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.
Highlights
The classic source to skin distance (SSD) technique for breast cancer treatment combines from 2 to 7 irradiation fields
This lower monitor units (MU) number is due in part to the fact that the single isocentric technique (SIT) needs less numerous beams and in part to the collimator opening which is different for SIT
Isodose curves In the transversal plans, we observed that both 95% and 100 % isodose curves enclose the same fraction of planning target volumes (PTV) using the two techniques
Summary
The classic source to skin distance (SSD) technique for breast cancer treatment combines from 2 to 7 irradiation fields. An operator needs to enter the treatment room and change the patient position for the treatment field. This technique is rather time consuming, and introduces potential positioning errors that may lead to over or under-irradiation.[1, 2] This technique looks rather old fashion at the present time where up to date, linear accelerators allow complex volume irradiations from a single isocenter as for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Breast cancer is a frequent condition, it looks inappropriate to design randomized comparative trials having clinical issues as end point, to solve this question. A comparative analysis based on dosimetric comparative tools originated from quality assurance methods could help to solve this problem and provide to the radiation oncologists’ clues solid enough to switch for SIT with or without adaptations of their prescriptions references
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