Analysis of dosimetric verification results of stereotactic body radiotherapy

Objective To analyze the patient-specific dosimetric verification result of stereotactic body radiotherapy (SBRT) plans, and to investigate the sensitivity of the result to three factors: interpolation of measured data, size of dose calculation grid and assessment threshold. Methods The dosimetric verification results of SBRT plans of 50 patients were retrospectively analyzed to evaluate the impact of the following factors. The linear interpolation (1.00 mm) and non-interpolation (7.62 mm) were applied to measured data respectively. Three dose calculation grid sizes of Eclipse planning system, i. e., 1.0 mm, 2.5 mm and 4.0 mm were compared respectively. The threshold of dose assessment was selected as 10%, 20% and 30%, respectively. Three criteria of γ analysis were selected: 2%/2 mm, 3%/2 mm and 3%/3 mm. Results Under criteria of 2%/2 mm, 3%/2 mm and 3%/3 mm, the average γ passing rates were (86.3±7.3)% and (93.7±5.6)%, (94.1±4.4)% and(97.7±3.9)%, (97.7 ±2.2)% and (99.1±1.7)%, respectively, with and without linear interpolation. Relative to the 1.0 mm reference grid, the grids of 2.5 mm and 4.0 mm significantly decreased γ passing rates by 3.8%, 1.9%, 0.8% (t=8.41, 9.06, 5.30, P<0.05) and by 6.5%, 6.0%, 3.5% (t=-13.76, -13.15, -9.80, P<0.05) under criteria of 2%/2 mm, 3%/2 mm and 3%/3 mm, respectively. Relative to the 10% reference threshold, the shresholds of 20% and 30% significantly decreased γ passing rates by 2.4%, 1.0%, 0.6%(t=-8.60, -5.86, -4.68, P<0.05) and by 4.0%, 1.7%, 0.9% (t=-9.45, -6.66, -5.06, P<0.05) under criteria of 2%/2 mm, 3%/2 mm and 3%/3 mm, respectively. Conclusions Interpolation, dose calculation grid size and dose assessment threshold are influential factors of dose verification result, and need to be considered during dosimetric verification of stereotactic radiotherapy patients. Key words: Stereotactic body radiotherapy; Dosimetric verification; Interpolation; Dose calculation grid size; Dose assessment threshold