A comparative dosimetric evaluation of two image-guided calibration strategies for multi-isocenter total body irradiation: integrated fan-beam CT–based alignment versus sequential cone-beam CT–guided localization

Bo Gao¹Yaling Hong²Junjin Yan¹Xiwei Chen³Suhua Tan²Yutao Zhu²Yan Liu²Zhaojie Yao¹Lizhen Wu¹Haiyan Chen²Zhanquan Lei^1*

• ¹Department of Radiation Oncology, Fujian Children's Hospital（Fujian Branch of Shanghai Children's Medical Center）, Fuzhou, China
• ²Shanghai Children's Medical Center Affiliated to Shanghai Jiaotong University School of Medicine, Shanghai, China
• ³Hunan Provincial Hospital of Integrated Traditional Chinese and Western Medicine, changsha, China

Background: Image-guided radiotherapy (IGRT) is key to making sure the radiation dose is accurate when treating kids with multi-isocenter total body irradiation (TBI). Ensuring dose accuracy in pediatric multi-isocenter TBI. Shanghai Children’s Medical Center employed low-dose fan-beam CT (FBCT) for integrated multi-isocenter calibration, whereas Fujian Children’s Hospital used cone-beam CT (CBCT) for sequential isocenter alignment. Objective: To evaluate FBCT versus CBCT guidance effects on target coverage and organ doses across two regimens: 12 Gy in 6 fractions and 3 Gy in 1 fraction. Methods: Retrospective analysis of 34 pediatric TBI patients (21 FBCT, 13 CBCT) treated with identical field setups (three upper-body isocenters; two/three lower-body isocenters; junction at upper femur third). Pre-treatment FBCT/CBCT images were registered to planning CTs; doses recalculated using original plans. Metrics: PTV V90%, V100%, V110%, mean doses; homogeneity index (HI); conformity index (CI); mean lung and kidney doses. Results: In the 12 Gy group, FBCT guidance improved PTV coverage: V90% increased from 96.11% to 97.14%, V100% from 90.40% to 92.81%, and V110% decreased from 20.73% to 16.67% (all P < 0.01). HI decreased from 0.25 to 0.16, CI increased from 0.77 to 0.89, and mean PTV dose rose from 12.33 to 12.57 Gy (all P < 0.01). Mean lung dose fell from 8.61 to 8.47 Gy, and mean kidney dose from 8.24 to 8.10 Gy (both P < 0.01). In the 3 Gy group, FBCT guidance also improved PTV coverage: V90% increased from 96.32% to 97.82%, V100% from 91.44% to 93.97%, and V110% decreased from 17.43% to 13.72% (all P < 0.01). HI decreased from 0.21 to 0.13, CI increased from 0.77 to 0.87, and mean PTV dose rose from 3.08 to 3.12 Gy (all P < 0.01). Mean lung dose decreased from 2.34 to 2.25 Gy, and mean kidney dose from 2.09 to 2.06 Gy (both P < 0.01). Conclusion: FBCT guidance gave better target dose conformity and homogeneity, and lower lung doses, than CBCT guidance—both for the 12 Gy myeloablative regimen and the 3 Gy low-dose regimen. These results suggest that FBCT guidance is a better option for image-guided total body irradiation in children.