Development of a phantom for assessing the precision of setup in skin mark-less surface-guided radiotherapy

Surface-guided radiotherapy (SGRT) is adopted by several institutions; however, reports on the phantoms used to assess the precision of the SGRT setup are limited.

The purpose of this study was to develop a phantom to verify the accuracy of the irradiation position during skin mark-less SGRT.

An acrylonitrile butadiene styrene (ABS) plastic cube phantom with a diameter of 150 mm on each side containing a dummy target of 15 mm and two types of body surface-shaped phantoms (breast/face shape) that could be attached to the cube phantom were fabricated. Films can be inserted on four sides of the cubic phantom (left, right, anterior and posterior), and the center of radiation can be calculated by irradiating the dummy target with orthogonal MV beams. Three types of SGRT using a VOXELAN-HEV600M (Electronics Research&Development Corporation, Okayama, Japan) were evaluated using this phantom: (i) SGRT_CT-a SGRT set-up based solely on a computed tomography (CT)-reference image. (ii) SGRT_CT + CBCT-a method where cone beam computed tomography (CBCT) matching was performed after SGRT_CT. (iii) SGRT_Scan-a resetup technique using a scan reference image obtained after completing the (ii) step.

Both the breast and face phantoms were recognized in the SGRT system without problems. SGRT_Scan ensure precision within 1 mm/1° for breast and face verification, respectively. All SGRT methods showed comparable rotational accuracies with no significant disparities.

The developed phantom was useful for verifying the accuracy of skin mark-less SGRT position matching. The SGRT_Scan demonstrated the feasibility of achieving skin-mark less SGRT with high accuracy, with deviations of less than 1 mm. Additional research is necessary to evaluate the suitability of the developed phantoms for use in various facilities and systems. This phantom could be used for postal surveys in the future.