2083 - Effects of Biology Tracking Zone Size on Biology-Guided Radiotherapy with and without Setup Margins for PET-Avid Bone Lesions
Presenter(s)
C. Han1, G. Gibbard2, Y. J. Chen1, W. T. Watkins1, T. M. Williams1, and A. Liu1; 1Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 2RefleXion Medical, Inc., Hayward, CA
Purpose/Objective(s): On a biology-guided radiotherapy (BgRT) machine with real-time positron emission tomography (PET) guidance, emissions from lesions with high F-18 fluorodeoxy glucose (FDG) metabolic activity are detected within a biology tracking zone (BTZ) to improve delivery accuracy. This treatment planning study evaluates effects of BTZ size on treatment time, dosimetric plan quality, and BgRT feasibility whether setup margins are used or not for FDG-avid bone lesions.
Materials/Methods: Based on a review of all patients treated on the BgRT machine at our institution, five patients with at least 1 cm distance from an FDG-avid bone lesion to the skin were selected. The prescribed dose ranged from 10 to 27 Gy in one to three fractions. A gross target volume (GTV) was drawn for the FDG-avid volume (average GTV volume: 6.6±3.8 cm3) based on PET imaging. Five BgRT plans were first created for each patient in which the PTV was obtained by adding a 5-mm setup margin to the GTV, and the BTZ was obtained by adding a margin to the GTV ranging from 10 mm to 6 mm in 1-mm steps. Ten additional BgRT treatment plans were created with no setup margins and the GTV-to-BTZ margin ranged from 10 mm to 1 mm in 1-mm steps. All 15 BgRT plans for each case used identical plan optimization settings. BgRT plans were evaluated for the conformity index (CI), dose drop-off based on CI80 and CI50 (ratio of the 80% and 50% isodose volume to the PTV volume, respectively), and homogeneity index (HI). The activity concentration (AC) and normalized target signal (NTS) were calculated using FDG concentration within the PTV and surrounding the BTZ. P-values from paired t-tests were used to evaluate statistical significance.
Results: With a 5-mm GTV-to-PTV setup margin, the BgRT plans had an average beam-on time of 1,997.8±674.9 sec, CI of 1.23±0.10, CI80 of 2.68±0.49, CI50 of 7.56±1.21, HI of 1.35±0.08, AC of 17.88±13.93 kBq/ml, and NTS of 10.96±4.60 with a 10-mm GTV-to-BTZ margin; with a 6-mm GTV-to-BTZ margin, the plans had an average beam-on time of 1,672.0±582.3 sec, CI of 1.20±0.08, CI80 of 2.49±0.37, CI50 of 6.76±0.89, HI of 1.35±0.09, AC of 15.78±11.51 kBq/ml, and NTS of 7.62±1.01 (p-values < 0.05 for beam-on time, CI80, and CI50). For plans without a setup margin, beam-on time, AC, and NTS all decreased monotonically with decreasing GTV-to-BTZ margins. AC and/or NTS fell below BgRT feasibility thresholds in three cases when the GTV-to-BTZ margin was at 3 or 1 mm. At a 4-mm GTV-to-BTZ margin, CI, CI80, and CI50 were all lower compared to those at a 10-mm GTV-BTZ margin despite no statistical significance; the beam-on time decreased from 1,898.6±703.9 to 1,501.6±675.6 sec.
Conclusion: A smaller BTZ for bone lesions can reduce beam-on time and could improve target conformity and dose drop-off outside the PTV with or without a setup margin. However, a small BTZ may negatively impact BgRT feasibility if the BTZ is too close to the FDG-avid GTV. This study suggests that zero setup margins in BgRT planning for bone lesions may be feasible if an adequate GTV-to-BTZ margin is used.