Single Iso Lung SBRT for Multiple Lesions: Rotation Error effects on Plan Clinical Acceptability

Purpose or Objective: Multi-target, single-isocenter SBRT treatments are not as robust against rotational errors because at least one target is necessarily offset from the point of rotation. The amount of target displacement increases in proportion to its distance from the point of rotation, which results in a greater dosimetric effect. We investigated the target rotational errors (i.e., pitch, roll, and yaw) and evaluated the dosimetric impact of simultaneous treatment of multiple lung lesions with a single isocenter SBRT technique.

Material and Methods: We created a clinical reference plan (CRP), which has optimised dose coverage, conformity, homogeneity, and step-wise fall-off to zero dose. We then applied orthogonal-to-spherical coordinate system transformations about the isocenter, to each individual beam’s parameters that simulated the multiple rotational errors for this research. For each patient (n=10) we generated 24 experimental treatment plan (ETP). For each ETP, DVHs were computed for the PTVs, Whole lung, Esophagus, Heart, Spinal cord, Bronchial tree, Great vessels. From all of the DVH data, critical analytical indices were computed and compared to the CRP. The indices include: Coverage, conformity, Dose Homogeneity, and Dose Gradient. The clinical suitability of each plan was evaluated. The accumulated indices were scored and weighted to obtain a single value, the Unified Dosimetry Index (UDI). The UDI represents a plan’s suitability vis-avis the CRP.

Results: From the computed UDIs we observed that ETPs with 1 - 2 degrees of rotation remain within the defined range of suit-ability for clinical treatment. Rotation errors greater than 2 degrees resulted in UDI outside the clinically acceptable range.

Conclusion: Isocenter rotations of 3 degrees or more significantly reduces the quality of clinical SBRT treatments. Small lesions are especially vulnerable to loss of dosimetric coverage. SBRT treatment setup should have a combined rotational error less than 2.5 degrees to effectively deliver the prescribed radiation dose using multiple lung lesions with mono-isocentric technique.