Verification of the junctional dose for irradiation of the chest wall and supraclavicular regions under the circumstances of advanced technologies

A single-isocenter half-beam technique is commonly used when irradiating the chest wall and supraclavicular regions in patients with high-risk breast cancer. However, several studies have reported that underdosage can occur at the junction of the chest wall and supraclavicular regions due to a "tongue-and-groove" effect. This study verified the efficacy of an open leaf technique (OL-tech) that involves placing a multileaf collimator 5 mm outside from the beam central axis to remove the effect of the multileaf collimator in a single-isocenter half-beam technique. We compared the junction doses of the OL-tech with those of a conventional technique (C-tech) in square and clinical plans, using 4 and 10 MV x-rays in the Clinac iX and 6 and 10 MV x-rays in the Trilogy accelerators (Varian Medical Systems, Palo Alto, CA). EBT3 radiochromic films were used for measurements. Measurements were performed at a depth of 3 cm when verifying field matching. The EBT3 films in the square plan indicated junction doses for the C-tech of 78.3% with the Clinac iX accelerator and 73.6% with the Trilogy accelerator. By contrast, the corresponding doses for the OL-tech were 107.2% and 99.8%, respectively. In the clinical plan, the junction doses for the C-tech were 76.5% with the Clinac iX accelerator and 72.6% with the Trilogy accelerator; the corresponding doses for the OL-tech were 108.3% and 101.7%, respectively. As with the square plan, variations in the junction dose were much smaller using the OL-tech than using the C-tech. Our results suggest that the OL-tech can be useful for improving dose homogeneity at the junction of the chest wall and supraclavicular regions.