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Assessing localized dosimetric effects due to unplanned gas cavities during pelvic MR‐guided radiotherapy using Monte Carlo simulations
Author(s) -
Shortall Jane,
Vasquez Osorio Eliana,
Chuter Robert,
McWilliam Alan,
Choudhury Ananya,
Kirkby Karen,
Mackay Ranald,
Herk Marcel
Publication year - 2019
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.13857
Subject(s) - imaging phantom , monte carlo method , beam (structure) , transverse plane , radiation treatment planning , dosimetry , nuclear medicine , dose volume histogram , photon , materials science , physics , path length , radiation therapy , optics , medicine , mathematics , radiology , statistics
Purpose It has been proposed that beam modulation and opposing beam configurations can cancel effects of the Electron Return Effect (ERE) during MR‐guided radiotherapy (MRgRT). However, this may not always be the case for unplanned gas cavities outside of the target in the pelvic region. We evaluate dosimetric effects, including effects in the rectal wall, due to unplanned spherical air cavities during MRgRT. Methods Nine virtual cuboid water phantoms containing spherical air cavities (0.5–7.5 cm diameter) and a reference phantom without air were created. Monte Carlo dose calculations of 7 MV photons under the influence of a 1.5 T transverse magnetic field were produced using Monaco 5.19.02 Treatment Planning System (TPS) (Elekta AB, Stockholm, Sweden). Cavities in the path of a single and multiple beam plans were considered. Dose distributions of phantoms with and without air cavities were compared (ΔD % ) using a spherical coordinate system originating in the center of the cavity. Effects in the rectal wall were quantified by comparing dose volume histogram (DVH) parameters for solid and gaseous filling from simulated rectal wall structures. Results Max(ΔD % ) of ~70% and 20% were observed around large cavities in the path of a single and multiple beam plans, respectively. Approximately 45 cm 3 of phantom surrounding the largest cavity in a single beam received dose changes of >10%. D mean in the rectal wall was unchanged when comparing gaseous and solid filling in the path of a single beam; however, D 1cc and D max increased by up to ~45% and ~63%, respectively. Conclusions Unplanned gas cavities in the path of a single beam during pelvic MRgRT with a 1.5 T transverse magnetic field cause dose changes which may impact toxicity in the rectal wall, depending on local dose and fractionation. Effects are reduced but not eliminated with a five‐beam plan.