Assessment of dose uniformity and optimal CT number for virtual bolus in breast VMAT planning
Authors:   Medical Physics Program, Department of Radiological Technology, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand., Chaisuttee Rusdchai , Kaewlek Titipong ,   Medical Physics Program, Department of Radiological Technology, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand., Department of Radiological Technology, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand.,Interdisciplinary Health and Data Sciences Research Unit, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand., Rattanarungruangchai Natch ,   Thailand Institute of Nuclear Technology, Bangkok, Thailand., Munsing Siwapon ,   Department of Radiology, Faculty of Medicine, Naresuan University, Phitsanulok Province, Thailand., Kumcharoenkun Orawan ,   Department of Radiology, Faculty of Medicine, Naresuan University, Phitsanulok Province, Thailand., Yabsantia Sumalee ,   Medical Physics Program, Department of Radiological Technology, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand., Department of Radiological Technology, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand.,Interdisciplinary Health and Data Sciences Research Unit, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok Province, Thailand.
Published: October 2024

Background: ICRU Report No. 83 proposes using the Flash Region in the strategic design of breast cancer treatment. However, concerns persist regarding the delivery of the designated radiation dose to breast cancer patients undergoing Volumetric Modulated Arc Therapy (VMAT) with virtual bolus in the irradiation plan. Objective: This study aimed to assess dose uniformity in breast VMAT treatment with a virtual bolus, validate the planning dose by comparing it with nanoDotTM measurements on a Rando phantom, and determine the optimal CT number for the virtual bolus in breast VMAT planning. Materials and Materials and methods: To assess dose uniformity in the breast VMAT plan, nine nanoDot™ dosimeters were placed on the breast of Rando phantom, followed by CT simulation and VMAT treatment planning. The clinical target volume (CTV) and organs at risk were contoured, and the planning target volume (PTV) boundaries were expanded by 5 mm and 10 mm for virtual bolus thicknesses of 10 mm and 15 mm, respectively. The CT number of the virtual bolus varied from 0 to -700 HU. The planning doses at 9 points were determined, and the coefficient of variation (%CV) was calculated. Additionally, measurements at these 9 points were performed using nanoDot™ dosimeters. The calculated and measured doses were then compared. Finally, VMAT treatment plans with a virtual bolus were implemented in 10 breast cancer patients, using the virtual bolus with varying CT numbers as in the phantom study to evaluate the optimal CT number of the bolus. Results: The doses among the 9 points for each plan were uniform, with a %CV of less than 4. For calculated dose validation, the percentage differences between the measured and calculated dose for all treatment plans, with variations in the CT number and the bolus thickness, were within ±5%. To determine the optimal CT number for the virtual bolus, the breast cancer treatment plan that met the dose criteria for tumors and organs at risk was the plan with a CT number of 0 HU for both virtual bolus thicknesses of 10 and 15 mm. Conclusion: Virtual bolus provides uniform dose distribution for breast VMAT planning, which measurements from nanoDotTM can validate. The appropriate CT number for the virtual bolus is 0 HU for both bolus thicknesses. In future studies, measurements should be conducted on actual patients.