Imaging and Treatment Isocenter Coincidence for a Linear Accelerator Using Oblique Projection Imaging

Modern radiotherapy treatments involve advanced patient imaging systems to allow for more accurate and reproducible patient positioning. This is known as image guided radiotherapy (IGRT). To conduct IGRT, the user needs to validate that the coincidence of the imaging system and the treatment beams' center is within acceptable tolerances (\textless 2mm). The equipment under investigation was a double oblique kV projection imaging system and a C-arm gantry mounted medical linear accelerator (linac). Using radiochromic film, the coincidence of the linac mechanical center, and radiation beam center was found to be 0.91mm. The imaging system center was calibrated to be \textless 1mm from linac mechanical center. Using IGRT, a hidden spherical target phantom was aligned. Marked radiochromic films were also inserted in the phantom to capture the target shadow made by the treatment beam. The test indicated an alignment error of 1.3\textpm 0.27mm between the imaging and treatment center, which was within stated tolerance. These experiments provide a conclusive and deconvoluted map of uncertainties using IGRT as well as understanding of quality assurance methods used by a medical physicist in a clinical setting.