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.