ESTRO 2021 Abstract Book

S334

ESTRO 2021

confidence about OARs position related to planning risk volume (PRV) for pancreatic cases. Results

The median treatment time over 28 sessions was 20±8 minutes. On average, beams had to be interrupted once (range: 0-5) per treatment sessions. Treatment time and number corrections were correlated (R=0.63). The CBCT image quality was in most cases: 4 (pre: 42%; post:39%) or 3 (pre: 39%; post: 31%). The Krippendorff's alpha among three independent RTTs showed that DIBH CBCT can produce sufficient image quality to detect OAR location accurately (Krippendorff-alpha=0.86; %=91) (Table 1) and can be used to reliably determine OARs position related to safety zone (PRV) (Krippendorff-alpha=0.96; %=99) (Table 1). There was a tendency of having larger gastric gas compared to baseline during treatment confirmed by interim and post CBCT. In one patient treatment adaptation was made due to substantial stomach volume changes. Apart from a single occurrence of Gr. 1 nausea no other acute side effect was noted.

Conclusion Marker-based SABR using DIBH technique with ABH+TI appears to be feasible on linac. The time efficiency and image quality are adequate allowing for safe treatment administration due to plan adaptation. During DIBH SABR increased gastric volume due to swallowed air could be observed. PH-0434 RTT verification of margin reduction strategy in rectal cancer short course radiotherapy L. Devlin 1 , G. Marshall 1 , S. O'Cathail 2 , H. Chemu 1 , P. McLoone 3 , A. Smith 1 , S. Corish 4 , A. Duffton 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom; 2 Institute of Cancer Sciences, University of Glasgow, Glasgow, United Kingdom; 3 Institute of Health & Wellbeing, University of Glasgow, Glasgow, United Kingdom; 4 Beatson West of Scotland of Cancer Centre, Radiotherapy, Glasgow, United Kingdom Purpose or Objective Short course radiotherapy for rectal cancer (SCRTrc) delivers large doses per fraction. Planning target volume (PTV) margins include set up error to encompass clinical target volumes (CTV). There are limited data on set- up error in rectal cancer using modern techniques, and the impact soft tissue motion has on PTV margins. We aimed to assess the safety of reducing PTV margins in SCRTrc. Objectives 1. Calculate the setup error margin required when using an offline imaging protocol. 2. Describe the effect of reduced margins on PTV volumes. 3. Can reduced PTV margins of 7 or 5mm maintain CTV coverage on daily CBCT? Materials and Methods Patients were treated with VMAT 25Gy/5 fractions using a PTV10mm margin. Daily CBCT images were registered to bone with all shifts applied. RTT’s assessed PTV10mm target coverage and OAR structures before treatment delivery. The group systematic error (Σ) and random error (σ) were calculated for the AP, SUP/INF and R/L shifts. Set-up error margin was calculated for an offline protocol using Van Herk et al. equation (2.5 Σ+0.7 σ). PTV7mm and PTV5mm were created retrospectively and volumes of these recorded. Two RTT’s retrospectively assessed the coverage of all PTV margins on daily CBCT images. Results Forty patients (CBCT n=200) were analysed with a median age of 70 (IQR 60.75-77). Rectum level was upper (n=10), mid (n=8) and lower (n=22). Using an offline protocol the set-up error margin was calculated as 1.1, 0.8 and 1.0cm on the AP, SI and RL respectively. PTV volumes are shown in table 1. and distribution in figure 1. For PTV7mm and PTV5mm CTV coverage failed in 2.5% (5/200) and 4.5% (9/200) of fractions respectively. Occurring for 7.5% (n=3) and 15% (n=6) of patients; with PTV7mm 2/5 fractions (n=2), and 1/5 (n=1); with PTV5mm in 2/5 fractions (n=3), and 1/5 (n=3).