ESTRO 2023 - Abstract Book

S1083

Digital Posters

ESTRO 2023

Results Between April 2021 and March 2022, 27 patients underwent treatment. Median age was 69 (range 43-85). All had ECOG performance status of 0-1. Locally advanced disease was most common (63%), followed by post-surgical local recurrence (19%), medically inoperable (11%) and borderline resectable (7%). All but 2 patients had 40Gy in 5 fractions. 85% had induction chemotherapy (CT) (FOLFIRINOX – 78%). Median duration of CT was 14 weeks. Online adaptive radiotherapy was delivered for every fraction. At median follow-up of 15 months, the incidence of grade 3+ toxicity from SABR was 7% (n=2) with no treatment-related mortality. Median, 1-, 2-year estimates OS were 18 months (95% confidence interval: 12-24 months), 76% and 21%. 1- and 2-year estimates of LC were 75% and 69%, median not reached. Conclusion MR-guided online adaptive radiotherapy has been successfully implemented. Our data suggests that toxicity is minimal with durable local control.

PO-1352 Is EDIC a metric to predict lymphopenia in oesophageal cancer patients?

F. Apostolopoulos 1 , O. Nicholas 2 , R. Slinger 1

1 South West Wales Cancer Centre, Swansea Bay University Health Board, Department of Medical Physics and Clinical Engineering, Swansea, United Kingdom; 2 South West Wales Cancer Centre, Swansea Bay University Health Board, Department of Oncology, Swansea, United Kingdom Purpose or Objective The spleen has not traditionally been considered an organ at risk (OAR) when treating oesophageal cancer (OEC) with radiotherapy. However, studies have shown that mean spleen dose (MSD) higher than 10 Gy is associated with increased rates of infection and infection-related mortality and patients that receive doses higher than 40 Gy are at a substantially higher risk [1]. Radiation-related lymphopenia is one of the risks associated with spleen irradiation during radiotherapy and is related to poorer overall survival [2]. The purpose of this study is to evaluate a metric which could predict the grade of lymphopenia and the overall survival rate of oesophageal cancer patients based on this data. Materials and Methods Patients at our centre from August 2018 to December 2019 (n = 41) who were treated with VMAT for OEC > 45 Gy and 25 fractions were audited and analysed. Effective Dose to Immune Cells (EDIC) [3] and MSD were collected to observe any correlation between lymphopenia grade, lymph blood values pre and post treated (PRTL and POTL respectively) or PTV volume. EDIC is calculated based on the major blood containing organs in the human body (lungs, heart, liver) and the integral dose which in our study was calculated by truncating the external contour 3 slices above the lungs and 3 slices below the kidneys to maintain consistency in the calculations.

For our statistical analysis we used two methods: the Spearman Correlation and Linear Regression.

Results Total of 41 patients. 78% of them (32/41) had a mean splenic dose > 10 Gy with no patient receiving more than 40 Gy. MSD (median = 1527.5 cGy) and PTV volume (median = 468.9 cm3) were collected for all patients, the EDIC was calculated (median = 630.2 cGy) and the data is represented in the table below.

Variable Spearman Correlation POTL (p-value) Linear Regression POTL (p-value) MSD 0.442 0.747 EDIC 0.935 0.924 PTV 0.248 0.607

Conclusion No significant associations were found no matter the statistical method used. In both approaches the p-value is significantly large which indicates no correlation between the analysed variables. The main limitation is the small sample, and there is no clear indication that higher spleen dose nor EDIC is associated with higher grade of lymphopenia. Previous work has shown EDIC > 4 Gy results in G4 lymphopenia but this is not replicated in this study. No indication that there is value in reducing splenic dose to improve lymphopenia rates. References