Formalisms for MU calculations, ESTRO booklet 3 versus NCS report 12

Although the relevance and importance of quality assurance and quality cont rol in radiotherapy is generally accepted, only recently, methods for monit or unit (MU) calculation and verification have been addressed in recognized recommendations, published by the European Society of Therapeutic Radiatio n Oncology (ESTRO) and by the Netherlands Commission on Radiation Dosimetry (Dutreix A, Bjarngard BE, Bridier A, Mijnheer B, Shaw JE, Svensson H. Moni tor unit calculation for high-energy photon beams. Physics for clinical rad iotherapy. ESTRO Booklet No. 3. Leuven: Garant, 1997; Netherlands Commissio n on Radiation Dosimetry (NCS). Determination and use of scatter correction factors of megavoltage photon beams. NCS report 12. Deift: NCS, 1998). Bot h documents are based on the same principles: (i) the separation of the out put factor into a head and a volume (or phantom) scatter component; (ii) th e use of a so-called mini-phantom to measure and verify the head scatter co mponent; and (iii) the recommendation to use a single reference depth of 10 cm for all photon beam qualities. However, there are substantial differenc es between the approach developed in the IAEA-ESTRO task group and the NCS approach for MU calculations, which might lead to confusion and/or misinter pretation if both reports are used simultaneously or if data from the NCS r eport is applied in the algorithms of the ESTRO report without careful cons ideration. The aim of the present paper is to discuss and to clearly point out these differences (e.g. field size definitions, phantom scatter paramet ers, etc.). Additionally, corresponding quantities in the two reports are r elated where possible and several aspects concerning the use of a mini-phan tom (e.g. size, detector position, composition) are addressed. (C) 2001 Els evier Science Ireland Ltd. All rights reserved.