Evaluation of Repeated Lung Stereotactic Body Radiotherapy Using a Radiotherapy Dose Accumulation Routine

Abstract

<h3>Purpose/Objective(s)</h3> Repeated lung SBRT has been used for recurrent or metachronous non-small-cell lung cancer. Comprehensive evaluation of cumulative dose is essential for safe reirradiation to minimize the risk of pneumonitis and esophageal toxicity. We evaluated an in-house script named RADAR (Radiotherapy Dose Accumulation Routine) designed to provide accurate dose assessment that accounts for registration uncertainties, and improve the efficiency of clinical workflow. <h3>Materials/Methods</h3> In this pilot study, we analyzed a cohort of 20 patients who underwent 2-5 courses of repeated lung SBRT. Lung volume changed 8±8% between courses. Planning CT images from earlier treatment courses were registered onto the final course in two steps: first, a rigid registration based on alignment of spine and rib cage, then a deformable image registration (DIR) using a ‘demons' algorithm in a planning system. The resulting deformable vector fields were used by RADAR to transform voxels for automated dose accumulation. User can choose to output either physical dose or equivalent dose in 2Gy fractions (EQD2, α/β=3) with a discount factor (Paradis, ARO 2019). RADAR provides three approaches for dose accumulation: 1) direct sum that outputs voxel level dose summation; 2) unrestricted search that incorporates DIR uncertainty by conservatively assigning the maximum dose within a search radius (3mm) to the voxel of interest for dose accumulation; 3) restricted search, which performs the same dose accumulation as in 2) but imposes structure label matching in the search radius, therefore including only voxels within the same structure. Accumulated dose distribution was written back to the planning system for evaluation. We extracted mean lung dose (MLD) and esophagus D5cc (EsoD5cc) with respect to various accumulation approaches for comparison. <h3>Results</h3> MLD and EsoD5cc were significantly higher when accounting for DIR uncertainties (<i>p</i><0.001 two-tailed t-test). MLD has less uncertainty than EsoD5cc due to its averaging nature. For radiotherapy with >2 courses, RADAR considerably shortened the processing time by auto-accumulating dose from multiple courses simultaneously, as opposed to the cumbersome manual approach of creating plan sums iteratively. Typical time for dose accumulation and evaluation using RADAR is < 10 mins. <h3>Conclusion</h3> RADAR is a convenient and efficient tool to calculate and evaluate cumulative dose for lung reirradiation cases. It identifies higher OAR doses due to dose accumulation uncertainties, for example coming from the registration. Our study provides initial data to establish clinical guidance for design and optimization of repeated lung SBRT.