High-Dose Rate Prostate Brachytherapy

PURPOSE To evaluate the impact of interfraction catheter movement on dosimetry in prostate high-dose-rate (HDR) brachytherapy. METHODS AND MATERIALS Fifteen patients were treated with fractionated HDR brachytherapy. Implants were performed on day 1 under transrectal ultrasound guidance. A computed tomography (CT) scan was performed. Inverse planning simulated annealing was used for treatment planning. The first fraction was delivered on day 1. A cone beam CT (CBCT) was performed on day 2 before the second fraction was given. A fusion of the CBCT and CT was performed using intraprostatic gold markers as landmarks. Initial prostate and urethra contours were transferred to the CBCT images. Bladder and rectum contours were drawn, and catheters were digitized on the CBCT. The planned treatment was applied to the CBCT dataset, and dosimetry was analyzed and compared to the initial dose distribution. This process was repeated after a reoptimization was performed, using the same constraints used on day 1. RESULTS Mean interfraction catheter displacement was 5.1 mm. When we used the initial plan on day 2, the mean prostate V100 (volume receiving 100 Gy or more) decreased from 93.8% to 76.2% (p < 0.01). Rectal V75 went from 0.75 cm(3) to 1.49 cm(3) (p < 0.01). A reoptimization resulted in a mean prostate V100 of 88.1%, closer to the initial plan (p = 0.05). Mean rectal V75 was also improved with a value of 0.59 cm(3). There was no significant change in bladder and urethra dose on day 2. CONCLUSIONS A mean interfraction catheter displacement of 5.1 mm results in a significant decrease in prostate V100 and an increase in rectum dose. A reoptimization before the second treatment improves dose distribution.

Comparison of high-dose rate prostate brachytherapy dose distributions with iridium-192, ytterbium-169, and thulium-170 sources.

PURPOSE Recent studies have identified that among different available radionuclides, the dose characteristics and shielding properties of ytterbium-169 ((169)Yb) and thulium-170 ((170)Tm) may suit high-dose rate (HDR) brachytherapy needs. The purpose of this work was to compare clinically optimized dose distributions using proposed (169)Yb and (170)Tm HDR sources with the clinical dose distribution from a standard microSelectron V2 HDR iridium-192 ((192)Ir) brachytherapy source (Nucletron B.V., Veenendaal, The Netherlands). METHODS AND MATERIALS CT-based treatment plans of 10 patients having prostate volumes ranging from 17 to 92cm(3) were studied retrospectively. Clinical treatment of these patients involved 16 catheters and a microSelectron V2 HDR (192)Ir source. All dose plans were generated with inverse planning simulated annealing optimization algorithm. Dose objectives used for the (192)Ir radionuclide source were used for the other two radionuclides. The dose objective parameters were adjusted to obtain the same clinical target (prostate) volume coverage as the original (192)Ir radionuclide plan. A complete set of dosimetric indices was used to compare the plans from different radionuclides. A pairwise statistical analysis was also performed. RESULTS AND CONCLUSIONS All the dose distributions optimized with specific (192)Ir, (169)Yb, and (170)Tm sources satisfied the standard clinical criteria for HDR prostate implants, such as those for the Radiation Therapy Oncology Group clinical trial 0321, for combined HDR and external beam treatment for prostate adenocarcinoma. For equivalent clinical target volume dose coverage, the specific (169)Yb and (170)Tm sources resulted in a statistically significant dose reduction to organs at risk compared with microSelectron V2 HDR (192)Ir source. This study indicates that a (170)Tm or (169)Yb radionuclide source may be an alternative to the (192)Ir radionuclide sources in HDR brachytherapy.

Cold spot mapping inferred from MRI at time of failure predicts biopsy-proven local failure after permanent seed brachytherapy in prostate cancer patients: Implications for focal salvage brachytherapy

BACKGROUND AND PURPOSE (1) To establish a method to evaluate dosimetry at the time of primary prostate permanent implant (pPPI) using MRI of the shrunken prostate at the time of failure (tf). (2) To compare cold spot mapping with sextant-biopsy mapping at tf. MATERIAL AND METHODS Twenty-four patients were referred for biopsy-proven local failure (LF) after pPPI. Multiparametric MRI and combined-sextant biopsy with a central review of the pathology at tf were systematically performed. A model of the shrinking pattern was defined as a Volumetric Change Factor (VCF) as a function of time from time of pPPI (t0). An isotropic expansion to both prostate volume (PV) and seed position (SP) coordinates determined at tf was performed using a validated algorithm using the VCF. RESULTS pPPI CT-based evaluation (at 4weeks) vs. MR-based evaluation: Mean D90% was 145.23±19.16Gy [100.0-167.5] vs. 85.28±27.36Gy [39-139] (p=0.001), respectively. Mean V100% was 91.6±7.9% [70-100%] vs. 73.1±13.8% [55-98%] (p=0.0006), respectively. Seventy-seven per cent of the pathologically positive sextants were classified as cold. CONCLUSIONS Patients with biopsy-proven LF had poorer implantation quality when evaluated by MRI several years after implantation. There is a strong relationship between microscopic involvement at tf and cold spots.

TU‐D‐BRB‐03: Enforcing Maximum Dwell Times in High Dose Rate Brachytherapy Highlights the Tradeoff between Small Dwell Time Gradients and Dose Coverage

Purpose Applicator‐based dose optimization (forward planning) emphasizes dwell time homogeneity to avoid regions of exceptionally high dose concentration. Anatomy‐based dose optimization (inverse planning) relaxes this constraint in order to obtain a dose distribution that is more conformal the anatomy. We developed a version of inverse planning constrained by maximum dwell times. The relationship between dwell time homogeneity and the dose delivered to cancerous and healthy organs is examined. Methods and Materials A software method was implemented within the IPSA inverse planning algorithm to allow for (1) imposition of a user‐defined global maximum dwell time at any given dwell position and (2) homogeneity of dwell times within any given catheter. In‐silico studies were performed on six previously‐treated‐patient image datasets using the contours and catheter digitization of the clinically‐used plan. Two each of tandem‐and‐ring gynecological, tandem‐and‐ovoid gynecological, and prostate were examined. The plan used for treatment served as the control and had no restriction on dwell times. New plans were optimized by imposing either a global (Study 1) or catheter‐specific maximum dwell times (Study 2) for different time limits. Dosimetric indices were compared with the control plans. Results Clinically acceptable plans were achievable with modest restriction of the dwell times, but severe restriction lead to unacceptable plans. Across all studies, restriction of dwell times correlated with an increase of the final objective function value. A higher objective function means less adherence of the optimization to the dosimetric requirements imposed on the optimization by the user. Conclusion Imposing restrictions on a global maximum dwell and intra‐catheter dwell time homogeneity necessarily results in a worsening of the dosimetric optimization objective function. However, less severe restrictions can allow for some tailoring of the dose distribution to avoid intra‐catheter dwell time heterogeneity with only a small decrease in target coverage. Research sponsored in part by Nucletron.

SU‐GG‐T‐63: Determination of Dose Objective Parameters and Dose Evaluation during Inverse Planned HDR Brachytherapy Based on a Global DVH‐Based Statistical Comparison

Purpose: Since brachytherapydose distribution contains steep dose gradients, plan evaluation over the whole DVH (rather than a few dose points) may be beneficial. We developed a rapid dose evaluation procedure to determine the optimization parameters, facilitate the planning process, and complete the dose distribution evaluation simultaneously for all regions of interest. Materials and Methods: After each dose calculation for a prostate implant, the DVH are automatically compared to the clinically validated reference data for prostate, urethra, rectum, bladder and bulb. The reference data was generated for each DVH from 50 HDR patients planned with IPSA, separated by volumes into three groups. Upper and lower 95% and 99% confidence intervals and the mean were computed for each structure from 10% to 200% of the prescribed dose. The data were entered into the OncentraBrachy planning system. Results: The observation of the color‐coded deviations provide an immediate visual cue indicating which organ(s) and what DVH dose intervals are above or below the preset reference range and require attention, and therefore the optimization parameters to be adjusted, such as the minimum or maximum dose objective values. The process is completed when all dose intervals for all structures are within the normal range in relation to the reference data. This approach is now clinically implemented. Conclusion: The similarity of the DVH, over a wide range of prostate volumes and shapes, illustrates the consistency of the dose distributions when IPSA is used. A global DVH‐based statistical procedure has been developed to quickly determine whether, and for which structure, a dose modification is needed during the dose optimization planning process. In addition to rapidly identifying which optimization parameters need adjustment, the comparison with clinically validated reference values provides an automatic, immediate, and global quantitative quality assurance. Research sponsored by Nucletron and DOD‐PCRP‐W81XWH‐04‐1‐0262 research contracts.

SU‐GG‐T‐53: Dosimetric Comparison of Iridium‐192, Ytterbium‐169, and Thulium‐170 Sources for HDR Prostate Brachytherapy

Purpose: Recent studies have identified that among different available radioactive isotopes, the dose characteristics and shielding properties of Ytterbium‐169 and Thulium‐170 sources can very well suit HDR brachytherapy needs. Based on the available studies of these new sources, the purpose of this work is to compare the dose distributions obtained with the specially designed Yb‐169 and Tm‐170 brachytherapy sources with standard Ir‐192 clinical dose distribution. Materials and methods: Ten patients having prostate volumes ranging from 17 to 92 cm3 were studied retrospectively. Original treatment of these patients involved 16 catheters and an Ir‐192 source. Dose distributions for all ten plans with each new source are optimized with an objective to achieve the best distribution, but with also an emphasis on ensuring that coverage is equivalent to the original Ir‐192 source plan. Results: The descriptive statistical analysis showed that the mean differences for average prostate V100 with Yb‐169 and Tm‐170 are significantly higher compared with Ir‐192. There are no statistical differences in D90 values for any of the paired three sources. The mean differences for Urethra V120, Rectum V75, and Bladder V75 values with Ir‐192 are significantly higher than Yb‐169 and Tm‐170. The overall patterns of dosimetric results are similar for nine out of the ten patients. Also, the Bladder V75, and Rectum V75 values for each patient are similar with all three sources but there is a wide range among the patients. Conclusion: All the three sources satisfied the clinical standard RTOG criteria to provide good PTV coverage for the prescription dose as well as maintaining the same dose to the target volume (D90). This study shows the possibility that the Tm‐170 and Yb‐169 sources can be used as a replacement for Ir‐192 source in HDR units. Research supported in part by Nucletron