2026 American Academy of Neurology Abstract Website

Objective:

We analyzed the relationship between Radiation Induced Contrast Enhancement (RICE), Relative Biological Effectiveness (RBE) and Linear Energy Transfer (LET) in a retrospective, single institution case-control study.

Background:

Proton radiation therapy (PRT) is an emerging modality for treatment of adult CNS tumors. PRT may lead to increased toxicity at sites of dose deposition.

Design/Methods:

150 consecutive adult patients with a histopathologic diagnosis of a CNS malignancy who underwent PRT between 2019 – 2023 with at >1 year of follow-up imaging were included. RICE was defined as new contrast enhancement lesion within the radiation field with no suspicion of tumor recurrence. Matched control group was selected based on age, gender, follow-up, tumor diagnosis, grade and location. The computed RBE dose and dose averaged LET (dLET) distributions of target and organs at risk (OARs) was performed using the Monte Carlo Method. Maximum, minimum and average RBE and LET were recorded.

Results:

RICE was identified in 11 patients. Diagnosis included 5 astrocytoma, 2 meningioma, 2 oligodendroglioma, 1 tectal glioma and 1 high-grade glioma. The median time after radiation for RICE was 9.5 months. Prescription dose was 54 Gy (RBE) (range, 45.0-60.0) with the fraction dose 1.8Gy or 2Gy. The median dose in RICE was 54.48 Gy (range 45.01-61.37 Gy). Median LET values in RICE was 2.8 keV/ μm (range of 2.3 to 4.7 keV/ μm. Median LET was 2.45 keV/ μm with the GTV (range 2.16 - 2.90 keV/ μm). Mean LET values to RICE was higher than to GTV or boost GTV volume in 8 of 11 cases. We are expanding the study to explore the interaction between radiation dose and LET with appearance of RICE.

Conclusions:

RICE appeared in high RBE and LET at the distal end of beams. Study was limited because of the small sample size and a retrospective cohort.