A comparative description of three multipurpose phantoms (MPP) for external audits of photon beams in radiotherapy:: the water MPP, the Umea MPP and the EC MPP

Aim: To present a technical description and intercomparison of three multipurpose phantoms (MPP) developed for mailed dosimetry checks of therapeutic photon beams in reference and non-reference conditions. Materials: The W-MPP is a water MPP, whereas the Ume(a) over dot-MPP, made of perspex (PMMA, Plexiglas), and the EC-MPP; made of polystyrene, are solid MPPs. The W-MPP uses only thermoluminescent dosimeters (TLD) for dosimetry checks. the EC MPP uses film and TLD; the Ume(a) over dot phantom uses film and TLD, and offers in addition the possibility for ionization chamber measurements. Either using TLD or films, the MPPs have been designed to check on-axis and off-axis the following irradiation conditions: square and rectangular fields, asymmetric fields, wedged beams, oblique incidence and, for the solid MPPs, also the influence of inhomogeneities. Results and discussion: The MPPs have been compared for different aspects going from their dosimetric performance (number of dosimetric parameters that can be checked) to some practical consideration in the use of the different MPPs (set-up time, stability, instruction sheets, etc.). From a comparison between the solid multi-purpose phantoms, it turns out that the EC-MPP is capable of checking the largest number of dosimetric parameters per beam, but has the longest set-up time (similar to 2 h) per beam according to the users. The Ume(a) over dot-MPP has a smaller number of set-ups thence a smaller average time) and also includes some parameters not checked with the EC-MPP (e.g. SSD accuracy. The major drawback however of the Ume(a) over dot-MPP is considered to be its high density (> 1.1 g/cm(3)) which increases the difficulty of the analysis with the treatment planning system. The W-MPP checks the smallest number of parameters, but is the fastest in set-up time, the easiest for mailing, and is water equivalent, which is advantageous for the TPS checks. The major drawback of this MPP is however the inability to check complete dose distribution (film) or inhomogeneities. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.