Journal of Medical Physics
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Year : 2004  |  Volume : 29  |  Issue : 2  |  Page : 48-54

Tongue And Groove Effect Of Varian Multileaf Collimator For Different Clinical Conditions

Correspondence Address:
S D Sharma

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Use of multileaf collimator (MLC) for shaping the treatment fields in external beam therapy is now a common practice. Tongue and groove arrangement is used in all commercially available multileaf collimators to limit interleaf leakage. The tongue and groove effect (TGE) is an under-dosing effect in the region of overlap between two adjacent leaves of a multileaf collimator. Mean dose reduction (MDR) and its spatial extent (in terms of FWHM) at the field junction defined by Varian Standard 80 leaves MLC for 6 and 18 MV photon beams were measured using radiographic and radiochromic films. Measured values of MDR and mean FWHM are presented for three different field geometries, namely, a narrow field, a medium size rectangular field and an irregular field at the depth of dose maximum (dm) as well as at 10 cm depth. These three fields correspond to the treatment fields used in intensity modulated radiotherapy (IMRT) and conformal/conventional radiotherapy. FWHMs are found to be independent of beam energy, depth of measurement and field geometry. However, MDR depends on beam energy, depth of measurement and field geometry. Maximum dose reduction of 17.6% with spatial extent of 2.8 mm was observed at the dm of 6 MV photon beam for the narrow field.

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