Session Item

The close proximity of the movable nozzle and range shifter to the patients’ surface in pencil beam scanning (PBS) proton therapy of shallow target volumes limits the use of intra fraction surface guidance systems (SGRT). We have optimized the range shifter position for five intracranial indications and four thoracic indications to allow surface guidance and have assessed the impact on plan quality.

Five patients treated for intracranial malignancies and two breast and two lymphoma patients were retrospectively retrieved from our clinical database. PBS intensity modulated proton therapy (IMPT) plans were generated using a standard minimal range shifter air gap of 7 cm (standard plan). The body contour and plan parameters (gantry rotation, couch rotation and isocenter) were imported into a raytracing simulation framework where the visibility of a region-of-interest (ROI) on the patients surface was calculated for varying range shifter positions. Two SGRT systems were considered: (a) the commercial three camera ceiling mounted configuration (3-SGRT) and (b) a novel conceptual five camera design (5-SGRT). For every treatment field the range shifter position closest to the patients’ surface was determined were at least 50 % of the ROI was visible. The optimal parameters were determined for the 3-SGRT and 5-SGRT configuration and used to create two additional treatment plans, as shown in figure 1. DVHs were compared between the three plans per indication.

The 3-SGRT system required a maximally retracted range shifter position for at least one field per plan, except for one patient. The median air gap was 31 cm (range 7 – 35 cm) for the 3-SGRT and 11 cm (range 7 – 17cm) for the 5-SGRT system. The mean pituitary dose was 12.2 Gy for the standard, 12.0 Gy for the 3-SGRT and 11.8 Gy for the 5-SGRT plan. Mean lens dose was 8.9 Gy (L) and 8.2Gy (R) for the standard, 12.0 Gy (L) and 10.8 Gy (R) for the 3-SGRT and 9.3 Gy (L) and 9.0 Gy (R) for the 5-SGRT plan. Mean lacrimal gland dose was 15.5 Gy (L) and 12.9 Gy (R) for the standard, 16.7 Gy (L) and 13.2 Gy (R) for the 3-SGRT and 15.7 Gy (L) and 13.0 Gy (R) for the 5-SGRT plan. The 5-SGRT plan had a mean dose sparing of 9.9% for the mean dose to the pituitary gland, 31.0 % and 21.6 % for the mean left and right lens dose, and 25.2 % and 25.6 % for the mean left and right lacrimal gland dose, over the 3-SGRT plan. The achievable dose sparing for the 5-SGRT is shown in figure 2 for two intracranial cases.

Surface guidance systems can be used during PBS treatment of superficial targets with minimal loss of plan quality when a dedicated, gantry specific five camera SGRT system is used. The conventional three camera SGRT system requires the range shifter to be retracted with a detrimental impact on OAR sparing.