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e14540 Background: Glioblastoma (GBM) is the most common primary central nervous system (CNS) malignancy. In recent years, molecular and histological analyses have identified GBM with primitive neuronal component (GBM-PNC) as a distinct entity with poor regional control and worse overall prognosis. GBM-PNCs are more prone to CNS dissemination and leptomeningeal (LM) metastasis, occurring in up to 40% of patients. Given the concern for distal CNS spread, we hypothesized that craniospinal irradiation (CSI) may be an effective treatment option for GBM-PNC. Methods: We performed a retrospective chart review of patients diagnosed with GBM-PNC treated at a large academic center with institutional review board approval. Clinical, histologic and treatment characteristics, as well as disease outcomes were obtained from the electronic medical record. Results: Ten patients diagnosed with GBM-PNC were identified in the period of 2005-2019. Median follow-up was 17 months. Overall, 80% of patients were men, median age was 49 years (range 20-64) and ECOG 0-1. Fifty percent of patients received CSI as opposed to involved field radiation therapy (IFRT). Four of five patients receiving CSI were treated with proton therapy. All patients, but one received concurrent Temozolomide. Both IFRT and CSI were tolerated well with minimal grade 3-4 toxicities. Patients treated with CSI were more likely to experience transient grade 3-4 lymphopenia (60% vs 0%). Eighty percent of patients experienced a recurrence after treatment with median progression free survival (PFS) of 13 months (range 8-26). PFS did not differ between patients receiving IFRT and CSI (14 vs 12.5 months, p = 0.9). Median overall survival (OS) was 30 months for patients who received IFRT, and median OS was not reached in the CSI group. Of the patients who received CSI as part of their treatment, all recurrences were within the high dose radiation area. On the contrary, 75% of patients who received IFRT experienced multiple recurrences at new intracranial sites and later in the leptomeninges or the spine, requiring multiple treatment courses. Conclusions: This hypothesis generating study demonstrates that CSI may prevent distant intracranial metastasis and LM disease progression. CSI delivered with proton therapy was well tolerated with minimal additional toxicity. A larger study is needed to guide the optimal treatment of patients diagnosed with glioblastoma with primitive neuronal component.

240 Background: The American Society for Radiation Oncology (ASTRO) ‘Choosing Wisely’ (CW) list notes radiation interventions lacking evidence of benefit in cancer care. This study characterizes adherence to the 2014 CW item to ‘not recommend radiation following hysterectomy in low-risk endometrial cancer’ (Stage I, Grade 1-2). No studies have previously evaluated adherence to this CW item. Methods: SEER records for patients age >65 at first primary diagnosis of Stage I (Grade 1-2) endometrial cancer between 2008-2011 were linked with Medicare files. Included cases had Medicare Part A and B coverage for ≥12 months before/after diagnosis and had a hysterectomy 1 month prior to 4 months following diagnosis. We used logistic regression to evaluate associations between socio-demographic and clinical characteristics (Table 1) and receipt of radiation therapy within 1 year of diagnosis. Results: Among 2,938 women meeting eligibility criteria, mean age was 74, 90% were white, and 55% lived in a ‘big metro’ area (population >1m). The non-adherent fraction was 24%. Among non-adherent cases, 76% received brachytherapy. In adjusted analyses (Table 1), only tumor grade was associated with non-adherence. Conclusions: More than 20% of low-risk endometrial cancer cases in our sample were non-adherent to ASTRO CW recommendations, and this only varied significantly by tumor grade. Therefore, use of radiation therapy following hysterectomy appears to be a widespread issue. This is important because non-adherence may result in side effects or substantial cost without commensurate clinical benefit. This study’s follow-up period was prior to the ASTRO CW list, but similar recommendations existed in ASCO, ESMO and NCCN guidelines at the time—suggesting work is need to align practice with guidelines. Our findings provide a baseline measure against which future studies can measure progress. [Table: see text]

e11584 Background: Brain metastases occur late in the course of metastatic breast cancer and the median survival from time of brain metastases has historically been ~4 months. Stereotactic radiosur...