Journal
JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS
Volume 16, Issue 1, Pages 159-169Publisher
MULTIMED INC
DOI: 10.1120/jacmp.v16i1.5108
Keywords
image-guided radiotherapy; head and neck radiotherapy; interfractional larynx setup error; quality assurance; target volume and organs-at-risk margins
Funding
- UICC American Cancer Society Beginning Investigators Fellowship - American Cancer Society
- National Institutes of Health/National Cancer Institute's Paul Calabresi Clinical Oncology Award Program [K12 CA088084-06]
- Clinician Scientist Loan Repayment Program [L30 CA136381-02]
- National Institutes of Health Cancer Center Support (Core) Grant [CA016672]
- SWOG/Hope Foundation
- General Electric Healthcare/MD Anderson Center for Advanced Biomedical Imaging In-Kind Award: an Elekta AB/MD Anderson Department of Radiation Oncology Seed Grant
- Center for Radiation Oncology Research at MD Anderson Cancer Center
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Larynx may alternatively serve as a target or organs at risk (OAR) in head and neck cancer (HNC) image-guided radiotherapy (IGRT). The objective of this study was to estimate IGRT parameters required for larynx positional error independent of isocentric alignment and suggest population-based compensatory margins. Ten HNC patients receiving radiotherapy (RT) with daily CT on-rails imaging were assessed. Seven landmark points were placed on each daily scan. Taking the most superior-anterior point of the C5 vertebra as a reference isocenter for each scan, residual displacement vectors to the other six points were calculated postisocentric alignment. Subsequently, using the first scan as a reference, the magnitude of vector differences for all six points for all scans over the course of treatment was calculated. Residual systematic and random error and the necessary compensatory CTV-to-PTV and OAR-to-PRV margins were calculated, using both observational cohort data and a bootstrap-resampled population estimator. The grand mean displacements for all anatomical points was 5.07 mm, with mean systematic error of 1.1 mm and mean random setup error of 2.63 mm, while bootstrapped POIs grand mean displacement was 5.09 mm, with mean systematic error of 1.23 mm and mean random setup error of 2.61 mm. Required margin for CTV-PTV expansion was 4.6 mm for all cohort points, while the bootstrap estimator of the equivalent margin was 4.9 mm. The calculated OAR-to-PRV expansion for the observed residual setup error was 2.7 mm and bootstrap estimated expansion of 2.9 mm. We conclude that the interfractional larynx setup error is a significant source of RT setup/delivery error in HNC, both when the larynx is considered as a CTV or OAR. We estimate the need for a uniform expansion of 5 mm to compensate for setup error if the larynx is a target, or 3 mm if the larynx is an OAR, when using a nonlaryngeal bony isocenter.
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