ESTRO 2023 - Abstract Book

S1765

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ESTRO 2023

Figure 2: Comparison of measured (CMOS) and expected (TPS) dose distribution.

Conclusion The vM2428 "LASSENA" CMOS detector demonstrates the viability of the technology for on-line verfication of clinical PBS proton systems. Development is ongoing to further reduce the dead time between frames.

PO-1991 Dosimetric Parameters for Localized Salvage Prostate Cancer Retreatments using Proton Scanning Beams

C. Shang 1 , T.R. Williams 1 , S. Ramirez 1 , G. Evans 1 , M. Raman 1

1 South Florida Proton Therapy Institute, Radiation Oncology, Delray Beach, FL, USA

Purpose or Objective To analyze the dosimetric planning parameters and organ volumetric doses for 34 localized salvage prostate cancer retreatments using intensity-modulated proton scanning beams with a near-term recurrence-free survival, PSA response, and toxicity assessment. Materials and Methods A total of 34 salvage radiation retreatments using proton beam scanning for locally recurrent prostate cancers for 30.6 -60 GyE over 17 - 30 fractions with a daily doses of 1.8 or 2.0 GyE were retrospectively analyzed. All the patients, at age of 77.4 (ranging from 56.4 to 88.3) years old with previous radiation treatments of either EBRT (30), Brachytherapy (3), or HIFU (1), were treated on a ProBeam Compact™ (Varian Medical, CA) with two coplanar oblique proton scanning beams under daily CBCT guidance. The local recurrences were confirmed pathologically except for one being diagnosed with Axumin-PET for elevated PSA. 32 cases had positive Axumin/PSMA-PET (7) or FDG-PET (25) which assisted in outlining GTVs. All the treatment plans employed 3-5 mm/3.5% dosimetry robustness and computed on a Varian Eclipse planning system using the single-field optimization and AcurosPT (Monte Carlo alternative) dose algorithm. PTVs, expanded from GTVs by about 3 mm based on the proximities to the rectum and/or bladder, are used for prescribed dose evaluations. Results In this series, PTV (11.89 ± 19.17 cm3) to GTV (4.78 ± 9.20 cm3) ratio is 3.18 ± 1.32. The D95% of PTV is (96.8 ± 2.1) % of prescribed doses. The volumetric doses of the organs at risk are listed in table 1, where values are grossly comparable with those treated with other modalities as published. In some complicated cases, the maximal volumetric dose values appear noticeably higher than the average. With a mean follow-up time of 16.4 ± 10.3 months, only mild GU and GI toxicities and no local recurrence were identified. 28 (or 82.4%) cases showed more than 50% of PSA reduction post-proton retreatments. Table 1. Volumetric Doses of Organ at Risks for Prostate Cancer Radiation Retreatments using Proton Beam Scanning (n =34) Dose Points Rectum (2cc) Rectum (20cc) Rectum (mean) Bladder (2cc) Bladder (15cc) Bladder (mean) Urethra (0.1cc) Average, GyE 19.46 1.19 2.93 13.22 5.05 1.40 26.79 Std, GyE 9.48 1.36 1.93 14.64 8.56 2.03 10.58 Max, GyE 37.34 6.57 9.06 50.84 36.07 10.13 52.67

Conclusion Our preliminary results, with over 16-month follow-up, suggest a good clinical efficacy in salvage proton scanning beam retreatment for localized recurrent prostate cancer post definitive localized radiation therapy using the dosimetric parameters as presented. The volumetric dosimetric values to the rectum, bladder, and urethra analyzed in this study may imply a practical reference for future prostate retreatments using intensity-modulated proton scanning beams.

Poster (Digital): Optimisation, algorithms and applications for photon and electron treatment planning

PO-1992 A new and practical approach to dose constraints in re-irradiation.

S. Vieira 1 , J. Stroom 1 , A. Soares 2 , C. Greco 1

1 Champalimaud Foundation, Radiotherapy, Lisbon, Portugal; 2 Mercurius Health, Radiotherapy, Lisbon, Portugal

Purpose or Objective Re-irradiation has become more frequent, complex, and is not only palliative anymore. Different fractionation schemes are used making dose comparisons challenging, especially since 3D fraction-dose conversions are usually not commercially available. Furthermore, organ at risk (OAR) repair factors and validated dose constraints are still lacking. To start bringing some consistency to dose constraints for re-irradiations, we propose a simple method based on summation of relative values, independent of the fractionation scheme or constraint type used. Materials and Methods We considered 16 patients re-irradiated with different fractionations (f1 and f2) and in total 66 relevant OARs. Data selection was based on the variety of OARs and fractionation schemes (see Table). In our proposed percentage method (PM), the planned DVH dose at Vconst (i.e. Dplan(Vconst)) is determined, and subsequently divided by the constraint dose