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Study of Dosimetric Properties of Cadmium Free Alloy Used in Compensator Based IMRT

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A Tyagi

A Tyagi1*, S Kaushik1,2 , R Punia2 , M Singh3 , (1) Dr B L Kapoor Hospital, New Delhi, Delhi, (2) Guru Jambheshwar University of Science & Technology, Hisar, Haryana, (2) Guru Jambheshwar University of Science & Technology, Hisar, Haryana, (3) MMH College, Ghaziabad, Utter Pradesh


SU-F-T-533 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To study the dosimetric properties of cadmium free alloy which is used in compensator based IMRT.

Methods:A mixture of 30% of lead,52% of bismuth and 18% of tin was used to prepare alloy. We prepared slabs of different thicknesses ranging from 0.71 cm to 6.14 cm. Density of alloy was measured by Archimedes’ principle using SI-234 Denver instrument and water as buoyant liquid. Transmission, linear attenuation coefficient (μ), tissue phantom ration (TPR), beam hardening, surface dose (Ds), percentage depth dose (PDD) and effect of scatter were measured and analyze for different field size and different thickness of compensator for 6 MV photon beam. Measurements were carried out at 100 cm SSD and 160 cm SSD.

Results:Density of alloy was found to be 9.5456 gm/cm3. Melting point of alloy is 90-95 °C. For a field size of 10X10 cm2 μ was 0.4253 cm-1 at 100 cm SSD. Calculated TPR was found to be within 3 % of measured TPR. Ds was found to be decreasing with increasing thickness of compensator. 1cm, 1.98 cm and 4.16 cm thick compensator slab decreased surface dose by 4.2%, 6.1% and 9.5% respectively for a field size of 10x10cm2 at 100 cm SSD. As field size increases Ds increases for a given compensator thickness. This is due to increase in amount of scattered dose from wider collimator opening. For smaller field size, PDDs are increased from 3.0% to 5.5% of open beam PDDs as compensator thickness increases from 1 cm to 6.14 cm at a depth of 10 cm in water. For larger field size variation in PDDs is not significant.

Conclusion:High degree of modulation can be achieved from this compensator material, which is essential in compensator based IMRT. Dosimetric properties analyzed in this study establish this alloy as a reliable, cost effective, reusable compensator material.

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