Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_15_3263 - Calculation of Head and Neck Treatment Doses on Cone-Beam CT

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Calculation of Head and Neck Treatment Doses on Cone-Beam CT
L. Qiu, N. P. Joshi, N. M. Woody, S. Koyfman, and A. R. Godley; Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH

Purpose/Objective(s): To develop a dose calculation method on daily CBCTs of head and neck (H&N) cancer patients that is sufficiently accurate in determining planning target volume (PTV) coverage and hence decide if adaptive replanning is needed.

Materials/Methods: Ten H&N cancer patients with an adaptive replan during their treatment course were selected for this study. Each patient’s planning CT, replanning CT and the CBCT imaged on that replanning day were registered. The treatment beams were then copied to the replanning CT and CBCT. The beams calculated on the replanning CT provided the reference dose i.e. the dose that was delivered that day's anatomy. The beams were also recalculated on the CBCT, with the electron density of the CBCT set to water within the external contour(homogeneous calculation). This CBCT calculated dose is all that would typically be available to determine if a replan is needed. The CBCT external contour however is limited by the field of view (FOV) of the CBCT, which misses the shoulders and posterior neck. The external contour was therefore corrected to estimate the true external. The doses calculated on the CBCT using the limited and corrected external were then compared to the reference dose using 3D gamma. Gamma was calculated only within the combined high and low dose PTV (tumor plus nodes).

Results: The table presents the patient average 3D gamma within the PTV using different tolerances for both the limited FOV external, and the corrected external contour. The corrected external improves the agreement of the CBCT calculated dose with the reference dose. To get 98% 3D gamma agreement requires a tolerance of 5%/3 mm, which defines the accuracy limit of the CBCT homogenous dose calculation. Our methods would therefore allow detection of PTV coverage reduction below 95%. The agreement using the limited FOV external is never sufficient.

Conclusion: Using a corrected external contour is critical to accurate dose calculation on CBCTs. An alternative is to use a larger FOV scan, requiring a full arc, rather than the partial arc used by typical H&N CBCT protocols. It is recommended to use a full arc scan if the coverage of a PTV extending inferior to the neck is required. The PTV coverage can then be sufficiently well estimated using a homogenous dose calculation.
3D gamma within PTV
External contour (3%/3 mm) (4%/3 mm) (5%/3 mm)
Limited FOV 61.4% 87.0% 80.4%
Corrected 73.0% 91.8% 97.8%

Author Disclosure: L. Qiu: None. N.P. Joshi: None. S. Koyfman: Research Grant; Merck. A.R. Godley: None.

Lei Qiu, PhD

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