title: Design and dosimetric characteristics of a new endocavitary contact radiotherapy system using an electronic brachytherapy source.
abstract: OBJECTIVE
  To present design aspects and acceptance tests performed for clinical implementation of electronic brachytherapy treatment of early stage rectal adenocarcinoma. A dosimetric comparison is made between the historically used Philips RT-50 unit and the newly developed Axxent(®) Model S700 electronic brachytherapy source manufactured by Xoft (iCad, Inc.).
  
  
  METHODS
  Two proctoscope cones were manufactured by ElectroSurgical Instruments (ESI). Two custom surface applicators were manufactured by Xoft and were designed to fit and interlock with the proctoscope cones from ESI. Dose rates, half value layers (HVL), and percentage depth dose (PDD) measurements were made with the Xoft system and compared to historical RT-50 data. A description of the patient treatment approach and exposure rates during the procedure is also provided.
  
  
  RESULTS
  The electronic brachytherapy system has a lower surface dose rate than the RT-50. The dose rate to water on the surface from the Xoft system is approximately 2.1 Gy∕min while the RT-50 is 10-12 Gy∕min. However, treatment times with Xoft are still reasonable. The HVLs and PDDs between the two systems were comparable resulting in similar doses to the target and to regions beyond the target. The exposure rate levels around a patient treatment were acceptable. The standard uncertainty in the dose rate to water on the surface is approximately ±5.2%.
  
  
  CONCLUSIONS
  The Philips RT-50 unit is an out-of-date radiotherapy machine that is no longer manufactured with limited replacement parts. The use of a custom-designed proctoscope and Xoft surface applicators allows delivery of a well-established treatment with the ease of a modern radiotherapy device. While the dose rate is lower with the use of Xoft, the treatment times are still reasonable. Additionally, personnel may stand farther away from the Xoft radiation source, thus potentially reducing radiation exposure to the operator and other personnel.
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO 63110, USA. srichardson@radonc.wustl.edu
authors.forename: Susan
authors.initials: S
authors.lastname: Richardson
authors.forename: Jose
authors.initials: J
authors.lastname: Garcia-Ramirez
authors.forename: Wei
authors.initials: W
authors.lastname: Lu
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Myerson
authors.forename: Parag
authors.initials: P
authors.lastname: Parikh
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation*; D004867:Equipment Design; D006801:Humans; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted