ESTRO 2025 - Abstract Book

S1355

Clinical - Lung

ESTRO 2025

A total of 232 lung lesions from 167 patients were identified with sufficient data for analysis. The median PTV was 17cc [1.3-81.70], and the median GTV was 3.4cc [0.01-24.30] excluding outliers. The relative dose difference between the prescription dose and the mean GTV dose (Figure 1) was highest for lesions from the 1822 cohort and 1945 OligoRare, with a median difference of 29.1% and 21.8%, respectively, with differences up to 52.6%. The lesions from the 1702 HALT showed a lower median difference of 19.5% [6.5-35.3], while the 22113 Lungtech study had the smallest median difference of 13.7% [3.9-24.5]. When categorized by dose calculation algorithms (Figure 2), lesions calculated with Type B algorithms had a median relative dose difference of 18.0% [0.7%- 41.4%] while those calculated with Monte Carlo-based algorithms had a higher median difference of 26.5% [0.1%-52.6%]. Conclusion: The results indicate significant variability in GTV dose distribution when using PTV for dose prescription in lung SBRT. Possibly reflecting their greater sensitivity to non-homogeneous environments and PTV margin variations, Monte Carlo-based algorithms showed a greater difference. These findings support a potential shift towards using the PTV as an optimization tool rather than for dose prescription in lung SBRT. A direct GTV-based prescription approach could help reduce dose discrepancies, enhance treatment precision by mitigating the influence of surrounding low-density tissue on final dose evaluation, and provide a more accurate reflection of dose delivery within the tumour. References: [1] “Prescribing, Recording and Recording of Stereotactic Treatments with small Photons Beams” The ICRU Report No 91, 2017. [2] “GTV-based prescription in SBRT for lung lesions using advanced dose calculation algorithms” T. Lacornerie, A. Lisbona, X. Mirabel, E. Lartigau, N. Reynaert - Radiation oncology, 2014. [3] “Adapted Prescription Dose for Monte Carlo Algorithm in Lung SBRT: Clinical Outcome on 205 Patients” J. Bibault, X. Mirabel, N. Reynaert, E. Lartigau, PloS ONE 2015. [4] “About the non-consistency of PTV-based prescription in lung” S. Lebredonchel, T. Lacornerie, E. Rault, A. Wagner, N. Reynaert, F. Crop. Physica Medica, 2017. Keywords: SBRT, Dose Prescription, GTV Phoebe Lacoste 1 , Danya Abdulwahid 2 , Kathryn Banfill 2 , Claire Barker 2 , Neil Bayman 2 , Joanna Coote 2 , Catherine Harris 3 , Margaret Harris 2 , Jennifer King 2 , Laura Pemberton 2 , Hamid Sheikh 2 , David Woolf 2 , Corinne Faivre-Finn 4,2 , Clara Chan 2 1 School of Medical Sciences, University of Manchester, Manchester, United Kingdom. 2 Department of Clinical Oncology, The Christie NHS Foundation Trust, Manchester, United Kingdom. 3 Department of Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom. 4 Division of Cancer Sciences, University of Manchester, Manchester, United Kingdom Purpose/Objective: Reirradiation, defined as a new course of radiotherapy which overlaps with a previously irradiated volume, or causes concern for toxicity due to cumulative doses without overlap of volumes, is being utilised increasingly for patients with lung cancer. However there remains inconsistency as to the best way to select patients and estimate cumulative dose and toxicity. This retrospective study aimed to assess compliance to our internal reirradiation guidelines and assess toxicity and response to thoracic reirradiation in patients treated at our institution. 2469 Digital Poster Reirradiation for Lung Cancer: Investigating Toxicity and Outcomes