Decreasing the dosimetric effects of misalignment when using a mono-isocentric technique for irradiation of head and neck cancer

Purpose: The purpose of this study was to quantify and develop methods to d ecrease 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 h alf-blocked field edge and quantified potential sources of systematic match line error. Then, two methods were used to evaluate matchline doses. The fi rst used film dosimetry data from a half-beam field and a spreadsheet. Dupl ication and reversal provided two columns, each representing a beam-split f ield edge. Summation simulated perfect abutment and shifting created variou s gaps and overlaps. The second method involved obtaining dose profiles at midfield along the ray perpendicular to abutted, overlapped, and gapped bea m-split fields on six linear accelerators. To enlarge the penumbra, we desi gned several field edge modifiers, then re-evaluated matchline doses. The f ield edge modifiers applicability to a 3-field head and neck treatment tech nique 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 b eam-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 inhomo geneities nearly 30% above or below the intended dose. A customized penumbr a generator decreased the magnitude of these inhomogeneities to 20% and 10% , respectively, Conclusion: The two methods of evaluating matchline dose described above ga ve similar results. When using the mono-isocentric half-field technique, sm all 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. (C) 2000 E lsevier Science Inc.