ESTRO 36 Abstract Book

S65 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective Clinical treatment planning of proton therapy assumes that the relative biological effectiveness (RBE) of protons is uniformly 10% higher than photons. However, the energy deposition of protons is considerably different from photons resulting in an increased linear energy transfer (LET) at the distal end and laterally of the proton beam Bragg Peak. In addition, the distal end of proton beams is often located in or very close to normal tissues / organs at risk. This gives rise to concern of an increase in the biological effects of the normal tissues. We are currently initiating a normal tissue complication probability modelling project on outcomes following proton therapy of prostate cancer. As a first step, we have in this study investigated the dose-averaged LET d distributions in the normal tissues (rectum and bladder) for spot scanning proton therapy of prostate cancer using the two most common field arrangements for these patients. Material and Methods Spot scanning proton therapy plans (3 mm spot size with 1 mm spacing) were created in the treatment planning system PyTRiP on CT scans of seven prostate cancer patients (without rectal balloons or spacers). The CTV included the prostate gland, while CTV–to–PTV margins of 4 mm axially and 6 mm in the superior and inferior directions were used. The PTV was planned to receive a dose of 78 Gy (RBE). Treatment plans with two opposing lateral fields (90°/270°) and with two lateral oblique fields (70°/290°) were created for all cases. Dose and LET d distributions were calculated in PyTRiP for all plans and visualised as dose and LET d volume histograms (DVHs/LET d -VHs). Results The LET d distribution patterns for both the lateral opposing and the lateral oblique fields were overall similar for the seven patients. For the two lateral opposing fields the median (range) of the mean LET d in the prostate was 2.1 keV/µm (2.0–2.2 keV/µm) while somewhat higher LET d was found in the high-dose areas outside of the prostate (Fig. 1). For the rectum, 6% (median; range 4–13%) of the volume had an LET d higher than the mean LET d in the prostate, compared to 27% (median; range: 8–36%) for the bladder. For the lateral oblique fields the mean LET d in the prostate was 2.1 keV/µm (2.0–2.1 keV/µm) while the LET d more than doubled compared to the plans with lateral opposing fields (Fig. 2). For the rectum, 15% (median; range 12–24%) of the volume had an LET d higher than the mean LET d in the prostate, compared to 23% (median; range: 2–35%) for the bladder.

Conclusion High LET d

volumes outside the target were seen in both rectum and bladder when treating with spot scanning proton therapy. In particular, the rectum was subject to high LET d volumes when irradiated with lateral oblique fields compared to lateral opposed fields.