Cone-beam computed tomography (CBCT) for adaptive image guided head and neck radiation therapy

Adaptive (ART) and image guided radiation therapy (IGRT) can improve target coverage and reduce unnecessary irradiation of organs at risk (OAR). The purpose of this study is to report the results of using mid-course imaging and a novel workflow with cone-beam computed tomography (CBCT) surveillance of dose to OAR to trigger adaptive replanning in head and neck radiotherapy.

Impact of radiation therapist (RTT) managed match protocol and mid-course imaging was assessed in two cohorts of consecutive patients receiving RT to the head and neck region, using computed tomography (CT) and CBCT-based dose verification respectively. In the CBCT cohort, patients at high risk of xerostomia received weekly dose surveillance, while low-risk patients received a mid-course CBCT review. For weekly surveillance, predicted total doses to parotid glands, spinal cord and brainstem were calculated. If predicted mean dose to parotid glands increased by >2 Gy or constraints to brainstem or spinal cord were exceeded, replanning was performed.

None had replanning triggered by mid-course imaging. In the CBCT cohort, weekly surveillance of 40 patients yielded minimal reduction in mean dose to parotid glands of 0.65 Gy (range0.4-1 Gy ) for three patients. Patients were surveilled averagely 4.5 times during treatment. Time consumption per CBCT/week was 22 min (range 17-38). Number of patients needed to see to achieve any dose reduction to parotid glands was 13 or the equivalent of 22 working-hours.

The tested dose surveillance algorithm resulted in a minimal dose reduction ( ≤1 Gy) to parotid glands for three of 40 patients. The proposed algorithm and workflow is thus not sustainable. Mid-course dose verification did not provide added benefit and can be safely omitted in the presence of closely monitored daily IGRT. Daily image guidance and match protocol is a safe and efficient method for identifying patients requiring adaptive replanning.