(P 306) USE OF PENCIL BEAM SCANNING PROTON RADIOTHERAPY IN MOBILE SPINE CHORDOMAS

Objective: The mobile spine is the rarest site for chordomas and management of these tumors is not uniform involving surgery, radiotherapy or a combination of both. High doses of radiation are required to optimally control these tumors and due to the proximity to sensitive structures proton beam radiation offers an optimal approach. This is an initial review of the use of pencil beam scanning proton therapy (PBPT) in mobile spine chordomas from our institution, presenting tumor response and treatment-associated toxicities.

Methods: Mobile spine chordoma patients treated with PBPT from 3/2016-12/2022 were identified from a prospectively maintained database. A total of 41 primary tumor patients were identified. PBPT was delivered posteriorly for lumbar and thoracic spine tumors typically using 2-3 fields with cervical spine tumors having anterior fields as well. Medical records were retrospectively reviewed for clinical characteristics, surgical details: treatment plan parameters, toxicity, and outcomes under an IRB approved protocol. Descriptive statistics were used to collate patient and treatment characteristics as well as treatment response and toxicity. Recurrences were recorded based on serial imaging.

Results: Location of the chordomas were cervical in 16, thoracic 8, and lumbar 17 patients with a median age of 62 years (range:18-80). All were conventional chordomas except 2 undifferentiated and 1 poorly differentiated. The most common presenting symptom was localized pain (n = 23, 56%) followed by cord compression/cauda equina syndrome (20, 48%) and neuropathic symptoms (radiating pain, numbness, weakness, n= 16, 39 %). Bone and soft tissue disease was present in all cases but one with only soft tissue disease. One patient had local nodal metastases at diagnosis. Mean initial gross tumor volume was 83 cc (range 3-1041). Surgery was performed in 36(88%) patients with radiation done postoperatively in 24, preoperative in 3 and both in 8. Seven patients received radiation as definitive management after biopsy alone. Low-dose photon radiation was used in 5 cases prior to surgery for cord compression. Three patients had P-32 plaque therapy at the time of surgery delivering 8-10 Gy to 1 mm depth along the dura. Surgery was enbloc in 10, marginal resection in 9 and debulking in 17 patients. Most (36, 88%) patients had 2-3 treatment volumes and hypofractionation up to 2.45 Gy/fraction was utilized in 16 patients, one patient was treated b.i.d.

Doses data is outlined in Table 1. Median follow-up after final treatment was 35 mo. (range 0-76). 11 local recurrences within the radiation volumes were documented at a median of 21 mo.(range 1-53). 3 patients had distant disease: 2 in the lungs and one in lymph nodes. A total of 3 patients have died of disease, all with local recurrences. Treatment-associated toxicities included radiation dermatitis, ongoing pain/stiffness related to treatment, and persistent neuropathies. Chronic pain after treatment was seen in 6 patients.

Conclusion: Mobile spine chordomas are challenging tumors with a variety of treatment approaches. PBPT is an effective component of local therapy often used in combination with surgery. High doses can be achieved utilizing hypofractionation. Optimal integration of radiation with surgery and better standardization of the sequencing needs to be coordinated between surgical and radiation oncology team members.