Journal of Medical Physics
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Year : 1998  |  Volume : 23  |  Issue : 1  |  Page : 9-15

Dosimetric Aspects Of Small Circular Fields Of 4, 6 And 10 MV Photon Beams For Radiosurgery


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Correspondence Address:
K Parthasaradhi


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Dosimetric data are collected for 4, 6 and 10 MV photon beams of small circular fields of 12.5,20, 30 and 40 mm diameters. Tissue Maximum Ratios (TMR) are measured in a mini 6-inch diamater cylindrical water phantom and in a 25 x 25 cm2 polystyrene phantom using 0.02 cc ionization chamber. Output factors (OPF) are measured in polystyrene phantom using ionization chamber, films, TLDs and MOSFET (Metal Oxide Field Emission Transistor) detector. Profiles are measured using films at 10 cm depth. The location of TMR dm= depth, shifts by 1 mm as the field size increases from 12.5 to 40 mm diameters for 4 MV beam, whereas it shifts by 3 and 6 mm, respectively, for 6 and 10 MV beams as the field size increases. The output factors measured with ionization chamber are in better agreement with those measured with films even at 12.5 mm dia field size for 4 MV photon beam than those for 6 and 10 MV beams. The profiles of 4 MV beam are in better conformity with the expected field shapes than those of 6 MV beam, and in tum, 6 MV beam profiles are better than those of 10 MV beam, particularly in the region of shoulders. The estimated 80% isodose volumes for 1 to 10 non-coplanar converging arcs of 4 MV beam for a typical 20 mm diameter field size are slightly larger than those of 6 and 10 MV beams. The isodose volume decreases as the number of arcs increases and then remains, more or less, the same. On the whole, it is noticed that the measured dosimetric data of 4 MV beam give less uncertainty in dose calculations and in delivery to the exact tumor size for routine radiosurgery.


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