Intrafractional tumor position stability during computed tomography (CT)-guided frameless stereotactic radiation therapy for lung or liver cancers with a fusion of CT and linear accelerator (FOCAL) unit

PURPOSE: To evaluate intrafractional tumor position stability during computed tomography (CT)-guided frameless stereotactic radiation therapy (SRT) for lung or liver cancers, we checked repeated CT scanning, with a fusion of CT and linear accelerator (FOCAL) unit. METHODS AND MATERIALS: The FOCAL unit is a combination of a linear accelerator (Linac), CT scanner, X-ray simulator (X-S), and carbon table, and is designed to achieve CT-guided SRT with daily CT positioning followed by immediate irradiation while patients keep reduced shallow respirations. To evaluate intrafractional tumor position stability, 50 lung or liver lesions in 20 patients were checked by repeated CT scanning just before and after irradiation, and the obtained images were compared. RESULTS: There was no case with the intrafractional error judged to be greater than 10 mm. In 68% of cases, the intrafractional positioning errors were negligible (0-5 mm). CONCLUSIONS: Using the FOCAL unit, SRT for lung or liver cancers could be performed with intrafractional positioning errors not greater than 10 mm.