Develpoment Of Calculation Approach For The Determination Of Central Axis Depth Doses In Water For Cobalt 60 Beam.

ABSTRACT Radiotherapy is a complicated procedure with several steps, and these procedures have a great influence on the treatment outcome. Exact determination of dosimetric functions that are used to calculate dose within a patient undergoing external beam radiotherapy to realize the intent of treatment is very crucial for any radiotherapy treatment technique. One of the dosimetric functions, which is of great importance, is central axis percentage depth dose, which are usually measured with automated systems. The aim of the current study was to develop and propose a semi-empirical formula that can be used to determine central axis percentage depth doses through manual calculations. This could also be used as a quality assurance tool to check uncertainties associated with measured central axis depth doses. Linear attenuation coefficients of water were measured for water with beams from a Theratron Equinox 100 cobalt 60 teletherapy unit using field sizes ranging from 4 cm x4 cm to 30 cm x30 cm, to determine the effective linear attenuation of water within a formula obtained from literature to express the primary component of percentage depth dose (PDD). The linear attenuation coefficient measurements were done in air with a 0.125 cc Semiflex ionization chamber with its build-up cap on. With the expression for the primary component of the percentage depth doses (PDD) and measured PDD data obtained during commissioning of the telecobalt machine, a semi-empirical formula was developed and proposed for the determination of PDD, which considered both the primary, and the scatter components of the PDD. The PDDs calculated with the proposed semi-empirical formula compared very well with their measured counterpart with percentage differences between measured and calculated PDDs ranging from 0.10% to 2.01% (mean of 1.24±0.88%), which is within the 2% acceptable limit for central axis parameter constancy (PDD, TAR) as recommended in the report of the TG-40 The use of the proposed formula for calculating PDD is recommended for clinical application, but one needs to be circumspective in the use of the formula as some few PDDs (2%) calculated with the formula compared to their measured counterparts were found to have discrepancies beyond the tolerance of 2% recommended for the determination of central axis depth doses.