2196 - A Dosimetric Comparison between BgRT-SABR Plans and C-Arm Linac-Based IGRT-SABR Plans for Lung and Bone Lesions

02:30pm - 04:00pm PT

Hall F

Screen: 24

POSTER

Presenter(s)

Daniel Pham, PhD, CMD - Stanford Health Care, Stanford, CA

D. Pham¹, L. Tran², L. Vitzthum³, B. Loo¹, A. L. Chin³, B. Han¹, M. Surucu¹, and N. Kovalchuk¹; ¹Department of Radiation Oncology, Stanford University, Stanford, CA, ²MD Anderson, Houston, TX, ³Stanford University School of Medicine, Stanford, CA

Purpose/Objective(s):

Biology-guided Radiotherapy (BgRT) uses PET emissions from the patient to guide the radiotherapy treatment to tumors. BgRT received FDA clearance for lung and bone cancer patients in February 2023, and our institution commissioned the first clinical installation of BgRT linac, RefleXion X1, for SCINTIX BgRT treatments. This study aims to compare dosimetric plan quality between X1 BgRT-SABR and corresponding C-arm linac-based IGRT-SABR plans.

Materials/Methods:

At our institution, a total of 16 patients underwent a functional modelling (PET-simulation) session on RefleXion X1 required for BgRT planning, 10 of which were for an emulated-treatment study, and 6 with intent for clinical treatment. Ten patients were treated to lung cancer with prescriptions ranging from 50 to 54 Gy in 3 to 5 fractions. BgRT PTV volumes were expanded 5 mm from the GTV. Six patients were treated for bone lesions with doses ranging from 18 to 30 Gy in 1 to 3 fractions. BgRT plans (BgRT-SABR) were generated using 6 MV FFF beam and dose calculated using a collapsed cone algorithm, with a dose grid of 2.1 mm.

The corresponding C-arm linac-based (IGRT-SABR) plans were generated using a single partial arc, 10MV FFF, with a dose rate of 2400 MU/min. For lung tumors, the PTV was generated from an ITV or a 3mm expansion from GTV (breath hold). Acuros was used to calculate dose using a calculation grid of 1.5 mm or less.

PTV volume, conformity index (CI100), intermediate dose (CI50), the minimum dose to the target, and the dose to organs-at-risk (OAR) were compared between the BgRT-SABR and IGRT-SABR plans using paired t-tests with p<0.05 considered statistically significant.

Results:

The mean PTV volume for BgRT-SABR volumes was significantly smaller than IGRT-SABR volumes at 24.5 cc vs 36.2 cc (p = 0.01), respectively. On average, BgRT-SABR plans exhibited a higher CI_100 (1.23) compared to IGRT-SABR (0.98) (p<0.01), and a higher CI_50 (9.74 vs. 4.22, p<0.01). When comparing the 100% isodose volume size in lung tumors, BgRT-SABR plans averaged 21.5 cc vs 25.3 cc for IGRT-SABR plans (p < 0.01). In bone lesions, BgRT-SABR plans averaged 58.1 cc vs 45.9 cc in IGRT-SABR plans (p = 0.14).

There was no significant difference in the minimum dose to the GTV (103.9% for IGRT-SABR vs. 105.6% for BgRT-SABR, p=0.11). Heterogeneity indices were 1.31 for BgRT and 1.27 for IGRT (p=0.25). For BgRT-SABR, mean lung (minus GTV) dose averaged 3.1 Gy and lung V_20 was 3.9%, which did not differ significantly from IGRT-SABR (2.3 Gy, p=0.08 and 2.9%, p=0.06, respectively).

Conclusion:

BgRT-SABR and IGRT-SABR plans provided comparable target coverage and OAR sparing. While BgRT-SABR plans showed higher intermediate dose spillage, the absolute volume of the prescription dose was significantly lower in lung lesions compared to IGRT-SABR plans. Doses to critical organs remained within institutional guidelines. Overall, BgRT appears to be feasible for treating lung and bone tumors.