Journal List > Prog Med Phys > v.25(1) > 1098435

Lee, Choi, Oh, Jeun, Lee, Kim, Kim, Oh, Kim, and Yea: The Output Factor of Small Field in Multileaf Collimator of 6 MV Photon Beams

Abstract

The IMRT is proper implement to get high dose deliver to tumor as its shape and selective approach in radiation therapy. Since the IMRT is performed as modulated the radiation fluence by the MLC created the open shapes and its irradiation time, the dose of segment of radiation field effects on the cumulated portal dose. The accurate output factor of small and step shape of segment is important to improve the determination of deliver tumor dose as it is directly proportional to dose. This experiment performed with the 6 MV photon beam of Clinac Ex(Varian) from 3×3 cm2 to 0.5×0.5 cm2 small field size for collimator jaw in MLC free and/or for MLC open field in fixed collimator jaw 10×10 cm2 using the CC01 ion chamber, SFD diode, diamond detector and X-Omat film dosimetry. As results of normalized to the reference field of 10×10 cm2 of MLC, the output factor of 3×3 cm2 showed 0.899±0.0106, 0.855±0.0106 for 2×2 cm2, 0.764±0.0082 for 1×1 cm2 and 0.602±0.0399 for 0.5×0.5 cm2. The output factor of MLC open field has shown a maximum 3.8% higher than that of the collimator jaw open field.

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Fig. 1.
Showing the small field size of MLC in cm for 10×10 cm2 collimator jaw. The pictures showed (a) 1×1 cm2 and (b) 0.5×0.5 cm2 on SAD 100 cm and SPD 95 cm.
pmp-25-15f1.tif
Fig. 2.
Showing the output factors of small fields in 6 MVX rays using the SFD diode, Xomat film, diamond chamber and CC13 ion chamber used for reference output dose. The field size delivered from the collimator jaws in cm for MLC free (a) and the MLC open in collimator jaws set of 10×10 cm2 (b).
pmp-25-15f2.tif
Table 1.
The averaged output factors obtained as function of the collimator field size for the MLC free.
  Jaw field (cm2) Output factor
MLC free 10×10 1.000±0.0000
5×5 0.920±0.0100
3×3 0.866±0.0093
2×2 0.830±0.0096
1×1 0.741±0.0022
0.5×0.5 0.550±0.0240

The averaged output factors were delivered from the CC01 ion chamber, SFD diode chamber, diamond chamber and Xomat film dosimetry.

Table 2.
The averaged output factors obtained as function of the MLC field size for the Collimator 10×10 cm2.
  MLC field (cm2) Output factor
JAW 10×10 cm2 10×10 1.000±0.0000
5×5 0.955±0.0035
3×3 0.899±0.0106
2×2 0.855±0.0106
1×1 0.764±0.0082
0.5×0.5 0.602±0.0399

The averaged output factors were delivered from the CC01 ion chamber, SFD diode chamber, diamond chamber and Xomat film dosimetry.

Table 3.
Comparison the output factors of experiments to that of derivered' from TPS.
  Jaw field (r) TPS∗ MUGy CBSF (r) Dnorm (X,Y)10 TMR∗∗ (r,10) TPS OPF Experiment
OPF diff (%)
MLC free 10×10 126 1.000 1.000 0.775 1.000 1.000 0.0%
5×5 139 0.979 0.887 0.737 0.949 0.955 -0.6%
3×3 146 0.950 0.820 0.707 0.942 0.899 4.8%
2×2 154 0.930 0.761 0.689 0.915 0.855 7.0%
1×1 180 0.909 0.636 0.662 0.815 0.764 6.7%
0.5×0.5 - - - - - 0.602 -

TPS∗ MUGy represents MU for 1 Gy at 10 cm depth in a given jaw field(r) and TMR∗∗ was derivered from PBC modeling in Eclipse TPS.

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