Purpose: Investigation of the effects of breathing motion- and misregistration-induced errors on the superficial dose in the treatment of breast cancer using helical tomotherapy (HT).
Material and methods: Surface dose measurements were performed with thermoluminescence dosimetry (TLD). Two treatment plans with different planning target volume (PTV) definitions of the left breast were used: PTVskin had its ventral border exactly on skin level, while PTVair included also a 10-mm extension ventral to the PTVskin. With a thoracic static phantom, misregistration errors in an HT were simulated. A dynamic phantom was used to simulate a breathing patient during HT. Surface doses of breast cancer patients were measured both for an HT (179 points) and a conventional three-dimensional conformal treatment (70 points).
Results: In the static phantom misregistration setup, dose deviations of -31.9% for PTVskin to +35.4% for PTVair could be observed. The dynamic phantom measurements resulted in surface dose deviations from those in a static position between 0.8% and 3.8% without a significant difference for the PTV definitions. The measured surface doses on patients averaged (mean +/- standard deviation) 1.65 +/- 0.13 Gy for the HT and 1.42 +/- 0.11 Gy for the three-dimensional conformal treatment.
Conclusion: HT enables a homogeneous and reproducible surface dose with small dose deviations in the treatment of breast cancer. HT is a feasible method to treat breast cancer under free shallow breathing of the patient using a treatment plan with a ventral PTV border on the skin level.