title: Development of a novel quality assurance system based on rolled-up and rolled-out radiochromic films in volumetric modulated arc therapy.
abstract: OBJECTIVE
  To develop a cylindrical phantom with rolled-up radiochromic films and dose analysis software in the rolled-out plane for quality assurance (QA) in volumetric modulated arc therapy (VMAT).
  
  
  METHODS
  The phantom consists of an acrylic cylindrical body wrapped with radiochromic film inserted into an outer cylindrical shell of 5 cm thickness. The rolled-up films with high spatial resolution enable detection of specific dose errors along the arc trajectory of continuously irradiated and modulated beams in VMAT. The developed dose analysis software facilitates dosimetric evaluation in the rolled-up and rolled-out planes of the film; the calculated doses on the corresponding points where the rolled-up film was placed were reconstructed into a rectangular dose matrix equivalent to that of the rolled-out plane of the film. The VMAT QA system was implemented in 3 clinical cases of prostate, nasopharynx, and pelvic metastasis. Each calculated dose on the rolled-out plane was compared with measurement values by modified gamma evaluation. Detected positions of dose disagreement on the rolled-out plane were also distinguished in cylindrical coordinates. The frequency of error occurrence and error distribution were summarized in a histogram and in an axial view of rolled-up plane to intuitively identify the corresponding positions of detected errors according to the gantry angle.
  
  
  RESULTS
  The dose matrix reconstructed from the developed VMAT QA system was used to verify the measured dose distribution along the arc trajectory. Dose discrepancies were detected on the rolled-out plane and visualized on the calculated dose matrix in cylindrical coordinates. The error histogram obtained by gamma evaluation enabled identification of the specific error frequency at each gantry angular position. The total dose error occurring on the cylindrical surface was in the range of 5%-8% for the 3 cases.
  
  
  CONCLUSIONS
  The developed system provides a practical and reliable QA method to detect dosimetric errors according to the gantry angle. Film dosimetry based on rolled-up and rolled-out techniques leads to dose verification in the subspaces of the 3D dose volume. The system can be employed as an alternative tool to detect the pitfalls of planar dose verification.
authors.affiliation: Department of Biomedical Engineering and Research Institute of Biomedical Engineering, College of Medicine, The Catholic University of Korea, Seoul 137-701, Korea.
authors.forename: Ji-Yeon
authors.initials: JY
authors.lastname: Park
authors.forename: Jeong-Woo
authors.initials: JW
authors.lastname: Lee
authors.forename: Kyoung-Sik
authors.initials: KS
authors.lastname: Choi
authors.forename: Jung Seok
authors.initials: JS
authors.lastname: Lee
authors.forename: You-Hyun
authors.initials: YH
authors.lastname: Kim
authors.forename: Semie
authors.initials: S
authors.lastname: Hong
authors.forename: Tae-Suk
authors.initials: TS
authors.lastname: Suh
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D005373:Film Dosimetry / Q000295:instrumentation*; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care / Q000379:methods*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation*