Clinical implementation of kilovoltage cone beam CT for the verification of sequential and integrated photon boost treatments for breast cancer patients.

OBJECTIVE: The objective of this study was to formulate a practical method for the use of cone beam CT (CBCT) for the verification of sequential and integrated tumour bed boosts for early breast cancer patients.
METHODS: Partial arc scan geometries were assessed on a treatment unit. Imaging dose measurements on an Elekta Synergy CBCT system were made in a CT dose phantom for scan parameters 100 kV, 25 mA and 40 ms with an S20 collimator. The protocol was used to verify the setup of a cohort of 38 patients, all of whom had surgical clips inserted in the tumour bed. Setup errors with and without an extended no action level (eNAL) protocol were calculated.
RESULTS: Arcs from 260° to 85° (left breast) and 185° to 15° (right breast) were found sufficient to image fiducial markers and anatomy whilst accounting for the physical limits of the equipment. A single treatment and imaging isocentre was found by applying simple constraints: isocentre <8 cm from midline and isocentre-couch distance <30 cm. Contralateral breast doses were ∼2 mGy per scan (right breast) and ∼12 mGy (left breast). Both mean population systematic error and mean population random error were 3 mm prior to correction. The systematic error reduced to 1.5 mm using an eNAL correction protocol, implying that a 5-mm setup margin could be achieved.
CONCLUSION: An image-guided verification protocol using CBCT for breast cancer boost plans was implemented successfully. Setup errors were reduced with an acceptable imaging dose to the contralateral breast.