ESTRO 2020 Abstract Book

S484 ESTRO 2020

dose (V5Gy – V20Gy), which have been analysed in two ways: continuous with absolute volumes, and dichotomised according to thresholds proposed in previous studies. Results Median overall survival was 14.2 months and RN occurred in 25 (19.8%) of treated patients. The median prescribed dose was a single fraction of 21 Gy to the 100% isodose, equivalent to EQD2 (a/b=2)87.7 Gy (range: 60 Gy - 156 Gy). Non-significant correlations with the occurrence of RN were found for male sex (p=0.066) and lung histology of the metastasis (p=0.165). The mean V12Gy was not lower in the RN group compared to the non-RN group, with 37.8 cc and 38.2 cc respectively (p=0.97). V12Gy dichotomised in a <10cc and >10cc group resulted in a p-value of 0.48 in a chi-squared test. Multivariate analysis showed an odds ratio for V12Gy of 1.012 (CI95%= 0.993, 1.031). Other volumes (V5Gy through V20Gy) gave similar results. The gross tumour volume (GTV), conformity index (CI), homogeneity index (HI) did not significantly correlate with the incidence of radionecrosis.

M. Harat 1 , M. Blok 2 , A. Harat 3 , K. Soszynska 4 1 The Franciszek Lukaszczyk Oncology Center, Radiotherapy, Bydgoszcz, Poland ; 2 bydgoszcz, Poland, ; 3 Public Health, Bydgoszcz, Poland ; 4 10th Military Hospital, Molecular Pathology, Bydgoszcz, Poland Purpose or Objective Changes in MGMT promoter methylation, IDH1 and IDH2 mutation, and 1p/19q co-deletion status in gliomas between first and subsequent resections and their associated clinical factors are poorly described. Here we assayed for these biomarkers in the clinical setting. Material and Methods We used multiplex ligation-dependent probe amplification to measure MGMT promoter methylation, IDH mutation status, and 1p/19q co-deletion in 45 paired tumor samples from patients undergoing resection and subsequent re- resections for gliomas. Results Molecular changes were present in 20 patients (44%). At least one molecular characteristic changed over time in 89% of patients with primary grade III tumors. Gliomas with IDH wild-type and/or non-co-deleted were stable, but IDH1/2 mutation and/or co-deletion were sometimes lost at the time of recurrence. In a multivariate analysis, adjuvant radiotherapy alone was independently associated (p = 0.02) with changes in molecular profile. Conclusion Molecular biomarkers change in gliomas during the course of the disease, most often MGMT methylation status. These changes in genetic profiles are related to adjuvant treatment with radiotherapy alone, which might be important for individualized treatment planning over the disease course. PO-0906 MRI contrast clearance analysis for follow-up of brain tumors treated with VMAT and SRT A. Toutaoui 1 , T. Baroudi 1 , S. Mahmoudi 1 , R. Louelh 1 1 Hôpital Chahids Mahmoudi, Département de Radiothérapie et d'Imagerie Moléculaire, Tizi Ouzou, Algeria Purpose or Objective Treatment response assessment maps (TRAMs) based on contrast clearance analysis calculated from delayed- contrast MRI is a novel methodology that enables reliable differentiation between tumor and non-tumoral tissues in brain tumor patients. The contrast clearance analysis maps are calculated by subtracting 3D-T1-MRIs acquired 5min (early time point) post-contrast injection from those acquired 60-105min (late point) later. TRAMs maps differentiate between contrast clearance and accumulation and support clinicians in separating tumor progression and treatment effects, such as radiation necrosis. Clinical studies affirm high positive predictive values (PPV) and outstanding sensitivity to active tumor. We studied potential applications of this methodology for post-therapeutic follow-up of patients in conventional/stereotactic radiotherapy. Material and Methods A total of 7 patient treated in our department from April 2018 to February 2019 were included in this study. 5 patients with glioblastoma and 2 patients with germinoma were scanned by the contrast clearance analysis protocol (T1-MRIs acquired 5min and 60-105min post contrast injection). The first scan, for each patient, was performed 4 months after the end of treatment. For three patients, three additional scans were performed every three months. Two other patients had two more scans every three months. Tumor/treatment-effects volumes were calculated from the TRAMs within regions appearing enhanced on T1-MRIs. Results Overall, 20 TRAMs maps were calculated for 7 patients included in the study. Once the maps are validated to

Conclusion We conclude that irradiated healthy brain volume is not associated with the risk of radionecrosis in patients receiving SRT for solitary brain metastases. This is in contrast with previous studies, which have shown a significant increased risk of RN with increasing V10Gy and V12Gy. Although ALARA comes first, our finding may force us to reconsider the role of V12Gy in SRT planning in patients with solitary brain metastases. A V12Gy higher than 10cc did not result in higher risk for RN in patients with single brain metastases. Future studies should analyse whether the prognostic role and suggested thresholds of irradiated brain volumes are dependent on patient specific factors, like number of metastases and type of primary tumour.

PO-0905 The impact of adjuvant radiotherapy on molecular prognostic markers in gliomas