cax (1.13) and DmeaS}Cax is the dose at a point at the same depth but on the central beam axis. 1.4.2 R e v i e w o f S t u d i e s o n A s y m m e t r i c F i e l d s Two documents have been published on monitor unit (MU) calculations in.high energy photon beams, an E S T R O booklet by an I A E A - E S T R O task group[8] and Report'12 of the Netherlands Commission on Radiation Dosimetry (NCS)[9]. Both of these documents support that the close rate at reference depth should be separated into the head and volume scatter components when calculating the number of mon-itor units necessary to deliver the prescribed dose. The head scatter factor can be measured by an ion chamber with a build-up cap of a size large enough to provide dose build-up. There are a large number of investigations on the dose rate (the dose per monitor unit) in asymmetric fields. Khan showed that beam data determined for symmetric fields (e.g.TMR, head scattering factor, off-axis ratio) can be used for calculations of asymmetric photon beams when off-axis softening is accounted for[10]. Chui and Mohan [11] improved Khan's method by separating the off-axis ratio into a boundary factor and a primary off-axis ratio. Tenhunen and Lahtinen separated the field into four quadrants and calculated the dose contribution of each quadrant using head scatter and volume scatter factors for symmetric fields [12]. Marinello and Dutreix separated the influence of each field-defining collimator when calculating the \"output in air\" of symmetric and asymmetric fields[13]. Kwa used Day's equivalent field size method by applying symmetric dose rates and T M R in asymmetric field [14]. Chapter 1. Introduction to Dose Calculation and Measurement Techniques 20 1.4.3 Previous Studies on Wedged A s y m m e t r i c Fie lds Venselaar and Welleweerd [4] recently showed for a number of commercial treatment planning systems, that the algorithms for calculating monitor units (MUs) in wedged asymmetric fields have their limitations. Deviations up to 13% between the calcu-lated and measured dose were observed at the thin and thick sides of the wedge. Compared with the criteria Venselaar proposed [3], improvements in M U calculation are required. Khan proposed an approach to calculate MUs for wedged asymmetric fields[15] in which close rates of asymmetric fields and off-axis ratios are used to relate the dose in the asymmetric field to the close under reference conditions. The number of monitor units for wedged asymmetric photon field can be calculated by: DreI x TPR(d,rd) x Sc(rc) x Sp(rd) x {SDcal\/SPDf x OAR