Impact of varying air cavity on planning dosimetry for rectum patients treated on a 1.5 T hybrid MR‐linac system

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. D(95)(PTV(45)–PTV(50)), D(95)(PTV(50)–aircavity), V(50)(PTV(50)–aircavity), D(max)(PTV(50)–aircavity), and V(110%)(PTV(50)–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 D(max) (%) and V(110%) (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 D(max) and V(110%) 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 D(max) 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.