C Liu¹*, T Ma², T Gray³, S Ahmed⁴, N Yu⁵, K Stephans⁶, G Videtic⁷, P Xia⁸, (1) Cleveland Clinic, Cleveland, OH, (2) Medstar Georgetown University Hospital, Midlothian, VA, (3) Cleveland Clinic, Cleveland, OH, (4) Cleveland Clinic, Cleveland, OH, (5) Cleveland Clinic, Cleveland, OH, (6) Cleveland Clinic, Cleveland, OH, (7) Cleveland Clinic, Cleveland, OH, (8) Cleveland Clinic, Cleveland, OH

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  C Liu

PO-GePV-T-373 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To investigate the impact of tumor displacement (TD) on target coverage in photon and proton stereotactic body radiation therapy (SBRT) treatments for lung cancer patients under daily IGRT.

Methods: For patients treated with photon SBRT for lung cancer in 2017-2019 in our institution, 16 fractions from 16 patients with daily iso-center shifts >1.5 cm in any directions were identified according to kV-cone-beam CT (kV-CBCT) guidance. TDs of these fractions were identified by the difference between the iso-center shifts of bony alignment and tumor alignment. Eight fractions from eight patients had TDs in any directions >3 mm. For each of these eight patients, one photon plan with VMAT technique and one proton plan with IMPT were generated retrospectively. Photon and proton plans were optimized to achieve >95% of the PTV and >98% of the ITV receiving the prescription dose, respectively. For dose recalculation with TD, we shifted the tumor contours according to the corresponding TD on the patient specific planning CT. HU override was applied to shifted ITV (0.7 g/cm³) and original ITV (0.26 g/cm³). We evaluated dosimetric impact of TD with the percentage volume of GTV and ITV receiving the prescription dose (V100%).

Results: The range of TDs was 3.58–12.18 mm, where the translational shifts was 0.24–4.45 mm in ML direction, 0.11–11.69 mm in AP direction and 0.9–10.57 mm in SI direction, respectively. For photon SBRT, all fractions had V100% of the GTV ≥99% and V100% of the ITV ≥98%. For proton SBRT, V100% of the GTV and ITV had a range of 12.60%–96.21% and 34.17%–83.75%, respectively.

Conclusion: Adequate target coverage was achieved in photon SBRT plans. TD had greater impact in proton SBRT plans, leading to target underdosage and adaptive planning was needed to provide adequate tumor dose coverage.

Funding Support, Disclosures, and Conflict of Interest: C Liu is supported by AVO and P Xia receives a research grant from AVO.

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