ESTRO 36 Abstract Book

S959 ESTRO 36 2017 _______________________________________________________________________________________________

patient is treated on outpatient basis on 10 fractions of 3.4 Gy over 5 days.The CT scans of these patients, taken before each treatment were separately imported in to the treatment planning system and registered with the initial CT scan respective to the applicator. Three radio channel markers of the applicator are used as reference points to conduct landmark registration on each CT scan. Difference in Max Dose received by skin, ribs and PTV(Planning target volume) during each treatment is recorded. Results Contours of any of the OARs were not exactly similar when CT images were fused on each other. Deduction in volumes of PTV and cavity was noticed. There was always a difference between received doses by the OARs and PTVs between treatments. Variations in received dose by Skin and ribs were statistically significant for 3 treatments and 2 treatments respectively under 5% level. Variations were statistically significant for 4 more fractions for both organs under 10% level. Some data indicates, few times patients received more than 145% of prescribed dose that breach the specific guidelines of APBI. Conclusion The difference recorded in volumes of OARs and iso-doses near the OARs between treatments indicate that the received doses to OARs differ from the prescribed dose in the initial treatment plan. Similarly PTV receiving a lesser dose than the prescribed dose affects the quality of the treatment. It appears that taking a CT scan before each treatment and re-planning is important at this stage to minimize the risk of delivering different doses than the prescribed. EP-1768 What is the proper dose in single fraction HDR brachytherapy as monotherapy for prostate cancer? S. Ruiz Arrebola 1 , A.M. Tornero-López 2 , J.M. De la Vega 3 , P.J. Prada 1 , D. Guirado 3 1 Hospital Universitario Marqués de Valdecilla, Department of Radiation Oncology, Santander, Spain 2 Hospital Universitario Dr. Negrín, Unidad de Radiofísica, Las Palmas de Gran Canaria, Spain 3 Complejo Hospitalario de Granada, Unidad de Radiofísica, Granada, Spain Purpose or Objective High dose rate brachytherapy (HDRBT) as monotherapy for prostate cancer is applied with several fractionation schedules. Usually the linear-quadratic (LQ) model is used to establish their equivalence. However, using the currently accepted value of α/β for prostate cancer, a significant discrepancy between the LQ model predictions and clinical outcomes is found for the only single fraction schedule in use with long-term results [1]. We aim to determine the value of the absorbed dose for an extreme hypofractionation regime of one fraction in HDRBT monotherapy leading to a biochemical failure rate similar to that of most widely used regimes. Material and Methods We used available published data from biochemical control at 5 years, for prostate cancer patients of low and intermediate risk treated with exclusive HDRBT: 7 clinical studies with 9 fractionation schedules, from 1 fraction of 19 Gy to 9 fractions of 6 Gy per fraction. To compare the different schedules, we used the equivalent total dose in 2 Gy fractions, and to describe the biochemical control (BC), we used the LQ model together with the logistics probability function: Electronic Poster: Brachytherapy: Prostate

being D the total dose, d the dose per fraction, α/β the LQ parameter that allows to quantify the sensitivity to the fractionation of prostate cancer, γ the maximum normalized dose-response gradient and D 50 the total dose needed to achieve 50% BC. To fit the model parameters, and to obtain its uncertainties, we used Monte Carlo methods. Results Firstly, the value of EQD 2 of each program was calculated. Figure 1 shows the value of BC versus EQD 2 if the currently accepted value for α/β=1.5 Gy is used in equation (1). The black square corresponds to the single fraction schedule [1]. The fit of equation (2) to clinical data produces the results in figure 2. The confidence intervals of the parameters correspond to 95%. Figure 2 also shows BC versus the new EQD 2 values, calculated with α/β=8.7Gy. From these results, the value of the absorbed dose for an extreme hypofractionation regime with one fraction in HDRBT monotherapy, allows us to obtaining a BC=90% at 5 years, is 21.9 [19.6,26.3] Gy.

Conclusion The α/β value obtained for a range of dose per fraction between 6 and 19 Gy is much greater than the one currently estimated for prostate cancer. The absorbed dose in HDRBT monotherapy for a regime with one fraction would allow us to obtain a BC=90% is 22 Gy. The value of α/β obtained here explains well the clinical data in the region of the doses per fraction considered. Nonetheless it is important to take into account some of the limitations of the model, which may be overcame by