ESTRO 36 Abstract Book

S184 ESTRO 36 _______________________________________________________________________________________________

DP (1 VMAT and 1Tomo) were observed. Multivariate analysis showed, for both cases, a significant correlation (p<0.05) between Homogeneity Index (HI) and both OAR dose sparing and PTV coverage. Irradiation techniques correlated with spinal cord sparing; however institutions using similar/same delivery/TPS techniques produced quite different dose distributions, highlighting the influence of the planner experience on the optimization process (figure 1).

Further, we made IMRT and VMAT plans with a prescription of 41.4Gy (preoperative RT) or 50.4Gy (definitive RT). For IMRT/VMAT, a DO strategy (i.e., assigning mass density to gas pockets in the pCT) with three settings was used: no DO (denoted as DO=0), DO=0.5, and DO=1 (equivalent to an adipose-muscle mixture), resulting in 6 plans per patient. Next, by copying the gas pockets derived from the available CBCT to the pCT, a fractional CT was simulated to calculate the fractional doses using all 6 plans. DVH parameters of the CTV and organs at risk (OARs) were compared between 1) the three DO settings, 2) IMRT and VMAT, and 3) fractional and planned dose. Dose distribution difference in the CTV between fractional and planned dose was also compared. Results The range of initial gas volume measured in the pCT was 56–732ml. The gas volume fluctuated over the treatment course with no time trend (range of mean: 33–519ml, range of standard deviation: 20–162ml). For the fractional dose, V 95% of the CTV was always >98% for VMAT but not for IMRT with DO=0 (Fig.1). For both IMRT and VMAT, DVH parameters of the CTV were significantly larger for DO=1 than for DO=0 and 0.5 ( p <0.05, Wilcoxon signed-rank test). For an increasing gas volume, an overdose (>3.5% higher than the planned dose) in the CTV was found in 72–88%/64– 77% cases for IMRT/VMAT with all three DO settings. The amount of overdose increased as the gas volume increased relative to the initial volume and was >5% when the increase was >100ml (Fig.2). For a decreasing gas volume, an underdose (>3.5% lower than the planned dose) in the CTV was found for IMRT/VMAT in 34%/23% cases with DO=0, 7%/0% cases with DO=0.5, and 0%/0% cases with DO=1. The underdose became more severe as the gas volume decreased for DO=0 and 0.5. An overdose (>3.5%) still existed in up to 28% cases for DO=1 when the gas volume decreased. DVH parameters of OARs in the fractional dose were almost the same as in the planned dose and below the clinical constraints for all scenarios. Conclusion For esophageal cancer RT, the use of VMAT with DO=0.5 in treatment planning is preferable to avoid an overdose/underdose in the CTV when the abdominal gas volume decreases during treatment. However, when the gas volume increases with >100ml, a DO strategy would result in an overdose >5%. Therefore, in that case re- planning may be a better solution.

Fig1 Box plot relative to the single metastasis case. 18 plans were computed using VMAT, 8 VMAT FFF (linac Free of Flattering Filter), 6 Ciberknife, 5 Tomotherapy, 7 IMRT, 1 3dCRT. Conclusion At our knowledge, this is the largest non-sponsored multicentre planning comparison. Differences in DVH binning among centres could explain minor violations. HI is a key factor for planning optimization: prescribing to lower isodose generally leads to better OAR sparing and higher PTV coverage. Results have a dependence on the irradiation technique, although the planner's experience plays a not negligible role. A multicentre analysis as proposed in this study can have an impact on the standardization of plan quality for spinal SBRT. OC-0348 Reducing the dosimetric impact of variable gas volume in the abdomen during RT of esophageal cancer P. Jin 1 , J. Visser 1 , K.F. Crama 1 , N. Van Wieringen 1 , A. Bel 1 , M.C.C.M. Hulshof 1 , T. Alderliesten 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective For middle/distal esophageal tumors, a varying gas volume in the upper abdomen could induce changes in the dosimetry of RT. In this study, we investigated the dosimetric impact of abdominal gas pockets as well as a density override (DO) strategy to mitigate dosimetric effects. Material and Methods We retrospectively included 1 patient with middle and 8 patients with distal esophageal cancer. For these patients, it was unclear whether re-planning was needed due to the varying gas volume during treatment. For each patient, we measured gas volumes in the planning CT (pCT) and 8–28 (median: 14) CBCTs to assess possible time trends.