Investigation of the skin dose using optically stimulated luminescent dosimeters (OSLDs) when utilizing virtual bolus in helical Tomotherapy treatment planning for post-mastectomy radiation therapy

Abstract

The objective of the study is to investigate the skin dose when using VB method for PMRT in Helical TomoTherapy treatment planning. The skin dose and dose distribution of the actual delivery plans were measured using OSLDs in the ArcCheck phantom. The target was localized on the ArcCHECK image by 3 mm margin from the phantom surface. The dimension of target, which includes the ArcCHECK’s detectors, with the 4.0 cm width and length 12.0 cm along the phantom. The five treatment plans were generated, one plan without VB application (NoVB) and the four plans with varying of VB thickness on the phantom surface by 0.5 cm (VB0.5), 1.0 cm (VB1.0), 1.5 cm (VB1.5), and 2.0 cm (VB2.0), in treatment planning but absent during irradiation. For measurement analysis, the OSLDs were used for skin dose measurement for the dose difference with the plan and the ionization chamber and the ArcCHECK detectors were used for point dose and dose distribution by investigating the percentage of dose difference and the gamma passing rate. The VB thickness 0.5, 1.0 and 1.5 cm showed acceptable value with 82.08% (VB0.5), 81.65%(VB1.0) and 85.11%(VB1.5) of the prescribed dose and less than 2% for dose difference by 0.37% (VB0.5), -0.11% (VB1.0) and -0.37% (VB1.5) at the center of ArcCHECK. The accuracy of dose distribution showed an acceptable gamma passing rate of 99.8% (VB0.5), 100% (VB1.0), and 90.2% (VB1.5) for gamma criteria by 3%/3mm for absolute dose analysis. However, the gamma passing rate of VB2.0 down to 71.2% of absolute mode for gamma criteria by 3%/3mm. The treatment plans with VB thickness less than 15 mm deliver doses that are comparable to treatment plans without virtual bolus based on gamma analysis. However, the deviation showed a trend increasing when VB thickness increased. The VB2.0 was not acceptable for point dose and dose distribution verification by more than 2% dose difference and less than 90% of gamma passing rate.