ESTRO 2023 - Abstract Book

S435

Sunday 14 May 2023

ESTRO 2023

Group (RTOG). The resulting planning target volume (PTV) is often large and includes considerable volumes of healthy brain tissue. The primary aim of this study was the analysis of a novel PTV delineation method based on fluid-attenuated inversion recovery (FLAIR)-alterations in magnetic resonance imaging (MRI). Materials and Methods A total of 89 patients were retrospectively included in this study. Postoperative MRI-scans at the time of radiation treatment planning were used to define the target volume according to EORTC, RTOG and the FLAIR method. For the latter, the gross tumor volume (GTV) was defined as the FLAIR-altered area, including the resection cavity and any contrast enhancing lesions. The clinical target volume (CTV) was defined as GTV + 2 mm and the PTV was defined as CTV + 3 mm. Patients were treated based on the EORTC volume with a total dose of 60 Gy in 30 fractions. Recurrences were delineated on follow up MRI-scans at their first occurrence. We performed a volumetric comparison of the defined PTVs and analyzed the recurrence pattern in relation to all three target volumes. Results The FLAIR target volume with a median of 179 ml was significantly smaller than the target volumes according to the EORTC and RTOG methods (292.9 ml and 452.9 ml, p<0.001). The recurrence pattern of the FLAIR target volume was significantly similar to that of the EORTC method (Kappa=0.7, p<0,001). The percentage of in-field recurrences was 79.5% vs. 80.8%, marginal recurrences occurred in 11% vs. 8.9% of patients, while out-of-field recurrences were noted in 9.6% vs. 10.3% of cases. With the RTOG method, there were fewer marginal (4.1%) and out-of-field (5.5%) recurrences compared to EORTC and FLAIR. Conclusion Even when considering the reduced volume of the FLAIR-PTV, the majority of recurrences occurred in field. The new target volume definition method yields comparable results with the commonly used EORTC method, while including significantly less surrounding tissue and can therefore be considered as valid option. A prospective investigation of this method for possible future consolidation of target volume reduction is planned. MO-0556 Reduction of the CTV margin in glioblastoma leads to dosimetric benefit without compromising outcome D. Di Perri 1 , D. Hartgerink 2 , K. Terhaag 2 , R. Houben 2 , I. Compter 2 , D. Eekers 2 1 Cliniques universitaires Saint-Luc, Radiation oncology, Brussels, Belgium; 2 Maastricht University Medical Centre+, Radiation Oncology (Maastro), Maastricht, The Netherlands Purpose or Objective Glioblastoma (GBM) is widely treated using large radiotherapy (RT) treatment margins (i.e., 2-3 cm around the surgical cavity/residual tumour) based on the fact that most GBM (i.e., ≥ 80%) recur within this area. Current ESTRO guidelines (Niyazi et al. 2016) advocate for a 20mm clinical target volume (CTV) expansion. However, accumulating data point towards a dose-dependent atrophy of cerebral structures after RT (Raschke et al. 2022; Nagtegaal et al. 2022), presumably associated with toxicity (e.g. cognitive decline). In this context, the question arises whether treatment volumes could be reduced. In 2018, the Dutch guidelines for GBM delineation were modified: CTV isotropic expansion was reduced from 20mm to 15mm (FLAIR hypersignal being still included within 20 mm around the GTV). We retrospectively analysed the impact of margin reduction on treatment volumes, dosimetric parameters for organs at risk (OARs), and patient outcome. Materials and Methods All adult patients with GBM treated between 01-2015 and 12-2020 with concurrent chemoRT (60Gy/2Gy or 59.4Gy/1.8Gy) with temozolomide were analysed (n=124). Patients treated with a CTV margin other than 20 or 15mm (n=5) or who did not complete RT (n=6) were excluded. Of the 113 remaining patients, 57 were treated using a 20mm CTV margin (CTV20), and 56 with a 15mm margin (CTV15). Both patient groups were compared for target volumes (i.e., GTV, CTV, PTV), OARs dose parameters (i.e. brain, hippocampi, brainstem, optic nerves, chiasm, cochleas, and pituitary gland), progression-free survival (PFS), and overall survival (OS)). Results Patient characteristics are described in Table 1. No difference was observed between CTV20 and CTV15 groups based on age, sex, tumour side, MGMT methylation, IDH mutation, resection extent (debulking/biopsy), or performance status. There was no difference in GTV volume either, with mean values of 37.8cm3 and 39.7cm3 (p=0.71) for CTV15 and CTV20 groups, respectively. In the CTV15 group, CTV and PTV were reduced from 238.9cm3 to 176.7cm3 (p=0.001) and from 292.6cm3 to 217.0cm3 (p<0.001), respectively. As a result, average brain mean dose (Dmean) and median left hippocampus Dmean were reduced from 25.2Gy to 21.0Gy (p=0.002) and from 43.7Gy to 10.6Gy (p=0.04), respectively. Significantly lower values were also observed for brainstem dose to 0.03cc (D0.03cc), cochleas Dmean and pituitary Dmean. There was no significant difference for other OARs (Table 1). Patient outcome was similar, i.e., median PFS was 8.0 (95% CI: 6.2-9.8) and 7.0 months (95% CI: 6.3-7.7) (p=0.80), and median OS was 11.0 (95% CI: 8.0-14.0) and 14.0 months (95% CI: 11.8-16.2) (p=0.61) for CTV20 and CTV15, respectively.