# Incidence and time course of new contrast-enhancing lesions on MRI after proton versus photon radiotherapy in glioma patients

## TL;DR

This study compares how often new MRI contrast-enhancing lesions appear after proton or photon radiotherapy in glioma patients, finding proton therapy leads to more frequent and earlier lesions.

## Contribution

The study is the first to show a significantly higher incidence and earlier onset of postradiogenic contrast-enhancing lesions after proton therapy compared to photon therapy in glioma patients.

## Key findings

- 74.2% of glioma patients developed new contrast-enhancing lesions after high-dose radiotherapy.
- Proton therapy resulted in a 47.5% incidence of postradiogenic lesions versus 15.6% for photon therapy.
- The median time to first postradiogenic lesion was 11.4 months after proton therapy versus 16.5 months after photon therapy.

## Abstract

Our aim was to evaluate the incidence and time course of new contrast-enhancing lesions (CEL) after high-dose irradiation therapy (RT) with protons or photons in adult glioma patients.

We retrospectively analyzed the MRI data sets of 240 consecutive, adult patients with gliomas who received high-dose photon (n = 141) or proton (n = 99) therapy. All available follow-up MRIs were analyzed for incidence of any new CELs. Postradiogenic contrast-enhancing lesions (pCEL) were distinguished from tumor progression either histologically (definite pCEL) or based on follow-up imaging demonstrating spontaneous complete or partial regression (probable pCEL).

During a median follow-up period of 30 months, we observed any new CEL in 74.2% of patients. Of these, 53.9% met the criteria for definite or probable tumor progression, while 28.8% met the criteria for definite or probable pCEL. The incidence of pCEL was significantly higher after proton therapy (47.5%, n = 47 of 99 patients) versus photon therapy (15.6%, n = 22 of 141 patients, p < 0.0001). The median time interval between end of RT and first evidence of pCEL was significantly shorter after proton therapy (11.4 months) compared to photon therapy (16.5 months, p = 0.035).

Postradiogenic CELs are a frequent finding in adult patients with gliomas, particularly in those treated with proton therapy. Awareness of the high incidence, variable morphology and time course of these treatment related changes is essential for correct interpretation of follow-up imaging findings and for appropriate patient management.

The online version contains supplementary material available at 10.1007/s00234-025-03829-1.