Investigating strategies to reduce toxicity in stereotactic ablative radiotherapy for central lung tumors

Backgound. Stereotactic radiotherapy for central lung tumors has a narrower therapeutic index than that for peripheral tumors. Tumor tracking strategies have been proposed to reduce treatment volumes and toxicity, however they need to consider uncertainties in tumor size and shape change throughout respiration to ensure optimal local control. We quantified these uncertainties and explored strategies to account for them. Material and methods. Ten patients with central tumors, PTV > 100 cm³, motion > 5 mm and a 10-phase 4DCT without significant artifact in the tumor region were evaluated. Uncertainties were quantified using GTV size in different phases, and the Hausdorff distance (HD) between the phase 50% GTV and other phases after soft-tissue rigid registration. An individualized internal target volume for tracking (ITVT) was generated from the union of the GTVs in all phases after rigid registration. This was compared to ITVs generated for tracking based on the phase 50% GTV alone or with isotropic margins of 3 or 5 mm for size and volume overlap. Results. Median free-breathing PTV size and motion were 162.1 cm³ (110-210) and 8.9 mm (6.1-14.1). Overall, median GTV size variation and HD were 4.7% (0.2-22.3) and 6.3 mm (3.9-17.6). Tracking using GTV 50% alone resulted in median volume overlap with ITVT of 71.7% (range 56.8-85.1). Isotropic margins of 3 or 5mm always resulted in a volume overlap less than 95% or a volume larger than the ITVT. Conclusions. Changes in size and shape of central lung tumors are substantial during respiration. These limit the ability to reduce treatment volumes with tracking, especially if isotropic margins are used. An individualized ITV for tracking, such as the ITVT is preferred.