ESTRO 2022 - Abstract Book

S261

Abstract book

ESTRO 2022

Materials and Methods The dose distribution in water around the INTRABEAM source was determined with Monte Carlo (MC) calculations using egs_brachy, a user code of EGSnrc. Following a modified version of the TG- 43 formalism (DeWerd 2015 Brachyther. 14 405-8), the dose to water at a reference point in water (1 cm from the applicator surface) is determined from the air-kerma rate at 50 cm from the source tip by means of a dose-rate conversion coefficient. The air-kerma rate was calculated with MC for each applicator and the dose-rate conversion coefficient, radial dose function and 2D anisotropy function were determined. The MC model of the source with applicators was validated by comparing calculated depth dose profiles with the calibration data provided by the manufacturer. Results The total uncertainties on the calculated depth dose curves in water along the beam central axis were all below 2.4%. The MC calculations agreed with calibration depth dose curves for all applicators within uncertainties. The calculated air- kerma rates were within the range (0.952-1.608)x10 -5 Gy/s, leading to dose-rate conversion coefficients in the range 182.6-683.5 for all applicators. The simulated radial dose functions (Figure 1) show the beam hardening effect caused by an internal aluminum filter present in the smallest applicators. The 2D polar anisotropy at 1 cm from each applicator’s surface is presented in Figure 2. A larger anisotropy is observed near the source axis indicating a strong contribution from primary and back-scattered components of the beam compared to the transverse dose.

Conclusion This work presents the MC calculated TG-43 parameters for the INTRABEAM with spherical applicators, which constitute the necessary data required by conventional brachytherapy TPS to accurately and rapidly generate 3D dose to water distributions in clinical IORT applications. This paves the way towards standardization of the dosimetry of eBT sources and is the first step towards the inclusion of IORT dose distribution in adjuvant treatment plans with complementary external beam radiotherapy techniques.

MO-0299 Determination of the optimal nadir PSA following partial gland ablation in prostate brachytherapy

B. Vanneste 1 , V. Skouteris 2 , L. Campos Pinheiro 3 , R. Stock 4 , N. Stone 5

1 MAASTRO, Radiation Oncology, Maastricht, The Netherlands; 2 Hygeia Hospital, Urology, Athens, Greece; 3 Hospital da Luz,, URology, Lisboa, Portugal; 4 The Icahn School of Medicine at Mount Sinai , Radiation Oncology, New York, USA; 5 The Icahn School of Medicine at Mount Sinai New York, Urology and Radiation Oncology , New York, USA Purpose or Objective Patients are seeking less invasive therapies when diagnosed with localized prostate cancer. Determination of a successful ablation of the clinical treatment volume (CTV) relies on knowledge of the new or nadir PSA (nPSA) following the ablation. We propose a novel method of determining this value. Materials and Methods The nPSA can be derived from the contribution of PSA from the prostatic epithelium (benign and cancer) of the treated (CTV) and the untreated volume. We calculated the contribution of PSA from the CTV by analyzing a cohort of 2861 men treated with prostate brachytherapy (PB) of which 2064 (72%) were without Phoenix failure Phoenix failure and were followed > 5 years. Delivered doses was converted to the biologically effective dose (BED) using the post-implant D90 and any external beam boost with an α / β of 2. BED levels of < 150, > 150-200 and > 200 Gy were compared to a last PSA <0.2 ng/ml by ANOVA from the PB database. Secondly, the PSA was determined for untreated non-cancerous prostate volume by analyzing another cohort of patients. 123 (55.1%) of 243 men with clinical suspicion of prostate cancer who underwent transperineal mapping biopsy and were found to have only benign pathology (no cance). PSA values for the men with benign disease were analyzed in 4 percentile groups for their PSAD (PSA density) values. Regression analysis with a linear model was used to determine which group had the highest R 2 value. An equation was constructed to determine the nPSA for a patient treated with partial gland ablation.

Results