Journal of Medical Physics
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ORIGINAL ARTICLE
Year : 2014  |  Volume : 39  |  Issue : 2  |  Page : 71-84

Monte Carlo study of MLC fields for cobalt therapy machine


1 International Cancer Center, Mahatma Gandhi Memorial Medical Trust, Peda Amiram, Bhimavaram, Andhra Pradesh, India
2 MNJ Institute of Oncology and Regional Cancer Center, Red Hills, Hyderabad, India

Correspondence Address:
Komanduri M Ayyangar
Mahatma Gandhi Memorial Medical Trust Hospital, Peda Amiram, Bhimavaram, Andhra Pradesh - 534 204
India
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Source of Support: This work is done as a part of the BRNS grant, “Development and implementation of automated multi-leaf collimator with treatment planning system as an add-on for telecobalt machine.” It is a social impact project sanctioned by BRNS in April 2011, Conflict of Interest: None


DOI: 10.4103/0971-6203.131279

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An automated Multi-Leaf Collimator (MLC) system has been developed as add-on for the cobalt-60 teletherapy machines available in India. The goal of the present computational study is to validate the MLC design using Monte Carlo (MC) modeling. The study was based on the Kirloskar-supplied Phoenix model machines that closely match the Atomic Energy of Canada Limited (AECL) theratron-80 machine. The MLC is a retrofit attachment to the collimator assembly, with 14 non-divergent leaf pairs of 40 mm thick, 7 mm wide, and 150 mm long tungsten alloy plates with rounded edges and 20 mm tongue and 2 mm groove in each leaf. In the present work, the source and collimator geometry has been investigated in detail to arrive at a model that best represents the measured dosimetric data. The authors have studied in detail the proto-I MLC built for cobalt-60. The MLC field sizes were MC simulated for 2 × 2 cm 2 to 14 × 14 cm 2 square fields as well as irregular fields, and the percent depth dose (PDD) and profile data were compared with ROPS treatment planning system (TPS). In addition, measured profiles using the IMATRIXX system were also compared with the MC simulations. The proto-I MLC can define radiation fields up to 14 × 14 cm΂ within 3 mm accuracy. The maximum measured leakage through the leaf ends in closed condition was 3.4% and interleaf leakage observed was 7.3%. Good agreement between MC results, ROPS and IMATRIXX results has been observed. The investigation also supports the hypothesis that optical and radiation field coincidence exists for the square fields studied with the MLC. Plots of the percent depth dose (PDD) data and profile data for clinically significant irregular fields have also been presented. The MC model was also investigated to speed up the calculations to allow calculations of clinically relevant conformal beams.


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