Novel technique for simulation and external beam treatment planning for obese patients

Purpose: In computed tomography (CT)-guided radiation treatment planning, an accurate assessment of a patient's external contour is needed to determine the number of monitor units appropriate to treat a target volume to the prescribed dose. When obese patients are imaged, even using a large-bore CT scanner, lateral tissues frequently fall outside the field of view of the CT scanner, which makes it difficult to accurately calculate the dose needed for lateral or oblique treatment fields. We aimed to develop a technique to capture the external contour of patients in whom traditional simulation techniques would fail to capture part of the anatomy. Methods and Materials: The following technique was used in multiple patients. The patient was centered on the CT couch, and an isocenter was placed. The initial scan was obtained with the isocenter at the central position on the table. Two subsequent scans were obtained with the patient first shifted right on the table and then shifted left. The 3 CT scans were fused using a CT-CT cross-correlation algorithm. The accuracy of the scans was reviewed manually. Using the fused scans, lateral tissues were contoured on the primary scan and assigned a density of 1 g/cm³. Results: With this technique, each patient's entire body contour was available on a single fused scan. The availability of the entire body contour on the scan allowed for treatment planning with a 360-degree freedom for external beam placement. Conclusions: This novel approach of fusing multiple CT scans allows for the external anatomy of obese patients to be completely captured. The increase in available beam angles made possible by this approach can optimize external beam treatment planning, including planning for intensity-modulated radiation therapy.