OS05.7.A RADIATION-INDUCED CONTRAST ENHANCEMENT AFTER PROTON THERAPY IN PATIENTS WITH GLIOMAS IN THE PRO-CNS STUDY

Abstract BACKGROUND Although proton radiotherapy has been used in a large scale for quite some time, published data from trials confirming the safety of proton therapy are still scarce. Yet, there are indications of radiation induced contrast enhancement (RICE) being more common after proton radiotherapy than photon radiotherapy. MATERIAL AND METHODS Between August 2015 and August 2020, adult patients receiving proton therapy for primary CNS tumours at the Skandion clinic in Sweden were invited to PRO-CNS, a prospective multicentre observational cohort study. We identified 120 patients with glioma WHO grade 1-3 in the PRO-CNS study. The patients received a total dose of 50.4 Gy (RBE), 54 Gy (RBE) or 59.4-60 Gy (RBE) in 1.8-2.0 Gy (RBE) fractions. Discrimination between RICE and true tumour progression was assessed by experienced neuroradiologists and neurooncologists. So far, only the 45 patients from Gothenburg have been analysed. RESULTS Preliminary data from the Gothenburg cohort show RICE in 70% of patients receiving a dose of 59.4 Gy (RBE). This is a considerably higher incidence of RICE than previously reported in patients treated with photons to the same dose. For comparison, 18% of patients receiving 50.4 Gy (RBE) and 21% receiving 54 Gy (RBE) developed RICE. Furthermore, RICE seemed to be slightly more common among patients treated for oligodendroglioma than astrocytoma, but the numbers are too small to draw conclusions. Analysis of the whole cohort of 120 patients is ongoing with the intention to be presented in 2024. CONCLUSION Our preliminary results raise concern on toxicity after proton treatment to 59.4 Gy (RBE) in glioma patients. These are preliminary results and further evaluation is needed.