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Can Dose Matrix Mapping to Kilo-Voltage Cone Beam CT Images Be Reliably Used to Monitor Weekly Dose Changes for Head and Neck Cancer Patients?

H Liu*, Q Shang, J Greskovich, S Koyfman, R Lotey, P Xia, The Cleveland Clinic Foundation, Cleveland, OH

SU-E-T-371 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: In external beam radiotherapy for head-and-neck cancer, the actual dose delivered to the patients could considerably deviate from the planned dose due to volumetric and anatomic variations. The purpose of this study was to investigate whether direct dose matrix mapping can be reliably used to monitor weekly dose changes when compared to recalculation method on weekly kilo-voltage CBCT images.

Methods:Ten head-and-neck patients, each with a planning CT, a mid-term replanning CT, and seven weekly CBCTs, were selected for this study. Contours of tumor volumes and radiosensitive normal structures were deformed from the planning CT to the CBCTs using a commercial program. These deformed contours were validated using the same day CT and CBCT. The daily doses were obtained by two methods: (1)mapping the planned dose matrix to the corresponding CBCTs after rigid image registration; (2)recalculating dose on the CBCT images with the same planning beam configurations. For a fair comparison, density override of the external contours was performed for both methods.

Results:No significant dose differences were seen comparing the dose matrix mapping and recalculation methods for both the target and sensitive structures(p>>0.05). From both dose mapping and recalculation methods, the difference in D95 between the dose of the day and planned dose was observed for the PTV with 1.7 ± 2.2% variations. Although no significant changes in the mean dose were observed for the other sensitive organs, the mean dose of the parotid glands increased 14.1 ± 14.9% for the right parotid and 7.5 ± 9.8% for the left parotid.

Conclusion:Dose matrix mapping is an effective and practical method to monitor weekly dose changes for patients with HN cancer. With the deformed contours from the planning CT, both the dose matrix mapping and recalculation methods can provide quantitative dose analysis and physician guidance for decisions on the adaptive replanning.

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