ESTRO 2022 - Abstract Book

S582

Abstract book

ESTRO 2022

Conclusion Various directions of movement were noted caused by the position of the bladder, rectum, bowels and patient position in relation to the CTV. Despite the effect of inter-fractional motion on the position of the CTV, the majority were in the ITV. However, a third of patients CTVs were not representative from the localisation scans, found to be patient specific and this suggests the need for individualised ITV margins for those patients to optimise their treatment further.

MO-0647 Relationships between Monte Carlo calculated dose parameters and lung densities in lung SBRT

Y. Tsang 1 , J. Duong 2 , G. Jackson 3 , R. DeSouza 1 , K. Ko 4 , P. Nariyangadu 3 , C. Clark 5

1 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom; 2 Mount Vernon Cancer Centre, Radiotherapy, Northwood , United Kingdom; 3 Mount Vernon Cancer Centre, Radiotherapy Physics, Northwood, United Kingdom; 4 University College London, Department of Medical Physics & Biomedical Engineering, London, United Kingdom; 5 UCLH NHS Foundation Trust, Radiotherapy Physics, London, United Kingdom Purpose or Objective Currently, no standard consensus in dose reporting in lung stereotactic body radiotherapy (SBRT) has been widely adopted yet and there is limited published literature on the correlation of Monte Carlo (MC) based lung SBRT dose distributions and patient’s physical lung density. Against this background, this study aims to investigate the relationships between the MC calculated dose parameters and the lung densities around target volumes of patients who received lung SBRT. Materials and Methods A retrospective review on lung function test results of all patients diagnosed with early-stage lung cancer (T1-2N0M0) who were treated in between January 2018 to December 2018 at our institution was conducted. All patients were treated with real-time motion tracking using a dedicated robotic stereotactic radiotherapy machine. All radiotherapy plans were optimised and calculated with MC calculation algorithms. For each lung SABR plan, mean hounsfield units (HUmean) of the gross tumour volumes (GTV), planning target volumes (PTV), and PTV minus GTV were used as surrogate volumes representing the low-density components around target volumes in the lung SBRT plans. Spearman correlation tests were performed to establish the correlations between the densities of surrogate volumes and patient’s physical lung function (FEV1). The HUmean of the surrogate volume with the best correlation to the FEV1 were used to divide the cohort in to two groups: <=median and >median. Mann Whitney U tests were used to investigate if there were any statistically significant differences in the dosimetric parameters between the two groups. Results This study included 42 lung SBRT plans for the analysis. The treatment schedules were either 54Gy in 3 fractions (83%) or 50Gy in 5 fractions (17%) with the range of prescription isodose from 71% to 77%. The median (range) of GTV and PTV volumes were 8.1cc (1.6 to 77.3) and 31.5cc (10.4 to 134.6) respectively. The median (range) HUmean of GTV, PTV and PTV minus GTV were -294.5 (-29.7 to -658.6), -537.6 (-179.5 to -832.7) and -622.5 (-229.1 to -854.2). PTV minus GTV was suggested to be the surrogate volume with the highest correlation (rs=0.42, p<0.05) to FEV1 (Figure 1).

As illustrated in table 1, statistically significant differences were found in GTV D50, GTV D2, PTV D2, gradient index and prescription isodose between the two groups of HUmean PTV minus GTV (p<0.05). It is noted that plans were usually prescribed to lower isodose to improve the dose coverage in the group of <=median HUmean PTV minus GTV.