ESTRO 2021 Abstract Book

S1672

ESTRO 2021

we calculated the distance between the GTV-T and GTV-N, and compared to the distance on the baseline scan. We used a random effect linear mixed model to explore whether the variation in COM position differed for GTV-N compared to GTV-T, as well as whether it differed in the pre-treatment compared to on-treatment scans. A similar model was used to explore the distance from GTV-T to GTV-N in pre-treatment compared to on-treatment scans. Results The patient population consisted of 10 patients (three male, seven female) with locally advanced rectal cancer (T2-4 N1-2), treated with long course RT (50.4 Gy in 28 fractions) and concomitant chemotherapy. The tumor was located in the lower rectum in 8/10 patients (remaining two in mid-rectum). Lymph nodes were all located in the mesorectal facia, either at or above tumor level. The mean COM position relative to baseline was 0.33 cm (SD 0.22; range 0.03 – 1.14) for the GTV-T and 0.33 cm (SD 0.20; range 0.06 – 1.00) for the GTV-N. See Figure 1 for change over time. The mean difference in distance from GTV-T to GTV-N relative to baseline was -0.16 cm (SD 0.30; range -0.85 – 0.58); and was significantly and numerically larger during treatment compared to pre-treatment (p=0.02). No significant difference was found in the variation of COM position for GTV-N compared to GTV-T, nor for pre-treatment versus on-treatment scans.

Conclusion While both GTV-T and GTV-N (in mesorectum) exhibited considerable variation in position relative to baseline, this did not differ between the volumes. GTV-N to GTV-T distance varied more from the baseline distance on the on-treatment than pre-treatment scans. PO-1965 Investigating the impact of Day Zero SABR appointments and role of the Therapy Radiographer (RTT) A. Webster 1 , K. Jones 1 , R. Steel 1 1 Guy’s & St Thomas’ NHS Foundation Trust, Department of Radiotherapy, London, United Kingdom Purpose or Objective There is sparse guidance on the appropriate additional imaging required for linear accelerator-based SABR treatments. This evaluation aimed to assess the impact of Day Zero “practice appointments” in SABR patients. A secondary aim of this evaluation was to assess the expanding role of Therapeutic Radiographers (RTTs) in this process. Currently, RTTs undertake the online image match and an offline image review is completed by both the RTTs and Clinical Oncologist (CO). Materials and Methods A service evaluation was approved conforming to the hospital’s clinical audit procedure. Patient and IGRT data were retrospectively collated. Patient variables included demographics and disease site. IGRT data included treatment machine, imaging frequency, technique and the offline image matching annotations made by both COs and RTTs. Descriptive statistics were used to characterise the data. Chi-squared was used to test for associations and agreement in image matching annotations between the COs and RTTs. Results 203 SABR patients treated between August 2015 and September 2020, Figure 1. Of these 203 patients, 29 did not have a Day 0.