Investigation of doses in irregular photon fields

Abstract

The mm of this rhesis work was to investigate the doses in irregul"r plmtonfields usually encountered ill routine mdiothaapy practices, In order to serve the purpose, 14 Irregular fields were simulated in solid acrylic phantoms, of which 10 were used in Ihe mvest1garion of doses for photon beam from wCo teletherapy unU and rhe rest 4 Were used in the case t;(6 MV photoll beam from Lmac - mevatroll 7445. It was expected Ihat, these fields would almost cover the different irregular fields enmuntered in daily mdlOtherapy praeli"es at diffaellt therapy estah/ishlllents, 11te direct measurement dose values in these irregular fields were presented m tables 3a - 3j for weo alld 4a - 4d for 6 AlV Iinac-m,evatron-7445 respertively, The direct meaSUremem dose values in irregular fields were compared with ealculared dose values ohtamed by Ihe use of Da)}'s method of dose calculation in irregul"r photon fields using hoth off axis dose ratios and " beam intensity fi,etors (ElF)", The tables coma in, directly measured dose values at vanous in/erested pomls in different Irregular fields, Ihe directly measured dose values ar those point, in the ('orresponding opell ,fields, percentage difference oI dose values berweell irregular (blocked) ami opell fields, calculaled dose values al re.\pecrlve poillts. percen/aKe difference of dose values betJ'l'een direclly measurrd and calculated values, 77le directly measured dose val"es at different polilts in iJ'regularfle/ds were normalized to the central a.xis dose vailles in the respective open fields. The lIl'eraged of the mean percelllaKe differe"ces with lsd hetween direclly measured dose values at diffrre/ll pm"ts in Irregular (blocked) fields and the corresponding dose values at those poims in open fields of 3.'1 - 3J for 60Co and 4A - 4D for 6 AlV Linac Mevalron 7445 were 6,85%:t 3.08 ( range 2.59% - 11,62%) and 4.04% :t 150 ( rangel.34% _ 704%) respectively. The mean mlue oI the coefficients of correlation (r) hetween the direcily measured dose valucs in irregular fields amI the calculated dose values ill the correspondingjields of}A - 3.1 for wCo and 4.1 - 4D for 6 AiV linac - mevatron 7445 were ,9929:t ,0034 alld .949 I ,06 while this value wa,<.935 I ,044 when RIF was used in place of off axis dose rarios ill Day's dose calculatioll formula, The averaged oI the meall differences With lsd Iletween direcrh' measured dose values and cal('ulated ,,. - - dose values by Day's me/hod m rhe case Cif 0°('0 Were 1,39%;} ,49 (range .1)2%- 4 16%) and 4.20%:f 1,05( rtmge 01% - J 2.77%) for 01' Co whde 2.63%;}.75 ( nmxe .59%" 5..'15%) and 2/8% :t.23 (nmge .02% - 5.67%jfor 6 MV Lina,. respectlve/y. The mean differences 6.85%:t 3 OR(ranging 2,59% . 11.61%) for 6Oeoand 4,04% :f: 1 50 ( rallgmg 1,34% - 7.04%) jr)r 6MV Linac Mevarron 7445 hetweell directly measured dose va/lies ar different pOllll,<'" Irregular ( blocked) fields and at the corresponding polms in respeclil'e open fields could be considered statistically significant ill Ihe CllSC of dose prescriplion ji)r a parlen! rcceivinr; radio/hempy treatment with irregularly shaped ph%n.fields Directly meusured dose values "I d,fferent pOl/lis in various irregular field, wac prescnled as iJormalized 10 the central axis dose value of the correspondinx open fields. This normalized cenlral axis dose data was expecled to be useful as reference dutu for dosimetry in routine radwtherapy practices wilh fields ilavmg //lore or less simJlar irregular c(Jnfiguralwn.