A quantitative method to implement and to assess the single isocenter technique for breast cancer radiation therapy

Abstract

Purpose: We propose a process of quality assurance to validate and implement the single isocenter technique for breast cancer radiotherapy. We evaluated the dosimetric and temporal gains using the single isocenter technique compared to classic source to skin distance (SSD) technique.

Methods: 6 patients of breast cancer localization were studied. For each patient 2 treatment plans were generated. In plan 1 the dose was calculated using SSD technique. In plan 2 the dose was calculated using single isocenter technique. To implement the plan 2 a dosimetric analysis including monitor units (MU), isodose curves, cumulative and differential dose volume histograms cDVH, dDVH respectively, coverage index, conformity index for planning target volume were used. The measurements using a PMMA phantom consist of measuring point dose by an ionization chamber and 2D dose distributions using 2D diodes arrays. Wilcoxon signed rank and Spearman’s tests were used to calculate p-value and correlation coefficient, respectively.

Results: The single isocenter technique reduced the MU by average on -30.1 ± 13.6%, (p = 0.03). We observed an improvement with statistical significance between the two techniques for the mean dose, minimum dose and volume receiving 95% of the prescribed dose without over-dosage. The analysis for dDVH showed that the dose distribution in the target volume calculated in the single isocenter technique is more homogeneous than the SSD technique. Wilcoxon test showed that the two treatment plans had the same quality (p > 0.05). The difference between calculated and measured dose was within 2.4 ± 3.3% for absolute point dose and the percentage of points passing gamma criteria was on average 99.8 ± 0.2%.

Conclusion: This method provides a quantitative evaluation and comparison of the two irradiation techniques for breast cancer and the consequences of the technical change on dose calculation.