Multileaf collimators (mlc) are now commonly used to shape treatment fields for conformal radiotherapy. In some of the clinical applications, a large area of extended leaves is uncovered. Such fields can be assumed to be mlc only defined fields. Percentage surface dose (psd), build up region, percentage depth dose (pdd), penumbra width (P80/20), central axis absorbed dose (caad) and total scatter factor (Scp) for radiation fields defined by jaw collimators (jc) only, jc and mlc together (jc + mlc), and mlc only were measured to quantify the differences in these parameters due to definition of fields by different combinations of jc/mlc. Measurements were carried out on a Varian Clinac 2100C medical linear accelerator and a Varian 80 leaf mlc with 6 (quality index, QI = TPR20,10 = 0.66) and 18 MV (QI = 0.78) photon beams. Radiation transmitted through jc alone, jc + mlc and mlc alone were also determined. psd and dose in build up region for mlc only fields are higher than jc/jc + mlc fields. Differences in psd of mlc only and jc/jc+mlc fields depend on beam energy and field size. These three types of fields have the same pdd while mlc penumbra (P80/20) is about 1 mm larger than jc penumbra. caad of mlc only field is also higher than jc/jc + mlc field and the difference in caad of these fields depend on field size, beam energy and distance of jaw collimators from mlc field boundary. Scp of mlc only field for field sizes greater than 10 × 10 cm2 are comparatively smaller than jc field. Radiation transmitted through mlc alone at 18 MV is 2.66%, which is about 6 times higher than transmission through either of the jaw collimators (≤ 0.44%).
Comparative dosimetric analysis of Varian MLC only and jaw collimator shaped fields
NOVARIO, RAFFAELE;CONTE, LEOPOLDO;
2004-01-01
Abstract
Multileaf collimators (mlc) are now commonly used to shape treatment fields for conformal radiotherapy. In some of the clinical applications, a large area of extended leaves is uncovered. Such fields can be assumed to be mlc only defined fields. Percentage surface dose (psd), build up region, percentage depth dose (pdd), penumbra width (P80/20), central axis absorbed dose (caad) and total scatter factor (Scp) for radiation fields defined by jaw collimators (jc) only, jc and mlc together (jc + mlc), and mlc only were measured to quantify the differences in these parameters due to definition of fields by different combinations of jc/mlc. Measurements were carried out on a Varian Clinac 2100C medical linear accelerator and a Varian 80 leaf mlc with 6 (quality index, QI = TPR20,10 = 0.66) and 18 MV (QI = 0.78) photon beams. Radiation transmitted through jc alone, jc + mlc and mlc alone were also determined. psd and dose in build up region for mlc only fields are higher than jc/jc + mlc fields. Differences in psd of mlc only and jc/jc+mlc fields depend on beam energy and field size. These three types of fields have the same pdd while mlc penumbra (P80/20) is about 1 mm larger than jc penumbra. caad of mlc only field is also higher than jc/jc + mlc field and the difference in caad of these fields depend on field size, beam energy and distance of jaw collimators from mlc field boundary. Scp of mlc only field for field sizes greater than 10 × 10 cm2 are comparatively smaller than jc field. Radiation transmitted through mlc alone at 18 MV is 2.66%, which is about 6 times higher than transmission through either of the jaw collimators (≤ 0.44%).
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