TY - JOUR
T1 - Decreasing the dosimetric effects of misalignment when using a mono-isocentric technique for irradiation of head and neck cancer
AU - Fabrizio, Patrick L.
AU - McCullough, Edwin C.
AU - Foote, Robert L.
PY - 2000/12/1
Y1 - 2000/12/1
N2 - Purpose: The purpose of this study was to quantify and develop methods to decrease inhomogeneities created with field edge mismatch when using a mono-isocentric beam-split technique.Methods and Materials: We validated techniques to determine dose across a half-blocked field edge and quantified potential sources of systematic matchline error. Then, two methods were used to evaluate matchline doses. The first used film dosimetry data from a half-beam field and a spreadsheet. Duplication and reversal provided two columns, each representing a beam-split field edge. Summation simulated perfect abutment and shifting created various gaps and overlaps. The second method involved obtaining dose profiles at midfield along the ray perpendicular to abutted, overlapped, and gapped beam-split fields on six linear accelerators. To enlarge the penumbra, we designed several field edge modifiers, then re-evaluated matchline doses. The field edge modifiers applicability to a 3-field head and neck treatment technique was also examined.Results: Film-determined dose profiles provide similar information across a beam-split field edge as an ionization chamber. With the mono-isocentric beam-split technique, a 4-mm overlap or gap produces inhomogeneities nearly 60% above or below the intended dose. A 2-mm overlap or gap produces inhomogeneities nearly 30% above or below the intended dose. A customized penumbra generator decreased the magnitude of these inhomogeneities to 20% and 10%, respectively.Conclusion: The two methods of evaluating matchline dose described above gave similar results. When using the mono-isocentric half-field technique, small misalignments produce worrisome regions of inhomogeneity. Our penumbra generator substantially decreases the magnitude of the dose inhomogeneities, although the volume receiving an inhomogeneous dose increases. Copyright (C) 2000 Elsevier Science Inc.
AB - Purpose: The purpose of this study was to quantify and develop methods to decrease inhomogeneities created with field edge mismatch when using a mono-isocentric beam-split technique.Methods and Materials: We validated techniques to determine dose across a half-blocked field edge and quantified potential sources of systematic matchline error. Then, two methods were used to evaluate matchline doses. The first used film dosimetry data from a half-beam field and a spreadsheet. Duplication and reversal provided two columns, each representing a beam-split field edge. Summation simulated perfect abutment and shifting created various gaps and overlaps. The second method involved obtaining dose profiles at midfield along the ray perpendicular to abutted, overlapped, and gapped beam-split fields on six linear accelerators. To enlarge the penumbra, we designed several field edge modifiers, then re-evaluated matchline doses. The field edge modifiers applicability to a 3-field head and neck treatment technique was also examined.Results: Film-determined dose profiles provide similar information across a beam-split field edge as an ionization chamber. With the mono-isocentric beam-split technique, a 4-mm overlap or gap produces inhomogeneities nearly 60% above or below the intended dose. A 2-mm overlap or gap produces inhomogeneities nearly 30% above or below the intended dose. A customized penumbra generator decreased the magnitude of these inhomogeneities to 20% and 10%, respectively.Conclusion: The two methods of evaluating matchline dose described above gave similar results. When using the mono-isocentric half-field technique, small misalignments produce worrisome regions of inhomogeneity. Our penumbra generator substantially decreases the magnitude of the dose inhomogeneities, although the volume receiving an inhomogeneous dose increases. Copyright (C) 2000 Elsevier Science Inc.
KW - Field matching
KW - Half-beam
KW - Head and neck
KW - Monoisocentric
KW - Split-beam
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U2 - 10.1016/S0360-3016(00)00775-6
DO - 10.1016/S0360-3016(00)00775-6
M3 - Article
C2 - 11121669
AN - SCOPUS:0034565928
SN - 0360-3016
VL - 48
SP - 1623
EP - 1634
JO - International Journal of Radiation Oncology Biology Physics
JF - International Journal of Radiation Oncology Biology Physics
IS - 5
ER -