Dosimetric studies of cadmium free alloy used in compensator based intensity modulated radiotherapy

Abstract

Aim of this study was to investigate dosimetric properties of cadmium free alloy which is used in compensator based intensity modulated radiotherapy (cIMRT). A mixture of lead, bismuth and tin was used to prepare the alloy whose melting point is 90–95°C. Slabs of different thicknesses ranging from 0.71cm to 6.14cm were prepared. Density of alloy was measured by Archimedes’ principle using water. For six megavolt (6MV) photon beam energy transmission, linear effective attenuation coefficient (µeff), tissue phantom ratio (TPR1020), beam hardening, surface dose (Ds), percentage depth dose (PDD) and effect of scatter has been measured and analyzed for different field sizes and different thickness of compensator. Effect of extended source to detector distance (SDD) on transmissions and µeff was measured. The density of alloy was found to be 9.5456g/cm3. At SDD of 100cm, µeff was observed 0.4253cm−1 for a field size of 10×10cm 2. Calculated TPR1020 was found to be within 3% of experimental TPR1020. It was found to be increasing with increasing thickness of compensator. Ds was found to decrease with thickness of compensator and increase with wider collimator opening due to increased scattered dose. Compensator slabs of 1cm, 1.98cm and 4.16cm decreased surface dose by 4.2%, 6.1% and 9.5% respectively for a field size of 10×10cm2 at 100cm SDD. For small field size of 3×3cm2 and 5×5cm2 PDDs are increased from 3.0% to 5.5% of open beam PDDs as compensator thickness increased from 1cm to 6.14cm at a depth of 10cm in water while variation in PDD is insignificant in for larger field sizes 10×10cm2 to 20×20cm2. A high degree of intensity modulation is essential in cIMRT and it can be achieved with this compensator material. Dosimetric properties analyzed in this study establish this alloy as a reliable, reusable, optimally dense and cost effective compensator material.