Cone-beam CT assessment of interfraction and intrafraction setup error of two head-and-neck cancer thermoplastic masks

Michael Velec, John N Waldron, Brian O'Sullivan, Andrew Bayley, Bernard Cummings, John J Kim, Jolie Ringash, Stephen L Breen, Gina A Lockwood, Laura A Dawson
International Journal of Radiation Oncology, Biology, Physics 2010 March 1, 76 (3): 949-55

PURPOSE: To prospectively compare setup error in standard thermoplastic masks and skin-sparing masks (SSMs) modified with low neck cutouts for head-and-neck intensity-modulated radiation therapy (IMRT) patients.

METHODS AND MATERIALS: Twenty head-and-neck IMRT patients were randomized to be treated in a standard mask (SM) or SSM. Cone-beam computed tomography (CBCT) scans, acquired daily after both initial setup and any repositioning, were used for initial and residual interfraction evaluation, respectively. Weekly, post-IMRT CBCT scans were acquired for intrafraction setup evaluation. The population random (sigma) and systematic (Sigma) errors were compared for SMs and SSMs. Skin toxicity was recorded weekly by use of Radiation Therapy Oncology Group criteria.

RESULTS: We evaluated 762 CBCT scans in 11 patients randomized to the SM and 9 to the SSM. Initial interfraction sigma was 1.6 mm or less or 1.1 degrees or less for SM and 2.0 mm or less and 0.8 degrees for SSM. Initial interfraction Sigma was 1.0 mm or less or 1.4 degrees or less for SM and 1.1 mm or less or 0.9 degrees or less for SSM. These errors were reduced before IMRT with CBCT image guidance with no significant differences in residual interfraction or intrafraction uncertainties between SMs and SSMs. Intrafraction sigma and Sigma were less than 1 mm and less than 1 degrees for both masks. Less severe skin reactions were observed in the cutout regions of the SSM compared with non-cutout regions.

CONCLUSIONS: Interfraction and intrafraction setup error is not significantly different for SSMs and conventional masks in head-and-neck radiation therapy. Mask cutouts should be considered for these patients in an effort to reduce skin toxicity.

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