Open AccessImpact of varying air cavity on planning dosimetry for rectum patients treated on a 1.5 T hybrid MR‐linac system
Author(s) -
Godoy Scripes Paola,
Subashi Ergys,
Burleson Sarah,
Liang Jiayi,
Romesser Paul,
Crane Christopher,
Mechalakos James,
Hunt Margie,
Tyagi Neelam
Publication year - 2020
Publication title -
journal of applied clinical medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.83
H-Index - 48
ISSN - 1526-9914
DOI - 10.1002/acm2.12903
Subject(s) - nuclear medicine , dosimetry , linear particle accelerator , supine position , field size , rectum , optics , materials science , physics , beam (structure) , medicine , surgery
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.