Prog Med Phys.  2021 Dec;32(4):137-144. 10.14316/pmp.2021.32.4.137.

Development of a Beam Source Modeling Approach to Calculate Head Scatter Factors for a 6 MV Unflattened Photon Beam

• Park SY
• Choi N
• Jang NY

• KMID: 2524160
• DOI: http://doi.org/10.14316/pmp.2021.32.4.137

Abstract

Purpose
This study aimed to investigate the accuracy of head scatter factor (S _c ) by applying a developed multi-leaf collimator (MLC) scatter source model for an unflattened photon beam.
Methods
Sets of S _c values were measured for various jaw-defined square and rectangular fields and MLC-defined square fields for developing dual-source model (DSM) and MLC scatter model. A 6 MV unflattened photon beam has been used. Measurements were performed using a 0.125 cm ³ cylindrical ionization chamber and a mini phantom. Then, the parameters of both models have been optimized, and S _c has been calculated. The DSM and MLC scatter models have been verified by comparing the calculated values to the three S _c set measurement values of the jaw-defined field and the two S _c set measurement values of MLC-defined fields used in the existing modeling, respectively.
Results
For jaw-defined fields, the calculated S _c using the DSM was consistent with the measured S _c value. This demonstrates that the DSM was properly optimized and modeled for the measured values. For the MLC-defined fields, the accuracy between the calculated and measured S _c values with the addition of the MLC scatter source appeared to be high, but the only use of the DSM resulted in a significantly bigger differences.
Conclusions
Both the DSM and MLC models could also be applied to an unflattened beam. When considering scattered radiation from the MLC by adding an MLC scatter source model, it showed a higher degree of agreement with the actual measured S _c value than when using only DSM in the same way as in previous studies.

Keyword

Head scatter factor; Unflattened photon beam; Multi-leaf collimator; Dual-source model

Figure

• 

  Fig. 1 Schematics of the geometrical relationship between the jaws and multi-leaf collimators (MLCs) in terms of the beam’s eye view, detector’s eye view, and scatter interface for (a) unaffected area, (b) affected area, and (c) head scatter factor (Sc) as a function of the MLC-defined square field size ranging from 4×4 to 15×15 cm2 at a fixed jaw setting of 15×15 cm2. The calculated Sc values are derived from the dual-source model.

• 

  Fig. 2 Comparison between the calculated and measured head scatter factor (Sc) values for (a) square fields ranging from 4×4 to 40×40 cm2; (b) rectangular fields with one pair of jaws fixed at 10 cm, while the other pair varied from 4 to 40 cm; (c) rectangular fields with one pair of jaws fixed at 4 cm while the other pair varied from 4 to 40 cm; and (d) rectangular fields with one pair of jaws fixed at 40 cm, while the other pair varied from 4 to 40 cm. The calculated Sc values are derived from the dual-source model.

• 

  Fig. 3 Comparison between the measured and calculated head scatter factor (Sc) values for fixed jaw defined fields of (a) 10×10 cm2, (b) 15×15 cm2, and (c) 20×20 cm2. Sc is calculated based on the dual-source model (DSM) and DSM in conjunction with the multi-leaf collimator (MLC) scatter source (DSM+MLC).

Reference

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