Case Report of a Grade 5 Toxicity Secondary to Single Fraction Cervical Spine SBRT

Summary: This case report describes a 55-year-old female who initially presented with neck pain and was found to have EGFR mutant stage IVB non-small cell lung cancer (cT4N3M1c). Osimertinib was initiated on 07/13/21, and stereotactic body radiation therapy (SBRT) was administered to C6 (35Gy/5fx SBRT) on 08/03/21 due to a painful pathologic fracture. She completed an additional course of cervical spine SBRT (20Gy/1fx) to C3 on 03/08/23. In March 2024, she developed throat pain and progressive hemoptysis. An endoscopy on 06/19/24 revealed a prevertebral fistula, which was debrided on 06/23/24. The patient then developed sepsis from the prevertebral fistula and ultimately passed away on 07/02/24. A comprehensive chart and literature review was conducted to understand the etiology and pathogenesis of this grade 5 toxicity. MIM was used to generate a plan sum to calculate cumulative dose to tissues, EQD2 and BED calculations. Literature search was performed using NIH PubMed searching for single fraction dose constraints for the pharynx, as well as any reported cases of pharyngeal fistula secondary to cervical spine SBRT. Plan sum evaluation revealed minimal field overlap with previous treatment (overlap of the 50 cGy isodose lines). Offline review of treatment images did not show any misalignment of treatment delivery beyond accepted tolerances. A max BED to the area of late tissue necrosis was calculated to be 7067cGy with max single fraction point dose of 1804 cGy. Single fraction mean dose to the pharynx was 402cGy, with D1cc = 1320 cGy (BED 6300cGy) and D5cc = 140cGy (BED 860cGy). There was no overlap between the PTV and pharynx OAR volume. Literature search did not identify any cases of late pharyngeal toxicity secondary to cervical spine SBRT. Furthermore, there were no published single fraction constraints for the pharynx. This case reports a rare and devastating complication of cervical spinal SBRT. Current international guidelines support the use of single fraction treatment in spinal metastases using an SBRT technique. Understanding of tissue tolerances of the head and neck using one to five fractions is currently being explored. Reported outcomes typically involve dysphagia rather than severe, life-threatening complications. While these constraints can be extrapolated, this case report underlies the importance of identifying and respecting tissue tolerances when using a single fraction technique. Lessons learned from this case may raise caution in the use of single fraction palliation in the cervical spine and help inform the development of future head and neck dose constraints. Lea J. Tan (she/her/hers), BA (Presenting Author) - UC Irvine School of Medicine; Nicholas Peterson, MD (Co-Author) - UC Irvine School of Medicine; Mark Linskey, MD (Co-Author) - UC Irvine School of Medicine; Jeremy Harris, MD (Co-Author) - UC Irvine School of Medicine