Metastatic Papillary Thyroid Cancer with Brain Metastasis Treated with Stereotactic Radiation Therapy

Summary: Papillary thyroid cancer (PTC) is the most common thyroid cancer, generally associated with a good prognosis and a low incidence of metastasis. Brain metastasis in PTC is extremely rare, occurring in less than 1% of cases. Herein, we present the case of a patient with metastatic papillary thyroid carcinoma (PTC) who presented with brain metastasis treated with stereotactic radiation therapy (SRT). This is a 51-year-old patient who was initially diagnosed with stage I (pT1aN1bM0) classic PTC. At the time of diagnosis, he underwent total thyroidectomy with a right lymph node dissection. Post-operative imaging identified pulmonary metastases, and the patient underwent multiple lines of therapy due to progressive thoracic disease. Next-Generation Sequencing later revealed a BRAF V600E mutation, leading to targeted therapy with Dabrafenib. Approximately 1.5 years from initial diagnosis, routine PET-CT showed stable pulmonary nodules but incidentally found a left temporal lobe intraparenchymal hemorrhage. Brain MRI confirmed a mixed cystic and solid mass (3.9 x 2.1 x 2.2 cm) in the left temporal lobe with mild mass effect and vasogenic edema; however, the patient was asymptomatic. After consulting Neurosurgery and Radiation Oncology, two treatment options were discussed with the patient including surgical resection with post-operative radiation therapy versus radiation therapy alone. Due to the size of the metastasis and the opportunity for pathologic confirmation, surgical resection followed by post-operative radiation was recommended. However, the patient opted against surgery. After detailed discussion, linac-based SRT was chosen. For SRT, the patient was simulated supine with a headrest and thermoplastic mask. A treatment planning brain MRI was used to delineate left temporal brain metastasis with a 1mm PTV expansion. The dose prescribed was 30 Gy in 5 fractions, with a 109% hotspot. Three coplanar partial arcs with 6MV FFF beam energy were used, while keeping brainstem, cochlea, and optic doses below acceptable tolerances. The patient tolerated the five-day treatment well and, at two-week follow-up, reported only mild alopecia at the treatment site. A follow-up MRI is scheduled for two months post-treatment (which has not been completed at the time of this submission, but results will be available at the time of the conference). Brain metastases in PTC are extremely rare, and optimal management and outcomes in such cases remain under-researched. This case highlights SRT as a feasible alternative for patients declining surgery. Continued follow-up will clarify the effectiveness of this approach in this patient, and more extensive research is needed to identify potential factors predisposing patients with PTC to brain metastasis and its impact on disease outcomes. Patricia Pius (she/her/hers), MD (Presenting Author) - University of Oklahoma; Taryn Kedizor, BS (Co-Author) - OU College of Medicine; Ozer Algan, MD (Co-Author) - OU Health Stephenson Cancer Center; Christina Henson, MD (Co-Author) - University of Oklahoma