Exquisite Radiosensitivity Leading to a Diagnosis of Ph+MPAL Mimicking Cerebrospinal Fluid Dissemination of Intraventricular Meningioma Following Stereotactic Radiotherapy.

We report a rare case of Philadelphia chromosome-positive (Ph+) mixed phenotype acute leukemia (MPAL) that initially radiologically mimicked cerebrospinal fluid (CSF) dissemination after resection and subsequent stereotactic radiotherapy (SRT) for an intraventricular meningioma. A 77-year-old woman underwent resection of an asymptomatic World Health Organization grade I lateral ventricular meningioma. Due to residual tumor growth, the patient received three sessions of SRT for seven lesions between 28 and 45 months postoperatively. Two of these lesions were radiologically diagnosed as CSF dissemination of meningioma. At 63 months postoperatively, 2 new frontal lobe lesions appeared and were similarly treated with SRT (21 Gy in 3 fractions). Follow-up magnetic resonance imaging revealed complete resolution of these new lesions only two months after this low-dose SRT. This remarkably rapid response was inconsistent with the typical radiobiology of meningioma, prompting diagnostic reconsideration. The patient was subsequently diagnosed with Ph+MPAL. Although chemotherapy (prednisolone and dasatinib) was discontinued after one month due to adverse effects, the patient achieved complete remission. No recurrence of Ph+MPAL was observed for 23 months after the discontinuation of chemotherapy, until the patient died of causes unrelated to the primary disease. The unexpectedly exquisite radiosensitivity should prompt clinicians to reconsider the initial diagnosis. Such rapid responses may indicate underlying hematologic malignancies rather than slow-growing tumors like meningioma. Even in elderly patients, early detection can lead to successful remission with short-term treatment.