Abstract Book

S1087

ESTRO 37

the reference arms, the number of cells able to divide ranged from 11% to 19%. For moderate hypofractionated scheme, with a dose per fx up to 3Gy, 5 fx/w were prescribed. With 4 and 3 fx/w, TCP would decrease respectively by 2% and 16%. 2 fx/w would lead to an unacceptable TCP of less than few %. For 3.4Gy per fx, 3 fx/w were prescribed in clinic. Our simulations showed that an increase to 4 or 5 fx would have slightly improved the TCP (2%). However, only 2 fx/w would have dropped by 45% the TCP. Finally, for severe hypofractionation (7.5Gy per fx), 2 fx/w were prescribed. More fx/w would have increased TCP by 4% to 5% but would have probably cause more toxicities. However, the impact of fx/w is clearly less important for such a high dose per fx. Conclusion The impact of the number of fx/w has been investigated. For moderate hypofractionated treatments, the number of fx/w should be at least 4 whereas it could be 2 for very high doses per fx. However those scheme shorten the overall duration of the course by 2 to 4 weeks which is not consistant with the LQ model. More radiobiological investigations are required to define optimized treatment schedules, including studies regarding toxicities. EP-1999 Linear energy transfer and related biological doses in focal prostate boosting with proton therapy P. Bræmer-Jensen 1 , L.P. Muren 2 , J. Pedersen 2 , A.G. Andersen 2 , J.B.B. Petersen 2 , J. Rørvik 3 1 Aarhus University, Physics and Astronomy, Aarhus, Denmark 2 Aarhus University Hospital, Medical Physics, Aarhus, Denmark 3 University of Bergen, Clinical Medicine, Bergen, Norway Purpose or Objective Focal tumour boosting strategies are being explored in radiotherapy of prostate cancer, with the aim of improving local control rates without increasing the risk of normal tissue morbidity. The dose shaping potential of spot scanning proton therapy could be beneficial for focal boosting, but there is emerging evidence of a spatially varying proton relative biological effectiveness (RBE), depending on the linear energy transfer (LET). In particular there is concern of an elevated LET at the distal end of the proton beams, often found near organs at risk. The aim of this study was therefore to investigate dose-averaged LET (LET d ) distributions for focal prostate boosting using spot scanning proton therapy and to derive RBE-weighted dose distributions using LET d -based Spot scanning proton plans were created for six prostate cancer patients in the treatment planning system, PyTRiP. All plans used the conventional two opposing lateral beam configuration (90◦/270◦ gantry angles), with a prescribed dose of 78 Gy(RBE 1.1 ) to the prostate and a total integrated dose of 95 Gy(RBE 1.1 ) to the index prostate tumour. The physical dose and LET d distributions were calculated and subsequently used to derive RBE- weighted dose distributions with three variable RBE models proposed by McNamara (MN), Wedenberg (WB) and Carabe (CB) as well as with the generic RBE=1.1. Results The spot scanning proton plans were highly conformal for all six patients, also for the focal boost (Fig. 1a). The LET d increased towards the end of the trajectory of each of the two opposing beams, resulting in ‘shells’ of high LET d surrounding the prostate as well as the index volume (Fig. 1b). As a result, both the rectum and the bladder had a region with elevated LET d values. The highest LET d in 1 cm 3 of the bladder ranged from 3.6 keV/µm for the patient with the smallest overlap between bladder and prostate to 5.3 keV/µm for the patient with the largest overlap. The highest LET d in 1 cm 3 of the rectum ranged from 3.5 to 4.1 keV/µm. The overall highest mean variable RBE models. Material and Methods

LET d values were found in the index lesion, with a tendency to increase with decreasing index volume (ranging from 2.7 to 3.7 keV/µm for the largest to the smallest volume). The RBE models (Fig 1c-e) increased the median mean dose of the prostate from 79.5 Gy(RBE 1.1 ) to 85.7 Gy(RBE MN ), with an even larger increase within the index, from 95.2 Gy(RBE 1.1 ) to 104.4 Gy(RBE MN ). The median mean dose for the bladder increased from 24.2 Gy(RBE 1.1 ) to 26.0 Gy(RBE MN ) and for the rectum from 6.5 Gy(RBE 1.1 ) to 7.0 Gy(RBE MN ). Conclusion The LET d distributions showed an increase towards the end of the proton beam trajectories. There were clear correlations between these distributions and the biological dose distributions from the RBE models. The RBE models predicted higher biological doses in the bladder, rectum, prostate and index compared to the generic RBE=1.1.