Premium
Output ratio in air for MLC shaped irregular fields
Author(s) -
Zhu Timothy C.,
Bjärngard Bengt E.,
Xiao Ying,
Bieda Mike
Publication year - 2004
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1767052
Subject(s) - collimator , isocenter , optics , monitor unit , physics , geometry , field (mathematics) , field size , dosimetry , mathematics , nuclear medicine , beam (structure) , imaging phantom , medicine , pure mathematics
For accurate monitor unit calculation, it is important to calculate the output ratio in air, S c , for an irregular field shaped by MLC. We have developed an algorithm to calculate S cbased on an empirical model [Med. Phys. 28 , 925–937 (2001)] by projecting each leaf position to the isocenter plane. Thus it does not require the exact knowledge of the head geometry. Comparisons were made for three different types of MLC: those with MLC replacing the inner collimator jaws; those with MLC replacing the outer collimator jaws; and those with MLC as a tertiary attachment. When the MLC leaf positions are substantially different from the secondary collimators (or the rectangular field encompassing the irregular field), one observes an up to 5% difference in the value of headscatter correction factor, HCF, defined as the ratio of output ratio in air between the MLC shaped irregular field and that of the rectangular field encompassing the irregular field. No collimator exchange effect was observed for rectangular fields shaped by MLC (e.g., 5×30 and 30×5 cm 2 diagonal) when the secondary collimators are fixed, unlike that for the rectangular fields shaped by the inner and outer collimator jaws, where it can be 1–2%. For the same MLC shaped irregular field, the value of S cincreases from the Elekta, to the Siemens, to the Varian accelerators, with an up to 4% difference. The calculation agrees with measurement to within 1.2% for points both on and off the central‐axis. The fitting parameters used in the algorithm are derived from measurements for square field sizes on the central‐axis.