To investigate the accuracy of dose measurement in mega voltage blocked beam y-radiation and to develop an imperial relation for treatment procedure

Abstract

Irregularly shaped fields are usually encounter~d in routine radiotherapy ,~hen radiation sensitive structures ar~ shielded Ii-om th~ primal)' beam or when the field extends beyond th~ irregularly shaped patient body ~ontour. The aim, of th~ present work were (a) to investigate the accuracy of d()';e measurement in irregular photon fields encountered 1n routine radiotherapy practiees, (b) to develop un Empirical relation fOf radiotherapy treatment procedur~_ For treatment planning in irregular photon fields usually encountered m romine radiotherapy practices eleven (11) irregular fields "ere simulated in solid perspex phantom to investigate the doses for photon beam of 6OCOteletherapy unit. It was expeded IhaL these fields would almost cover the different irregular rlelds encountered in daily radiotherapy practices at different therapy establishments, The direct measurement dose values in irregular fields were compared with calculated dose value,; obtained by the us~ of Clarkson's method and newly develop~d Empirical relation or dose calculation in irregular photon fields, The Tablcs 3(l-XI) contain. directly ll1easur~d dose values at variou, interested points in differenl inegular lield" the direclly measured dose values at tho,e points in the corresponding open fields. percentage dlfference of dose \'alues between irregular (blocked) and open fields, calculated dose values at respective points using botll Clarkson's method and Empirical relation, percentage difference of dose values between directly measured and calculated values. The calculated dose values of Empirical relation are in good agreement with the directly measured dose valu~s at dlfferent points in irregular fields, So that, dose estimation in irregular fields could be approximat~d with reasonable accuracy from the calculated dose values or Empirical relation normalized to central axis beam dose data in respecth-e llpen fields, The averaged of the mean percentage dilferences with bd between directly measured dose values al different points in irregular (blocked) fields and the corresponding dose values at those points in open fields of 3(i-xi) for 60CO was found to be 16.76% :t 9,12 (range 2.45% - 49,20%) for 6QCOteletherapy unit. The mean value of the coefficients of correlation (f) between directly measured dose values in irregular fields and the calculated dose values by Clarkson's method and Empirical relation in the corresponding fields of 3(i-xi) for oOCowere found to bc 0.999 and 0.999 respectively, The averaged of the mean percentage differences with lsd between directly measured dose • *. _. • values and calculated dose values of Clarkson., method and Empirical relation were found to be 2.52R%:t 1.622 (range 0.475% - 5.998%) and 2.527%:1: 1.623 (range 0.475%- 5.998%) respectively. The corresponding uncertainty :1:1.291%and :t1.291% between direclly measured dose values and calculated dose values by Clarkson's method and Empirical relation were found statistical!} satisfactory. because according to the International Commission on Radiation Units and measurements (ICRU) the dose delivered to the target volume should be at least within :l:5%[4ol.The averaged of the mean difference 16.76%::!: 9.12 (range 2.45% - 49.2%) bet".een directly measured dose values at different points in irregular (blocked) tields and the corresponding poinl<; in respeclive open tields could be considered staliSlically significant in case 01" Jo.<,e prescription for a patient recelvmg radiotherapy treatment with irregularly shaped photon fields. The important finding or this study is that the directly meawred dose values in irregular fields are in good agreement with calculated dose values of Empirical relation so that the dose eslimation in irregular fields could be approximated with reasonable accuracy from the calculated dose value~ of empirical relation normalized (0 central axis beam dose in open tields. This nonnalized central <lxi, dose data was expected to be useful as reference data for dosimetry of irregular fields in routine radiotherapy practice,. Moreover, il may be con,irlered that, our dosimetric results may he useful as general guidelines, to optimize the radiation doses to the organs al risk during: rJ.diothcrapy. On the ba,is of ALAR.J\ concept, exposure should be kept as low as reasonably achievable. It i~ suggested that shielding devices should be used in risk organs, especially gonad and len, of eye, whenever possible to reduce lhe potential risk due to (he scattered r<ldia(ion dose.