ESTRO 38 Abstract book

S157 ESTRO 38

be an association between heart irradiation and decreased survival following treatment. Consequently heart constraints have easily been met in recent trials of dose escalated concurrent CRT for NSCLC, such as the phase I/II isotoxic IDEAL-CRT study in which prescribed doses were the highest possible between 63 and 73Gy while meeting several normal tissue dose constraints. We have therefore carried out a planning study to determine the extent to which heart doses can be reduced without diminishing plan quality, in particular target coverage. Specifically, we investigated the feasibility of reducing mean heart doses, and the volume of left atrial wall receiving doses in excess of 63Gy, both of which have been associated with survival. Material and Methods 20 NSCLC patients planned using 4D-CT were selected for this study (IIIA (n=12) and IIIB (n=8), with an even number of left and right sided tumours), and re-planned following the IDEAL-CRT protocol using a VMAT technique. We identified new target levels for mean heart dose (MHD) and the volume of left atrial wall (LAW) receiving ≥ 63Gy (V LAW63 ). These levels are listed in Table 1 as ambitious, moderate and basic, and were identified from the 20 th , 50 th and 80 th percentiles of treated values reported in related trials. Patients were then re-planned, more highly prioritising heart and LAW dose constraints, and determining the extent to which heart and LAW irradiation could be reduced while still meeting the IDEAL-CRT protocol target dose coverage levels (PTV V98 > 90%; CTV V99 > 95%) and dose constraints on other normal tissues. Results Table 1 shows numbers of patients (n) achieving the ambitious, moderate and basic levels of the (a) V LAW63 and (b) MHD constraints, following baseline IDEAL-CRT planning and again after tightening of the V LAW63 and MHD constraints while continuing to meet all other IDEAL-CRT dose-constraints. After routine IDEAL-CRT planning, 8 patients met the ambitious level for V LAW63 ; however after prioritising constraints on LA Wall irradiation the ambitious level could be achieved for 19/20 patients. Similarly after routine IDEAL-CRT planning the ambitious level for MHD was met for only 3 patients, and 5 failed to meet the basic level; but after prioritising reductions in MHD, the ambitious level was achieved in 8 patients and none failed to meet the basic level.

differences at the same point in the treatment course. The #16sCT1 plans were also compared with #16CT2 plans. pCT and sCT plans were compared for equivalent dose and fractions. Results D95% to PTV1 was visually comparable between CT and sCT plans. There were no statistically significant differences in the mean cumulative dose difference between sCT and pCT plans for any of the OAR, at the weekly intervals (p<0.05). The #16sCT2 verification plans were comparable with the CT2 verification plans, with no statistically or clinically significant differences in the mean doses between the planning structures (Table 1). There were no statistically or clinically significant differences in the maximum dose to brainstem (p=0.19) or spinal cord (p=0.51) detected between the #16sCT1 and CT2 plans. Mean dose difference between the #16sCT1 and #16CT2 plans for brainstem was 4cGy (SD5.0) and spinal cord 0cGy (SD3.6). Where large contour changes arose through weight loss and mass reduction, visible air gaps between the body structure and skin surface were illustrated on cbCT and Velocity TM overestimated the soft tissue distortion (Figure 1). This occurred where gaps >1- 1.5cm were visible.

Figure 1: Overestimation of tissue by Velocity TM

Conclusion It appears feasible for sCTs to be used for dose calculation within Eclipse TM for select patients. Where weekly cbCT indicates large contour changes, a mid-point #16CT2 scan would be recommended. The process may reduce the number of repeat #16CT2 scans required during RT. OC-0307 Feasibility of cardiac sparing in isotoxic dose escalated radiotherapy for NSCLC L. Turtle 1 , A. Willett 2 , J. Leadbetter 1 , M. Brada 3 , J. Fenwick 3 1 Clatterbridge Cancer Centre NHS Foundation Trust, Radiotherapy, Wirral, United Kingdom; 2 Clatterbridge Cancer Centre NHS Foundation Trust, Physics, Wirral, United Kingdom; 3 University of Liverpool, Institute of Translational Medicine, Liverpool, United Kingdom Purpose or Objective Heart dose-volume constraints used in the planning of NSCLC radiotherapy have not changed greatly in the past two decades, despite increasing evidence that there may

Conclusion By setting more demanding and more highly prioritised heart dose constraints, MHD and LAW doses can be substantially reduced while continuing to meet the target coverage and normal tissue constraints of the IDEAL-CRT protocol, potentially improving survival.

Poster Viewing: Poster viewing 6: Radiobiological modelling and quantitative imaging