The use of a multipurpose phantom for mailed dosimetry checks of therapeutic photon beams: 'OPERA' (operational phantom for external radiotherapy audit).
OBJECTIVE
This study presents a technical description of the solid multipurpose phantom (MPP) 'OPERA' (operational phantom for external radiotherapy audit) for mailed dosimetry checks of photon beams in reference and non-reference conditions. The aim is to test the functionality of this phantom by comparing the results provided by the local treatment planning system (TPS) with the measurements by film and thermoluminescence dosimeters (TLDs) in a number of clinical conditions.


METHODS
The polystyrene MPP is constructed to check the following five irradiation conditions: square fields, asymmetrical fields, wedged beams, oblique incidence and influence of inhomogeneities in the field. The absorbed dose on the central beam axis is measured with TLDs for the first three irradiation conditions and the relative dose distributions are verified with film.


RESULTS
The 'OPERA' phantom and the corresponding instruction sheets were mailed to radiotherapy centres of 12 different countries, in order to verify its functionality. A good agreement between TPS and the film data is found: the mean deviations between relative dose profiles obtained from film and locally applied TPS data range between -1.2 and 0.4%, except for the short side of the asymmetrical field. The standard deviations (SDs) generally increase off-axis, with a large SD on the asymmetrical side. Considering the TLD results, a mean ratio of measured to stated dose of 0.995+/-0.014 (1 SD), 0.988+/-0.019 (1 SD) and 0.981+/-0.024 (1 SD) for set-up 1 (square field), 2 (asymmetrical field) and 3 (wedged field), respectively, has been found.


CONCLUSIONS
The 'OPERA' phantom can be useful for on- and off-axis verification of the TPS, as well as for mailed dosimetrical checks of photon beams in reference and non-reference conditions.
Swinnen
Ans
A
Division of Radiation Physics, Department of Oncology, University Hospital Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium.
Verstraete
Jan
J
Huyskens
Dominique
D
D005373:Film Dosimetry; D006801:Humans; D008485:Medical Audit; D019047:Phantoms, Imaging; D017785:Photons; D011874:Radiometry; D011878:Radiotherapy; D011879:Radiotherapy Dosage; D013819:Thermoluminescent Dosimetry