ESTRO 2021 Abstract Book

S854

ESTRO 2021

Conclusion Our study demonstrates a much higher proportion of p16+/HPV- patients (34%) than described by other institutions (~10-20%) and no statistically significant difference in outcomes for this population. This difference in results from previous studies could be related to differences in HPV identification (DNA-ISH versus RNA-ISH versus PCR), or could have resulted from the lower number of p16+/HPV- patients included in previous analyses. The divergence after 3 years warrants further investigation by pooling data from multiple centres, but from our data p16+ patients should not be treated differently due to their HPV status. PO-1025 Outcomes of re-irradiation of relapsed intracranic lesions with stereotactic radiotherapy R. Tummineri 1 , A. Fodor 2 , A. Sanchez Galvan 3 , S.L. Villa 3 , S. Baroni 3 , G. Mandurino 3 , P. Pacifico 3 , C.L. Deantoni 2 , F. Zerbetto 2 , A.M. Deli 2 , S. Broggi 4 , A. Del Vecchio 5 , N.G. Di Muzio 6 1 IRCCS San Raffaele Institute, Radiation Oncology, Milano, Italy; 2 IRCCS San Raffaele Scientific Institute, Radiation Oncology, Milano, Italy; 3 Milano-Bicocca University, Radiation Oncology, Milano, Italy; 4 IRCCS San Raffaele Scientific Institute, Medical Physics, Milano, Italy; 5 IRCCS San Raffaele Scientific Institute, ;edical Physics, Milano, Italy; 6 IRCCS San Raffaele Scientific Institute - Vita e Salute San Raffaele University, Radiation Oncology, Milano, Italy Purpose or Objective The treatment of relapses of already irradiated primary brain tumors and metastases is difficult, given the limited effectiveness of systemic therapy and the risks of surgery or re-irradiation. Here we present the results of salvage Stereotactic Radiotherapy (SRT) for the treatment of recurrent primary brain tumors and metastases (mts) after previous radiotherapy (RT). Materials and Methods From January 2018 to January 2021, 100 intracranial lesions in 24 patients (pts) were re-irradiated with robotic SRT. Six pts had recurrent mts of breast cancer, eight of NSCLC, one of prostate cancer, one of melanoma, three glioblastoma, two meningioma, one intracranial hemangiopericytoma, one oligodendroglioma and one pituitary adenoma. Previous RT on the same volume were performed with: GammaKnife in 8 pts, VMAT SRT in 5 pts, CyberKnife in 4 pts, whole-brain RT in 8 pts, post-operative IMRT in 4 pts, post-operative 3D-CRT in 3 pts. Six pts had multiple previous treatments. Median time from the previous RT was 14.5 (3-96.8) months. GTV was delineated on computed tomography (CT) and contrast-enhanced T1 magnetic resonance (MRI). Median GTV was 0.15 (0.01-36.8) cc. PTV was obtained adding an expansion to GTV of 1 mm (for brain metastases), or 3 mm (for glioblastoma). Median PTV was 0.45 (0.07-62.3) cc. Median prescribed dose was 35 (24-37.5) Gy in 1-5 fractions (median number of fractions was 5), at a median isodose of 78% (69-80%). Prophylactic corticosteroid therapy was prescribed to all pts and mannitol therapy to reduce intracranial pressure to 1 patient simultaneously treated on 21 lesions. The patients were followed up with contrast-enhanced MRI performed every three months. Results SRT was delivered on a median number of 2 (1-21) lesions; five pts were treated on > 5 lesions simultaneously (7, 9, 16, 19, and 21 lesions, respectively). Acute toxicity was G2 headache in three pts (GTV>1cc or >3