Purpose: 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 rolledout 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. VC 2011 American
Association of Physicists in Medicine. [DOI: 10.1118/1.3659706]