Investigating the impact of treatment delivery uncertainties on treatment effectiveness for lung SABR

Abstract

To quantify the impact of treatment delivery uncertainties on lung stereotactic ablative body radiotherapy (SABR) plans for step-and-shoot intensity-modulated radiotherapy (ssIMRT) and volumetric modulated arc therapy (VMAT). Baseline ssIMRT and VMAT treatment plans were generated for a cohort of 18 lung SABR patients. Modified plans were generated for each baseline plan by systematically varying gantry and collimator angles between − 5 and + 5 degrees, as well as multi-leaf collimator (MLC) leaf position errors of magnitude between 1 and 5 mm in both directions (i.e. leaf banks shifted either in the same (Type 1) or opposite (Type 2) directions). Planning target volume (PTV), spinal cord and healthy lung dose-volume histogram (DVH) metrics were compared between the modified and baseline plans. Collimator and gantry angle uncertainties did not significantly impact any of the PTV DVH metrics considered. MLC shifts of 5 mm resulted in average V_95% changes of \(-\,10\%\) (Type 1) and \(-\,33\%\) (Type 2) and average \(\text{V}_{100\%}\) changes of \(-\,17\%\) (Type 1) and \(-\,44\%\) (Type 2) for ssIMRT and VMAT plans. Comparatively, MLC shifts of − 2 mm resulted in average \(\text{V}_{95\%}\) changes of \(-\,1\%\) (Type 1) and \(-\,3.5\%\) (Type 2) and average \(\text{V}_{100\%}\) changes of \(-\,3\%\) (Type 1) and \(-\,12\%\) (Type 2) for ssIMRT and VMAT plans. ssIMRT gantry angle uncertainties impacted spinal cord DVH metrics the most, with increases in \(\text{D} 0.1 \text{cc}\) of \(9\%\) occurring for a 1 degree shift. Type 2 MLC modifications impacted all OAR DVH metrics substantially with differences in spinal cord \(\text{D}_{\text{max}}\) (ssIMRT) and healthy lung \(\text{V} 20 \text{Gy}\) (VMAT) exceeding \(40\%\) for 5 mm shifts. Uncertainties in MLC leaf positions affected target and OAR DVH metrics more than collimator or gantry angle uncertainties for lung SABR plans. Less patient-to-patient variation occurred from delivery uncertainties in VMAT than ssIMRT.

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