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Background  Despite combined modality treatment involving surgery and radiotherapy, a relevant proportion of skull-base chordoma and chondrosarcoma patients develop a local recurrence (LR). This study aims to analyze patterns of recurrence and correlate LR with a detailed dosimetric analysis.    Methods  222 patients were treated with proton radiotherapy for chordoma (n = 151) and chondrosarcoma (n = 71) at the PSI between 1998 and 2012. All patients underwent surgery, followed by pencil-beam scanning proton therapy to a mean dose of 72.5 ± 2.2Gy RBE . A retrospective patterns of recurrence analysis was performed: LR were contoured on follow-up MRI, registered with planning-imaging and the overlap with initial target structures (GTV, PTV high-dose , PTV low-dose ) was calculated. DVH parameters of planning structures and recurrences were calculated and correlated with LR using univariate and multivariate cox regression.    Results  After a median follow-up of 50 months, 35 (16%) LR were observed. Follow-up MRI imaging was available for 27 (77%) of these recurring patients. Only one (3.7%) recurrence was located completely outside the initial PTV (surgical pathway recurrence). The mean proportions of LR covered by the initial target structures were 48% (range 0–86%) for the GTV, 70% (range 0–100%) for PTV high  and 83% (range 0–100%) for PTV low . In the univariate analysis, the following DVH parameters were significantly associated with LR: GTV(V < 66Gy RBE ,  p  = 0.01), GTV(volume,  p  = 0.02), PTV high (max,  p  = 0.02), PTV high (V < 66Gy RBE ,  p  = 0.03), PTV high (V < 59Gy RBE ,  p  = 0.02), PTV high (volume,  p  = 0.01) and GTV(D95,  p  = 0.05). In the multivariate analysis, only histology (chordoma vs. chondrosarcoma,  p  = 0.01), PTV high (volume,  p  = 0.05) and GTV(V < 66Gy RBE ,  p  = 0.02) were independent prognostic factors for LR.    Conclusion  This study identified DVH parameters, which are associated with the risk of local recurrence after proton therapy using pencil-beam scanning for patients with skull-base chordoma and chondrosarcoma.

Dosimetric analysis of local failures in skull-base chordoma and chondrosarcoma following pencil beam scanning proton therapy