ESTRO 36 Abstract Book

S973 ESTRO 36 _______________________________________________________________________________________________

the limitations of the model, which may be overcame by adding additional clinical studies, not necessary of brachytherapy as monotherapy. Those limitations are: the fit is over conditioned by the only fraction datum, the data are heterogeneous, and its external validity may be limited, we do not properly know the associated uncertainties nor the dose distributions. References [1] Prada PJ et al. Radiother Oncol 2016;119:411-6. EP-1769 Hypofractionated EBRT and single fraction HDR brachytherapy for patients with prostate cancer. D.E. Kazberuk 1 , T.M. Filipowski 2 , A. Szmigiel-Trzcińska 3 , M. Niksa 3 , D. Hempel 4 , J. Topczewska-Bruns 2 , W. Nowik 5 , B. Pancewicz-Janczuk 6 1 Bialystok Comprehensive Cancer Center, Brachytherapy, Bialystok, Poland 2 Bialystok Comprehensive Cancer Centre, Radiotherapy, Bialystok, Poland 3 Bialystok Comprehensive Cancer Centre, Brachytherapy, Bialystok, Poland 4 Bialystok Comprehensive Cancer Center, Radiotherapy, Bialystok, Poland 5 Bialystok Comprehensive Cancer Centre, Physics, Białystok, Poland 6 Bialystok Comprehensive Cancer Centre, Physics, Bialystok, Poland Purpose or Objective To evaluate the short term efficacy, early toxicity and dosimetric aspects of combined HDR-BT with EBRT in the radical treatment of prostate cancer patients (PCPs). Material and Methods 40 PCPs underwent combined treatment including hypofractionated EBRT (37.5 Gy in 15 fractions over 3 weeks) and conformal HDR-BT between September 2013 and May 2015. The mean age was 69 years with average PSA 6,7 ng/ml and median Gleason score 6,8. T stage was distributed from T1 to T2c. Half of the patients received androgen deprivation. Treatment was delivered using IMRT with an 6- or 15-MV linear accelerator. HDR brachytherapy catheter insertion was performed under spinal anaesthesia. The median number of catheters was 17 (14-18). HDR brachytherapy was delivered using an Iridum-192 source (Nucletron) and treatment planning system: SWIFT 2.11.8 and Oncentra Prostate 3.0.9./4.0. Dose volume constraints included: prostate V 100 ≥95 %, V150 and V200 below 40%; maximal urethral dose ≤ 120% and average rectal dose ≤ 85% of the prescription dose. Patients were monitored weekly during radiotherapy and in 3 months intervals after treatment. Follow-up visit included clinical examination and PSA value assessment. The acute toxicities were graded according to the EORTC/RTOG scales. Results The median V100 was 93% and median D90 was 103%. All patients finished the scheduled therapy without interruption. The most common urinary symptoms were: urgency, frequency, dysuria and nocturia. The rectal symptoms (urgency, frequency) were rare. No grade 3 and 4 acute toxicities were recorded. No patient developed clinical or biochemical progression. The constant decrease of PSA level was observed during follow up. Conclusion Single fraction 15 Gy HDR-BT with hypofractionated EBRT enables dose escalation with excellent dosimetric parameters for the radical treatment of PCPs. The treatment was well tolerated by all patients with satisfactory disease control in the short and medium term.

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