Proton Therapy for Skull-Base Chordomas and Chondrosarcomas: Initial Results From the Beaumont Proton Therapy Center

Abstract

Background:Skull-base chordomas and chondrosarcomas are rare tumors that arise directly adjacent to important critical structures. Appropriate management consists of maximal safe resection followed by postoperative dose-escalated radiation therapy. Proton beam therapy is often employed in this context to maximize the sparing of organs at risk, such as the brainstem and optic apparatus.

Methods: This is a single-institutional experience treating skull-base chordomas and chondrosarcomas with postoperative pencil beam scanning proton therapy. We employed a simultaneous integrated boost to the gross tumor volume (GTV) for increased conformality. Demographic, clinicopathologic, toxicity, and dosimetry information were collected. Toxicity was assessed according to Common Terminology Criteria for Adverse Events (CTCAE), v. 4.0.

Results: Between 2017 and 2020, 13 patients were treated with postoperative proton therapy. There were 10 patients with chordoma (77%) and three with chondrosarcoma (23%). A gross total resection was achieved in six (60%) patients with chordoma and one patient with chondrosarcoma (33%). Nine patients (69%) received postoperative therapy, whereas four (31%) received treatment at recurrence/progression following re-excision. The median dose to the GTV was 72.4 cobalt-Gray equivalents (range, 70.0 to 75.8). The mean GTV was 3.4 cc (range, 0.2-38.7). There were no grade 3 or greater toxicities. One patient developed grade 2 temporal lobe necrosis. At 10.7 months' median follow-up (range, 2.1-30.6), the rates of local control and overall survival were 100%.

Conclusions: Proton beam therapy with pencil beam scanning and simultaneous integrated boost to the GTV affords excellent early local control with the suggestion of low morbidity. This method deserves consideration as an optimal method for limiting dose to adjacent organs at risk and delivering clinically effective doses to the treatment volume.

Keywords: pencil beam scanning; proton beam therapy; simultaneous integrated boost; skull-base chondrosarcomas; skull-base chordomas.

Figure 1. Representative (A) axial, (B) sagittal, and (C) coronal images of a patient treated with simultaneous integrated boost to 75.8 cobalt-gray equivalents to the gross tumor volume. Pertinent target volumes are denoted on the axial image.

Abbreviations: CTV, clinical target volume; GTV, gross tumor volume.

Figure 2. Plot of average maximum doses (0.03 cc) to the (A) brainstem surface, (B) optic chiasm, (C) left optic nerve, and (D) right optic nerve for each of the 13 patients with robustness evaluation and error bars representing standard deviations for the 26 dose scenarios.

Abbreviation: cc, cubic centimeter; cCGE, centi-cobalt-gray equivalent.