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 multip urpose 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 soli d MPPs. The W-MPP uses only thermoluminescent dosimeters (TLD) for dosimetr y checks. the EC MPP uses film and TLD; the Ume(a) over dot phantom uses fi lm and TLD, and offers in addition the possibility for ionization chamber m easurements. Either using TLD or films, the MPPs have been designed to chec k on-axis and off-axis the following irradiation conditions: square and rec tangular fields, asymmetric fields, wedged beams, oblique incidence and, fo r the solid MPPs, also the influence of inhomogeneities. Results and discussion: The MPPs have been compared for different aspects g oing from their dosimetric performance (number of dosimetric parameters tha t can be checked) to some practical consideration in the use of the differe nt MPPs (set-up time, stability, instruction sheets, etc.). From a comparis on between the solid multi-purpose phantoms, it turns out that the EC-MPP i s 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 sm aller 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 do t-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-MP P checks the smallest number of parameters, but is the fastest in set-up ti me, the easiest for mailing, and is water equivalent, which is advantageous for the TPS checks. The major drawback of this MPP is however the inabilit y to check complete dose distribution (film) or inhomogeneities. (C) 2000 E lsevier Science Ireland Ltd. All rights reserved.