ESTRO 36 Abstract Book

S14 ESTRO 36 _______________________________________________________________________________________________

(EQD2, α/β=3Gy) were collected. A deformable registration mapped CT fup to CT 0 using a multiresolution B- spline algorithm with constraints on rigidity and smoothness to avoid reshaping of infiltrations on CT fup . The median density change in Hounsfield Units (∆HU) within the ‘lungs minus GTV’ contour was then extracted per dose bin of 5 Gy from the difference image (HU fup -HU 0 ). Prognostic factors of ∆HU were obtained through linear regression. The studied covariates were CT fup timepoint (T fup ), total and ipsilateral lung volume (LV tot and LV ipsi ), mean lung dose, lung volumes receiving 5 Gy, 20 Gy and 40 Gy (V 5 , V 20 and V 40 ), mean heart dose (MHD), heart D max and PTV prim mean dose. Results The average density change response curve p er study arm is depicted in Figure 1. A saturation was only observed above 60Gy in Arm A. The higher response in A rm 0 above 40Gy suggested that local dose is not the on ly driver of local ∆HU risk. Prognostic factors of individ ual patient response at 20-25Gy, 40-45Gy and 60-65Gy were therefore searched in the combined dataset. Table 1 shows the significant covariates. Lower LV ipsi was highly prognostic of ∆HU in all dose bins. The dosimetric prognostic factors lung V 5 and MHD could explain the higher ∆HU in Arm 0 (standard RT was only delivered in presence of limiting constraint(s)): V 5 was on average 78.9%, 64.6% and 62.8% in Arm 0, A and B, respectively, while MHD was on average 17.9Gy, 13.4Gy and 13.1Gy, respectively. In multivariate analysis, LV ipsi , V 5 and T fup were independent prognostic factors for ∆HU in the 40-45Gy and 60-65Gy dose bins.

Conclusion The clinically estimated α/β values for prostate cancer from an isoeffective L-Q model were inversely related only to the proportion of patients receiving ADT. This is in accordance with in vitro and in vivo studies demonstrating ADT induced cell cycle arrest and induction of apoptosis in prostate cancer, both of which results in decelerating the tumour growth. Thus, ADT-primed prostate cancers cells could radiobiologically mimic late-responding normal tissues and display lower α/β values. This suggests a possible selective benefit of ADT in cancer prostate patients on HRT treatment protocols. OC-0037 Low dose volume effect is a critical determinant for radiation-induced lung fibrosis G. Defraene 1 , M. La Fontaine 2 , S. Van Kranen 2 , B. Reymen 3 , J. Belderbos 2 , J.J. Sonke 2 , D. De Ruysscher 1,3 1 KU Leuven - University of Leuven, Department of Radiation Oncology, Leuven, Belgium 2 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands 3 Department of Radiation Oncology MAASTRO GROW - School for Oncology and Developmental Biology, Maastricht University Medical Centre, Maastricht, The Netherlands Purpose or Objective Severe normal lung tissue damage after (chemo) radiotherapy, presenting as fibrotic changes, occurs in 20% of patients and often coincides with clinical symptoms. The mechanism of formation and/or propagation of these lung infiltrations is not well understood. The aim of this study was to quantify and explain lung tissue density increase assessed by CT scans as a surrogate of lung damage in a dataset in which unusually large prescription doses resulted in a wide range of lung tissue doses. Material and Methods The dataset consisted of 75 stage I-III non-small cell lung cancer patients from 3 institutions treated in the PET- Boost trial: a randomized phase II study (NCT01024829). The randomization arms were a dose escalation to the primary tumour (PTV prim ) as a whole (Arm A) and an integrated boost painted to the 50% FDG PET SUV max subregion of PTV prim (Arm B), both delivered in 24 fractions. When dose constraints prevented escalation ≥72 Gy, standard treatment of 66 Gy in 24 fractions was delivered (Arm 0). The planning CT (CT 0 ), follow-up CT (CT fup ) +/-3 months post RT and planned dose maps (IMRT or VMAT) corrected to equivalent doses in 2 Gy fractions

Conclusion This study indicates that the low dose volume effect is critical for the induction of severe lung fibrosis in high dose regions. This supports the hypothesis of the importance of residual lung volumes spared from low dose for optimal repair within the whole lung. OC-0038 Patterns in ano-rectal dose maps and the risk of late toxicity after prostate radiotherapy E. Onjukka 1 , C. Fiorino 2 , F. Palorini 3 , A. Cicchetti 3 , I. Improta 2 , C. Cozzarini 4 , C. Degli Esposti 5 , P. Gabriele 6 , R.