Shielding blocks for protection of vital organs within a radiation fieid give rise to changes in dose distribution of open fieid due to decreased scattered radiation from the shielded areas. Several methods have been used to calculated depth dose values for irregular fields. However, vaiidity ofsuch methods shouid be verified before routine use in ciinic.
In this study measured percentage depth doses for selected irregular abdomen and pelvis fields have been compared with percentage depth dose for fields obtained using equivalent sguares (negative fieid) and [4x(A/P)] techniques. Dose rates for blocked and unblocked fields have been measured. Aiso, dose profiles for open fields at 0.5 cm and blocked fields for three depths -0.5, 5 and 10 cm depth - have been obtained and the effect of biocking on dose profile for open fieid has been investigated.
When measured percentage DD for the selected fields were compared with percentage DD for equivalent squares technique,the percentage difference were found 1.38 % and 1.47 % since the block edge was dose to centrai axis. When measured percentage DDs were compared with percentage DD for [4x(A/P)] technique, the percentage difference were found 1.08 0% and 1.21 % for the abdomen and pelvis fields respectively. But both techniques were convenient for percentage DD except points at block edge. When out-put values for selected biocked fields were compared with those of open fields, if biocked fields were less than 25 %, out-put of open fields could be used for irregular fields. The dose under the block for selected fieid was about 10 % of dose at d max.
Key words: Irreguiar fieids, totai abdomen, pelvis, dosimetri, cobalt-60, radiotherapy.
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