Radiologic Patterns of Necrosis After Proton Therapy of Skull Base Tumors

Abstract

Proton beam therapy (PBT) is used for the treatment of skull base tumors to precisely deliver high-doses of radiation to the tumor volume while sparing normal brain tissue and other organs at risk such as the optic apparatus, the brainstem and the pituitary gland. It offers superior dose distribution as compared to photon radiation therapy. The clinical advantage of PBT over photon techniques is the marked reduction in the integral dose to the patient due to the absence of an exit dose beyond the proton Bragg peak. Proton beam therapy has shown benefits in the treatment of skull base tumors, uveal melanoma, optic pathway gliomas, pituitary adenomas, acoustic neuromas, nasopharynx and paranasal sinus tumors and spinal cord tumors. [1][2][3][4] . A rare yet important complication following brain radiation therapy is radiation necrosis 5-7 , i.e. the appearance of a new ABSTRACT: Background: Discrimination between radiation necrosis and tumor progression after radiation therapy represents a radiologic challenge. The aim of our investigation is to identify patterns of radiation necrosis on brain magnetic resonance imaging (MRI) and positron emission tomography (PET) with Fluoroethyltyrosin (FET) after proton beam therapy (PBT) for skull base tumors. Material and Methods:Five consecutive patients with extra-axial neoplasms were included, presenting a total of eight radiation necrosis lesions (three clival chordomas; two petroclival chondrosarcomas; two women; mean age: 49 ± 18.2 years). Radiation necrosis was defined as the appearance of abnormal enhancement on MRI after PBT decreasing over time, and additional histopathologic confirmation in one patient. MRI and PET imaging were retrospectively analyzed by two experienced radiologists in consensus. Results: All lesions were localized close to the primary tumor in the field of irradiation. Three patients showed bilateral symmetrical lesions. All lesions showed T2 hyperintensity and T1 hypointensity. Cerebral blood volume (CBV) was reduced in all available studies. None of the lesions showed a restricted diffusion. FET-PET (three patients) showed a higher uptake in four out of five lesions; three of which had a mean tumor-to-background (TBRmean) uptake lower than 1.95 and FET uptake increasing over time and were correctly classified into radiation necrosis. Conclusions: Most radiation necroses were in direct continuity with the primary tumor mimicking tumor progression. The most consistent imaging findings for PBT radiation necrosis are low CBV without restricted diffusion and FET-PET TBRmean lower than 1.95 or increasing uptake over time. Bilateral symmetric involvement may be another indicator of radiation necrosis.RÉSUMÉ: Critères radiologiques de la nécrose après protonthérapie des tumeurs de la base du crâne. Contexte : La distinction entre la nécrose due à l'irradiation et la progression de la tumeur après l'irradiation est un défi au point de vue radiologique. Le but de notre étude était d'identifier le profil radiologique de la nécrose radio-induite à l'IRM et...

Positive and Negative Predictive Value of PET-CT in Skull Base Lesions: Case Series and Systematic Literature Review

“… 17 Others have investigated fluoroethyltyrosin-PET (FET PET) for its value in skull base imaging. In employing FET PET along with MRI after proton beam therapy for skull base tumors, Korchi et al 18 described patterns indicative of radionecrosis. Using FET PET mean tumor-to-backsground ratio < 1.95 or a pattern of increasing uptake over time, they suggested that FET PET may be a useful adjunct for differentiation of radiation necrosis from tumor recurrence.…”

High control rates of proton‐ and carbon‐ion–beam treatment with intensity‐modulated active raster scanning in 101 patients with skull base chondrosarcoma at the Heidelberg Ion Beam Therapy Center

“…Because temporal lobe reactions and necrosis have been described after carbon-ion 23 and proton therapy, 24 it is possible that other cases of necrosis may occur during further follow-up in both subgroups. They saw a sharp increase in the rate of necrosis when the temporal lobe volume receiving 60 Gy (RBE) exceeded 5.5 cm 3 or when the temporal lobe volume receiving 70 Gy (RBE) exceeded 1.7 cm 3 .…”

“…Even though she was treated with cortisone, which decreased the necrosis, the patient was able to walk only short distances and was dependent on a wheelchair. Because temporal lobe reactions and necrosis have been described after carbon-ion 23 and proton therapy, 24 it is possible that other cases of necrosis may occur during further follow-up in both subgroups.…”

Diagnostic Challenges of Radiation Necrosis

“…1 Studies are often retrospective, have small sample size, and involve patients only from a single center. There is lack of uniform histologic confirmation of diagnosis.…”

“…In this issue, Korchi et al describe imaging appearances of five consecutive cases of RN from 73 skull-based tumors treated with proton-beam radiotherapy (RT) in Switzerland. 1 An important feature of this study is the selection of patients with extra-axial tumors that tend to recur locally. As such, intra-axial lesions detected following RT more likely represent radiation injury rather than TPR-an interesting model to study imaging findings of RN.…”