MONITOR UNIT CALCULATION IN 6 MV IRREGULARLY SHAPED BEAMS - ACCURACY IN CLINICAL-PRACTICE

The results of an investigation of the accuracy of monitor unit (MU) c alculation in clinical shaped beams are presented. Measured doses at t he reference depth on the beam central asis (isocentre) or on a beam a xis representative of the irradiated area when the isocentre lies unde r a block, yr near the edges oi the block's shadow) were compared with the expected doses when calculating MUs, by applying different method s normally used in clinical practice, Empirical (areas weighted, Wrede ) and scatter summation (Clarkson) methods as well as a pencil-beam ba sed algorithm were applied, 40 irregular fields (6 Ny X-rays, CLinac, Varian 6/100), divided into six categories, were considered. Dose meas urements were performed with a NE2571 ionization chamber in an acrylic 30 x 30 x 30 cm(3) phantom. The depths in acrylic were converted into water-equivalent depths through a correction factor derived from TMR measurements. The method of dose measurements in acrylic was found to be sufficiently accurate for the purpose of this study by comparing ex pected and measured doses in open square and rectangular fields (mean deviation + 0.2%, SD = 0.5%), Results show that all the considered met hods are sufficiently reliable in calculating MUs ill clinical situati ons, Mean deviations between measured and expected dose values are aro und 0 for all the methods; standard deviations range from 1% for the W rede method to 0.75% for the pencil-beam method. The differences betwe en expected and measured doses were within 1% for about: 3/4 of the fi elds when calculating MUs with all the considered methods, Maximum dev iations range from 1.6% (pencil-beam) to 3% (Wrede). Slight difference s among the methods of hall calculation were revealed within the diffe rent categories of blocked fields analysed, The surprising agreement b etween measured and expected dose values obtained by using empirical m ethods (area weighted and Wrede) is probably due to the fact that the reference points were positioned in a ''central'' region of the unbloc ked areas.