KEY POINTS
- Investigators retrospectively analyzed kidney stereotactic body proton therapy delivered with a deliberately proton-friendly immobilization setup that avoided abdominal compression and other material in the beam path.
- Treatment used a two-field SFO arrangement with posterior–anterior and ipsilateral lateral beams. Fields were duplicated for volumetric repainting, with repeat CBCT and patient repositioning between beam pairs.
- Across 92 treated fractions, the mean three-dimensional intrafraction repositioning shift was 2.6 ± 1.3 mm.
- Analytical margin calculation produced positioning uncertainties of approximately 3.0 mm right–left, 4.0 mm inferior–superior, and 2.6 mm posterior–anterior. The practical recommendation was therefore 4 mm I/S and 3 mm in the other two directions.
- Under modeled perturbations, using the directional margin and performing intrafraction repositioning significantly improved preservation of iCTV D95, D98, D99, and minimum dose compared with an isotropic 3-mm strategy delivered without repositioning.
- With intrafraction repositioning, even the isotropic 3-mm optimization maintained iCTV D99 ≥95% of prescription in 90/92 fractions (97.8%); analytical, 4-mm, and 5-mm approaches produced only small additional coverage gains.
- Larger robustness margins were not free: increasing positioning uncertainty significantly increased dose to uninvolved kidney tissue (p<0.01), arguing against simply using a generous isotropic 5-mm value.
- The analysis modeled positioning shifts rather than all sources of proton uncertainty; respiratory variation, range uncertainty, anatomical changes, deformation, and full interplay effects were not incorporated.
CLINICAL TAKEAWAY
For this kidney SBPT workflow, positioning uncertainty was direction-dependent rather than isotropic, with the greatest uncertainty in the cranio-caudal direction. Repeat imaging and repositioning between beam pairs may be at least as important as increasing robust margins, which otherwise comes at the cost of additional healthy-kidney exposure.