ESTRO 2023 - Abstract Book

S1239

Digital Posters

ESTRO 2023

Figure 2. A comparison of selected variables in different models

Conclusion The 2-year models failed to achieve satisfactory results, indicating that the models are not reproducible regardless of the algorithm used. For the 1-year outcome, the model based on LR, known as an explainable algorithm outperformed RF and SVM in external validation. Our findings demonstrate that a non-black box prediction model can still offer high performance to both patients and care providers.

PO-1524 SBRT boost is a viable alternative to HDR brachytherapy boost for prostate cancer patients

D. Grabec 1 , A. Majdi č 1 , J. Č arman 1 , B. Kragelj 1

1 Institute of Oncology Ljubljana, Radiotherapy, Ljubljana, Slovenia

Purpose or Objective The combination of 50.4Gy/28fx external beam radiotherapy (EBRT) and high dose rate brachytherapy (HDRB) boost with median prostate dose D90% of 19Gy/3fx resulted in PET/CT defined local failure incidence rate of 3/88 patients with the median follow up of 8 years. We hypothesise that similar efficiency is expected with stereotactic body radiotherapy (SBRT) boost. Here we develop the SBRT boost protocol focusing on the prevention of urethral toxicity, while escalating the dose on the dominant intraprostatic lesion. Materials and Methods Treatment plans for SBRT boost were constructed to meet the prescribed dose presented in table I, where GTVp is dominant intraprostatic lesion, CTVp is prostate and PTVp is prostate with margin. Urethra was defined with the fusion of urethra and sphincter urethrae membranaceae. For SBRT 2 mm expansion was used to define PRV. Dose to the urethra (HDRB) or urethra PRV (SBRT) was limited with Dmax < 110% for HDRB and D1% < 105% for SBRT. We retrospectively compared the dose distributions of HDRB and of SBRT boost plans for 10 patients.

Table I: Dose constraints for the SBRT boost planning and average achieved values for 10 SBRT boost plans. Dose constraints in red are obligatory and in green are optional. Results The average maximal dose (D1%) to urethra was 18.4 ± 0.1 Gy in SBRT boost vs 19.4 ± 0.1 Gy in HDRB boost. Average V18 Gy was 25.8 ± 8.7 % in SBRT boost vs 43.9 ± 13.2 % in HDRB boost and average V15.9 Gy was 58.3 ± 1.7 % in SBRT boost vs 62.4 ± 15.3 % in HDRB boost.