DIPG-52. EXTRAPONTINE PROGRESSION WITH STABLE PONTINE DISEASE POST-RADIOTHERAPY IN DIFFUSE INTRINSIC PONTINE GLIOMA (DIPG) ON A CONNECT TRIAL: IMPLICATIONS FOR RADIATION TREATMENT VOLUMES

Radiotherapy remains the only treatment modality proven to prolong survival and alleviate symptom burden for patients with DIPG. Upfront focal photon radiation to 54 Gy with 1-1.5cm margins is standard. Subventricular, leptomeningeal, and/or spinal dissemination is observed in up to 50% of cases at autopsy, with activation of metastasis-related pathways (eg, epithelial-mesenchymal transition) in a subset of tumors. Given potential for metastatic spread and critical implications for radiation field volumes, we analyzed the frequency of extrapontine disease progression with pontine tumor stability within a prospective cohort with standardized longitudinal imaging. In a CONNECT consortium-sponsored phase Ib trial with a radiosensitizing agent administered concurrently with focal radiation and adjuvantly, among 10 patients with DIPG who experienced progressive disease with available serial imaging, four (40%) had evidence of disease extension and/or non-contiguous dissemination with stable pontine tumor size at a median of 9 months (range: 5-15 months) from diagnosis. Specifically, extrapontine progression was observed in the spinal cord (n=3), cerebellum (n=2), basal ganglia (n=1), temporal lobe (n=1) and/or leptomeninges and ependymal surfaces (n=1), without radiographic pontine disease progression. Five of the remaining six patients (who came off treatment due to pontine progression) subsequently had radiographic evidence of extrapontine extension (cerebellar, spinal, thalamic, supratentorial). Comprehensive molecular profiling is underway on biopsy or autopsy specimens for three patients, focusing on metastasis pathways. In summary, within a small prospective cohort with longitudinal imaging, we observed extrapontine progression despite local pontine disease control following focal radiotherapy in 40% of patients with DIPG at a median of 9 months after diagnosis. Further research will be essential to identify biological predictors of dissemination and corroborate findings in a larger cohort, but results suggest at least a subset of patients with DIPG may benefit from larger radiation volume expansion into contiguous structures and white matter tracts.