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Megavoltage radiation field matching on uneven surface
Journal article   Peer reviewed

Megavoltage radiation field matching on uneven surface

Shirish K Jani, Edward C Pennington, Judith E Wacha and David H Hussey
International journal of radiation oncology, biology, physics, Vol.15(5), pp.1247-1250
1988
DOI: 10.1016/0360-3016(88)90212-X
PMID: 3141323

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Abstract

Large radiation fields adjacent to each other require proper field matching to avoid overdosage at their junction. Various methods of determining optimum field separation have been discussed in the literature. However, these methods consider a perfectly flat surface of incidence throughout the irradiated area. In reality, it is more common for the patient thickness to vary within large fields, thus creating an uneven surface of incidence. In this paper, we have discussed the approach to gap calculation for an uneven surface of incidence. Dose measurements were made at the junction of two fields for 60Co, and 4, 6, and 24 MV X ray beams to verify this approach. Beam profiles measured under uneven surfaces indicated that the 50 percent dose decrement line did not shift towards the central axis for frequently encountered variations in patient thickness. Thus, careful geometric considerations could provide an optimum field separation. Ignoring the surface inhomogeneity may lead to field overlap and overdose at the junction.
 Field matching Gap calculation Dosimetry

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