Session Item

Surface guided radiotherapy (SGRT) is reported to improve patient setup for whole‐breast radiotherapy. The purpose of this study is to evaluate the patient SGRT accuracy for pre-imaging setup considering two different IGRT workflows: orthogonal kV-MV images and CBCT guidance.

Patient setup accuracy during the first treatment session of 31 breast cancer patients was retrospectively evaluated. The workflow was performed in a TrueBeam unit and the optical SGRT system Align-RT (Vision-RT) as follows:

• First, patients were positioned by aligning tattoos to the room lasers and TPS isocentre shifts were applied to the couch. After this, patient arms and breast surface were matched to the simulation position with the aid of Vision RT postural video license and a ROI including the breast. From this, system delta couch shifts were performed only for the three directions: vertical, lateral and longitudinal, and recorded as ‘SGRT’ deviation.
• Orthogonal kV-MV images guidance were acquired. The register was done to bony structures (rib bone and sternum). Differences between initial (pre-imaging) and treatment couch positions were then recorded as ‘KV-MV’ deviation.
• Immediately after, a CBCT was performed and an automatic image registration to the PTV structure was done. Differences between initial (pre-imaging) and treatment couch positions were then recorded as ‘CBCT’ deviation.

If the deviations either with kV-MV or CBCT images were within 0.5 cm tolerance, no table shift was applied.

Figure 1 shows the couch shifts applied from SGRT system. From these values, the IGRT image registration deviations are displayed.

Except for one patient, IGRT deviation values were within tolerances, so no additional couch shifts were performed after SGRT setup. The exception was for KV-MV with 0.6 cm deviation in vertical axis, meanwhile the CBCT deviation was 0.2 cm.

For most patients the deviation obtained from CBCT registration was smaller (0.1±0.1, 0.0±0.1 and -0.1±0.2 in cm for vert, long and lat axes respectively) than the one obtained with orthogonal planar kV-MV imaging (0.0±0.2, -0.1±0.2, -0.1±0.2 cm vert, long, lat respectively). Even though the CBCT imaging was performed after kV-MV imaging and the patient could have moved. The percentages of patients with imaging derived differences within the interval [-0.2, 0.2] cm are shown in Figure 2.

Optical 3D surface imaging for breast cancer patient setup positioning showed an excellent agreement with CBCT images (less than 2 mm residual for most patients) and better than for kV-MV.

Positioning breast patients daily with SGRT could lead to a reduction to CTV to PTV margins without a cost of imaging dose linked to daily CBCT imaging.