RADT-40. RADIATION-INDUCED CONTRAST ENHANCEMENT FOLLOWING PROTON RADIOTHERAPY FOR LOW-GRADE GLIOMA DEPENDS ON TUMOR CHARACTERISTICS AND IS RARER IN CHILDREN THAN ADULTS

Proton beam radiotherapy (PRT) is used in the treatment of low-grade glioma (LGG) to mitigate long-term sequelae. Following PRT, increased rates of radiation-induced contrast enhancements (RICE) are suspected but poorly understood. We analyzed consecutive 227 patients (42 children and 185 adults) treated with PRT (54Gy RBE) for LGG from 2010 to 2020 and followed with serial clinical exams and magnetic resonance imaging for in median 5.6 years. Tumors were graded WHO 1 in a minority (n = 22, 12%) of adults, but a majority of children (n = 29, 69%). In contrast, tumors were graded WHO 2 in the majority (n = 160, 87%) of adults and a minority of children (n = 10, 24%). Five-year overall survival following PRT was 81% in adults and 91% in children. The risk of RICE was 5-fold more frequent in adults (25%) versus children (5%) (p = 0.0043). Also, within the adult cohort, RICE risk increases with age (p = 0,00128). In children and adults, RICE were symptomatic in 50% and 55% (n = 1 and 26) of cases with CTCAE grade 0 in 47% (n = 23), grade 1 in 25% (n = 12), 0% grade 2 (n = 0) and 29% grade 3 (n = 14), respectively. In adults, RICE risk was associated to WHO grading (8% in WHO grade 1 vs. 24% in WHO grade 2, p = 0.026), independent of age (p = 0.44) and irradiation dose (p = 0.005), but not independent of IDH mutational status. These data demonstrate effectiveness of PRT for LGG in both children and adults. The RICE risk is lower in children which are a main target group for PRT and differs with WHO grading.

RADT-41. IATROGENIC INFLUENCE ON PROGNOSIS OF RADIATION-INDUCED CONTRAST ENHANCEMENTS IN PATIENTS WITH GLIOMA WHO 1-3 FOLLOWING PHOTON AND PROTON RADIOTHERAPY

Radiation-induced contrast enhancement (RICE) is a common side effect following radiotherapy for glioma, but both diagnosis and handling are challenging. Due to the potential risks associated with RICE and its challenges in differentiating RICE from tumor progression, it is critical to better understand how RICE prognosis depends on iatrogenic influence. We identified 99 patients diagnosed with RICE who were previously treated with either photon or proton therapy for World Health Organization (WHO) grade 1-3 primary gliomas. Post-treatment brain MRI-based volumetric analysis and clinical data collection was performed at multiple time points. The most common histologic subtypes were astrocytoma (50%) and oligodendroglioma (46%). In 67%, it was graded WHO grade 2 and in 86% an IDH mutation was present. RICE first occurred after 16 months (range: 1 - 160) in median. At initial RICE occurrence, 39% were misinterpreted as tumor progression. A tumor-specific therapy including chemotherapy or re-irradiation led to a RICE size progression in 86% and 92% of cases, respectively and RICE symptom progression in 57% and 65% of cases, respectively. A RICE-specific therapy such as corticosteroids or Bevacizumab for larger or symptomatic RICE led to a RICE size regression in 81% of cases with symptom stability or regression in 62% of cases. RICE progression went along with a worsening in progression-free survival (p = 0.04). While with chemotherapy and re-irradiation a RICE progression was frequently observed, anti-edematous or anti-VEGF treatment frequently went along with a RICE regression. For RICE, correct diagnosis and treatment decisions are challenging and critical and should be made interdisciplinarily.

RADT-34. ANALYSIS OF A METHYLATION CLASS-GUIDED RADIOTHERAPY IN MENINGIOMA PATIENTS - AND THE PROSPECTIVE PHASE II MARCIE TRIAL WITH CARBON-ION BOOST RADIOTHERAPY

Meningiomas represent the most common primary brain tumor type, with a recurrence rate of approximately 20% following resection. Classification by molecular classes was demonstrated to be more accurate in identifying patients at high risk of disease progression in meningiomas, compared to WHO grading only. Novel radiotherapeutic modalities using protons or carbon ions provide an increased relative biological effectiveness (RBE) compared to photons, enabling a higher local dose deposition while reducing the radiation dose for organs of risks. The prospective single-arm phase II - MARCIE trial investigated a bimodal radiotherapy using photons (50 Gy in 25 fractions) and a carbon-ions boost with 18 Gy (RBE) in 6 fractions. Local progression-free survival after 3 years was defined as primary endpoint. In total, 33 patients were recruited before termination of the trial. After 3 years, local progression free survival was estimated at 94.7% and overall survival at 89.8%. Serious adverse events in the form of radiation necrosis were observed in 3 cases, requiring a subsequent treatment with bevacizumab. Post-hoc DNA methylation profiling revealed that relapses occur more frequently in high-risk meningiomas following radiotherapy, compared to the intermediate risk-group, confirming the prognostic relevance of the methylation-based meningioma classifier.Subsequently, a retrospective matched-pair analysis was performed to compare irradiated (n > 100) and non-irradiated meningiomas, stratified into the molecular risk groups (ben, int, high). The clinical benefit of postoperative radiotherapy and dose escalation with regard to the local control are currently being assessed for the respective molecular meningioma classes. Postoperative, bimodal radiotherapy with carbon-ion boost represent an auspicious treatment modality with an excellent local tumor control following resection. Molecular risk stratification is crucial in identifying patients who might benefit from postoperative radiotherapy.