Single-isocenter hybrid IMRT plans versus two-isocenter conventional plans and impact of intrafraction motion for the treatment of breast cancer with supraclavicular lymph nodes involvement

The purpose of this study was to compare the single-isocenter, four-field hybrid IMRT with the two-isocenter techniques to treat the whole breast and supraclavicular fields and to investigate the intrafraction motions in both techniques in the superior direction. Fifteen breast cancer patients who underwent lumpectomy and adjuvant radiation to the whole breast and supraclavicular (SCV) fossa at our institution were selected for this study. Two planning techniques were compared for the treatment of the breast and SCV lymph nodes. The patients were divided into three subgroups according to the whole breast volume. For the two-isocenter technique, conventional wedged or field-within-a-field tangents (FIF) were used to match with the same anterior field for the SCV region. For the single-isocenter technique, four-field hybrid IMRT was used for the tangent fields matched with a half blocked anterior field for the SCV region. To simulate the intrafraction uncertainties in the longitudinal direction for both techniques, the treatment isocenters were shifted by 1 mm and 2 mm in the superior direction. The average breast clinical tumor volume (CTV) receiving 100% (V<inf>100%</inf>) of the prescription dose (50 Gy) was 99.3% ± 0.5% and 96.4% ± 1.2% for the for two-isocenter and single-isocenter plans (p < 0.05), respectively. The breast CTV receiving 95% of the prescription dose (V<inf>95%</inf>) was close to 100% in both techniques. The average breast CTV receiving 105% (V<inf>105%</inf>) of the prescription dose was 32.4% ± 19.3% and 23.8% ± 13.3% (p = 0.08). The percentage volume of the breast CTV receiving 110% of the dose was 0.4% ± 1.2% in the two-isocentric technique vs. 0.1% ± 0.2% in the single-isocentric technique. The average uniformity index was 0.91 ± 0.02 vs. 0.91 ± 0.01 in both techniques (p = 0.04), but had no clinical impact. The percentage volume of the contralateral breast receiving a dose of 1 Gy was less than 2.3% in small breast patients and insignificant for medium and large breast sizes. The percentage of the total lung volume receiving > 20 Gy (V<inf>20Gy</inf>) and the heart receiving > 30 Gy (V<inf>30Gy</inf>) were 13.6% vs. 14.3% (p = 0.03) and 1.25% vs. 1.2% (p = 0.62), respectively. Shifting the treatment isocenter by 1 mm and 2 mm superiorly showed that the average maximum dose to 1 cc of the breast volume was 55.5 ± 1.8 Gy and 58.6 ± 4.3 Gy in the two-isocentric technique vs. 56.4 ± 2.1 Gy and 59.1 ± 5.1 Gy in the single-isocentric technique (p = 0.46, 0.87), respectively. The single-isocenter technique using four-field hybrid IMRT approach resulted in comparable plan quality as the two-isocentric technique. The single-isocenter technique is more sensitive to intra-fraction motion in the superior direction compared to the two-isocentric technique. The advantages of the single-isocenter include elimination of isocentric errors due to couch and collimator rotations and reduction in treatment time. This study supports consideration of a single-isocenter four-field hybrid IMRT technique for patients undergoing breast and supraclavicular nodal irradiation.