2046 - Volumetric Changes in High-Grade IDH Mutant Diffuse Gliomas Treated with Reduced Margin Adaptive Intensity Modulated Proton Therapy
Presenter(s)
A. Dasgupta1,2, R. Malhotra3, P. Chavhan3, A. Chatterjee3, P. Shinde4, M. Konthoujam4, R. Punde4, L. Meena4, A. Sahu5, A. Shah6, A. Sahay6, E. Sridhar7, and T. Gupta1; 1Department of Radiation Oncology, Advanced Centre for Treatment, Research and Education in Cancer, Mumbai, India, 2Homi Bhabha National Institute, Mumbai, India, 3Department of Radiation Oncology, Tata Memorial Hospital, Mumbai, India, 4Medical Physics, Advanced Centre for Treatment, Research and Education in Cancer, Mumbai, India, 5Department of Radiodiagnosis, Tata Memorial Hospital, Mumbai, India, 6Department of Pathology, Tata Memorial Hospital, Mumbai, India, 7Department of Pathology, Advanced Centre for Treatment, Research and Education in Cancer, Mumbai, India
Purpose/Objective(s): Patients with IDH mutant diffuse gliomas treated in our centre with proton beam therapy are considered for two-phase plans with reduced margins for the second phase. The current analysis was done to investigate the volumetric changes and early clinical outcomes using the two-stage adaptive treatment.
Materials/Methods: Adults with high-grade IDH mutant glioma receiving intensity modulated proton therapy, undergoing at least 1 interval MRI as part of an adaptive treatment workflow, were included in the retrospective analysis. Astrocytomas were treated with total dose of 59.4 CGE in 33 fractions and oligodendroglioma with 55.8 CGE in 31 fractions, along with concurrent temozolomide. Pre-radiation multiparametric MRI was done for contouring. Gross tumor volume (GTV) included the cavity and/or residual disease. Clinical target volume (CTV) included a 10 mm isotropic expansion edited from anatomical barriers. Between 4th and 5th week, a scheduled interval MRI was done. The GTV was modified based on interval imaging, and for phase 2, CTV margin reduction was done from 10 mm to 5mm based on the expansion of the adapted GTV. Phase 2 treatment was scheduled for the last 5 fractions, or in case of a major increase in volumes, immediate adaptation was considered.
Results: Fifty-one patients were included. The median age was 35 years, with 45% oligodendroglioma. The median time to adaptive MRI was 25 days from the start of radiation. The median GTV volumes at baseline and adaptation were 85.9 cc and 82.0 cc, respectively. All patients had morphological changes in the boundaries, proceeding with a scheduled change in contours. In the entire cohort, a decrease in GTV was seen in 53% of patients. Decrease of GTV was more commonly encountered in oligodendroglioma (65%) as compared to astrocytoma (38%) (p=0.10). The overall median change of GTV for patients with astrocytoma was 1.9% increase, while for oligodendroglioma, the median change was 3.4% decrease. Other factors, like time from surgery, time from radiation starting, and initial volumes, had no discernible effects on the trajectory of GTV changes. The median CTV volumes at baseline and adaptation were 194.1 cc and 135.5 cc, due to margin reduction. With a median follow-up of 9 months, no recurrences were seen. Treatment-related changes were seen in 10 patients, with 5 developing radiation-induced contrast enhancement (RICE) and radionecrosis in 5 (grade 3 in 2 patients).
Conclusion: Early clinical outcomes demonstrate the safety of reduced margin adaptive IMPT in IDH mutant gliomas. Interval MRI is critical for precision of IMPT, with all patients requiring morphological adaptation due to changes in volume or boundary of the GTV. Oligodendrogliomas have a higher likelihood of reduction of GTV, enabling reduction of overall GTV and CTV volume in the majority of patients, which reduce low dose irradiation to the normal brain.