Extended distance non-isocentric treatment in stereotactic body radiation therapy (SBRT) for lung cancer

Abstract

Purpose: To obtain the maximum differential non-coplanar beams angle for a faster dose dropping outside Plan Target Volume (PTV) for lung cancer treated by Stereotactic body radiation therapy (SBRT), an extended distance non-isocentric (EDNI) treatment method was explored and developed. Methods: The EDNI requires delivering of the treatment beam at 120 cm or farther for sauce axial distance (SAD) instead of standard 100 cm. This change provides a more compact dose distribution around PTV and the lower toxicity to organs at risk (OAR) due to benefit of 120 cm SAD and more choice of beam and couch angle. A hand calculation formula for the translation between 100 SAD and EDNI was used to verify the treatment plan results. A phantom for end to end study based on this EDNI technique was used to compare with standard 100 SAD deliveries for SBRT. Three patients who underwent SBRT treatment were randomly chosen to demonstrate the benefits of EDNI technique. These treatment re-plans were applied to EDNI and evaluated for conformal index (CI) of PTV, R50% of PTV, 2 cm distance (D2cm) of PTV and Maximum dose (Dmax) of OARs to compare with original clinical plans. Results: All of the cases delivered by the EDNI technique satisfied dose requirements of RTOG 0263 and showed a faster dose dropping outside of PTV than standard SAD deliveries. The distance from PTV after 1.5 cm for the EDNI technique had a smaller maximum dose and much lower standard deviation for dose distribution. The EDNI applied plans for patients showed less R50% and D2cm of PTV (P≤ 0.05), also similar results for Dmax of esophagus, trachea and spinal cord. Conclusion: The EDNI method enhances the capabilities of linear accelerators as far as the increased gradient of dose drop-off outside of PTV is concerned. More angular separation between beams leads to more compact dose distributions, which allow decreasing volume of high dose exposure in SBRT treatments and better dose distribution on sensitive organs to minimize the treatment toxicity.