Dosimetric analysis of local failures in skull-base chordoma and chondrosarcoma following pencil beam scanning proton th
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RESEARCH
Dosimetric analysis of local failures in skull‑base chordoma and chondrosarcoma following pencil beam scanning proton therapy Lucas Basler1* , Robert Poel1, Christina Schröder1, Alessandra Bolsi1, Antony Lomax1, Stephanie Tanadini‑Lang2, Matthias Guckenberger2 and Damien C. Weber1,2,3
Abstract 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 pat‑ terns 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 ± 2.2GyRBE. 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, P TVhigh-dose, PTVlow-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 PTVhigh and 83% (range 0–100%) for PTVlow. In the univariate analy‑ sis, the following DVH parameters were significantly associated with LR: GTV(V
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