he aim of this study is to investigate the effect of low magnetic field on dose distribution in the partial-breast irradiation (PBI). Eleven patients with an invasive early-stage breast carcinoma were treated prospectively with PBI using 38.5 Gy delivered in 10 fractions using the ViewRay^Ⓡ system. For each of the treatment plans, dose distribution was calculated with magnetic field and without magnetic field, and the difference between dose and volume for each organ were evaluated. For planning target volume (PTV), the analysis included the point minimum (D_min), maximum, mean dose (D_mean) and volume receiving at least 90% (V_90%), 95% (V_95%) and 107% (V_107%) of the prescribed dose, respectively. For organs at risk (OARs), the ipsilateral lung was analyzed with D_mean and the volume receiving 20 Gy (V_{20 Gy}), and the contralateral lung was analyzed with only Dmean. The heart was analyzed with D_mean, D_max, and V_{20 Gy}, and both inner and outer shells were analyzed with the point D_min, D_max and D_mean, respectively. For PTV, the effect of low magnetic field on dose distribution showed a difference of up to 2% for volume change and 4 Gy for dose. In OARs analysis, the significant effect of the magnetic field was not observed. Despite small deviation values, the average difference of mean dose values showed significant difference (p＜0.001), but there was no difference of point minimum dose values in both sehll structures. The largest deviation for the average difference of Dmax in the outer shell structure was 5.0±10.5 Gy (p=0.148). The effect of low magnetic field of 0.35 T on dose deposition by a Co‐60 beam was not significantly observed within the body for PBI IMRT plans. The dose deposition was only appreciable outside the body, where a dose build‐up due to contaminated electrons generated in the treatment head and scattered electrons formed near the body surface.

KeywordsLow magnetic field, Dose distribution, Partial breast irradiation, MRI-guided radiation therapy system, IMRT