LAUSR

Abstract Purpose To investigate the dosimetric impact of magnetic (B) field on varying air cavities in rectum patients treated on the hybrid 1.5 T MR‐linac. Methods Artificial air cavities of varying diameters (0.0, 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 cm) were created for four rectum patients (two prone and two supine). A total of 56 plans using a 7 MV flattening filter‐free beam were generated with and without B‐field. Reference intensity‐modulated radiation therapy treatment plans without air cavity in the presence and absence of B‐field were generated to a total dose of 45/50 Gy. The reference plans were copied and recalculated for the varying air cavities. D95(PTV45–PTV50), D95(PTV50–aircavity), V50(PTV50–aircavity), Dmax(PTV50–aircavity), and V110%(PTV50–aircavity) were extracted for each patient. Annulus rings of 1‐mm‐diameter step size were generated for one of the air cavity plans (3.0 cm) for all four patients to determine Dmax (%) and V110% (cc) within each annulus. Results In the presence of B‐field, hot spots at the cavity interface start to become visible at ~1 cm air cavity in both supine and prone positioning due to electron return effect (ERE). In the presence of B‐field Dmax and V110% varied from 5523 ± 49 cGy and 0.09 ± 0.16 cc for 0 cm air cavity size to 6050 ± 109 cGy and 11.6 ± 6.7 cc for 5 cm air cavity size. The hot spots were located within 3 mm inside the rectal‐air interface, where Dmax increased from 110.4 ± 0.5% without B‐field to 119.2 ± 0.8 % with B‐field. Conclusions Air cavities inside rectum affects rectum plan dosimetry due ERE. Location and magnitude of hot spots are dependent on the size of the air cavity.