title: On the usefulness of portal monitor unit subtraction in radiation therapy.
abstract: In order to avoid additional dose to patients caused by portal imaging with megavoltage x-rays, portal monitor units (MUs) are frequently subtracted from the actual treatment MUs. This study examines the usefulness of portal MU subtraction in radiation therapy. For 11 prostate cancer patients treated with 23 MV photons, dose to prostate due to portal filming with 6 MV photons was determined. In all 11 patients subtraction of portal MU values from the actual treatment MUs resulted in a small underdosing of the prostate with an average treatment error of -0.5%. Portal filming without MU subtraction would cause small overdosing of the prostate with an average treatment error of 1.2%. The results of this study indicate that the benefits of portal MU subtraction are in doubt if (a) the energy of treatment x-rays is much higher than that of the portal x-rays and/or (b) when radiotherapy is performed with physical wedges. Based on the obtained results, we argue against unconditional use of the portal MU subtraction method to eliminate the dose from portal imaging.
authors.affiliation: Radiation Oncology Service, J A Haley Veterans Hospital, Tampa, FL, USA. vadimkuperman@yahoo.com
authors.forename: Vadim Y
authors.initials: VY
authors.lastname: Kuperman
authors.forename: Leslie M
authors.initials: LM
authors.lastname: Lubich
mesh_terms: D019583:Dose Fractionation, Radiation; D005373:Film Dosimetry / Q000379:methods*; D006801:Humans; D018499:Linear Energy Transfer; D008297:Male; D008954:Models, Biological; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D012062:Relative Biological Effectiveness; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D017211:Treatment Failure