Optimization of Clarkson's Method for Calculating Absorbed Dose under Compensator Filters used in Intensity-modulated Radiation Therapy

Abstract

Background: Intensity Modulated Radiation Therapy (IMRT) is extensively used in the treatment of malignancies. Clarkson's method is one of the leading methods for dose calculation at open points present in irregular fields.

Objective: The aim of this study is to generalize the Clarkson's method for dose calculation at points under compensator filters in IMRT method and its application in IMRT quality control as well.

Material and methods: In this experimental study, compensator filters were designed in two forms: flat filter and block piled-up compensator. The measurements for the compensator filters and open fields in 5 and10 cm depths at energy levels (6, 10 and 18 MV) and in fields with different dimensions were performed using "Mapcheck2" dosimeter. The aim of performing calculations is to derive the theoretical dose by the generalized Clarkson's equation and comparing it with data resulted from the measurement for confirming the Clarkson's equation presented.

Results: These results demonstrate the data derived from the generalized Clarkson's method are in good agreement with the data resulted from measurement; the highest error of the proposed equation was 3% for flat filter, and less than 5% for block-piled-up filter. Higher error in the block-piled-up filter compared with the flat filter was due to the presence of leakage between these blocks.

Conclusion: The results of this study demonstrated that the presented equation is reliable and valid, and the proposed equation can be applied for dose calculation at all points under the compensator filter or the shielded areas.

Keywords: Absorbed Dose; Clarkson’s Method; Compensator; Radiation Dosage; Radiotherapy; Radiotherapy, Intensity-Modulated.

Figure 1

Clarkson’s drawing for the points existing in the field.

Figure 2

Schematic design of closed stepped wedge form fields ”brass cubic blocks with dimension 1 × 1 cm³ were arranged for making stepped wedge filter and Perspex blocks with dimension 1 × 1 cm³ were used for filling the empty spaces inside the compensator box made of aluminum”.

Figure 3

Comparison chart of theoretical and measured doses at 6 MV energy, 10 cm depth, 15 × 15cm² field dimensions, for (a) 1 cm thickness, (b) 5 cm thickness of flat compensator filter.

Figure 4

Comparison chart of theoretical and measured doses at 10 MV energy, 10 cm depth, 15 × 15 cm² field dimensions, for (a) 1 cm thickness, (b) 5 cm thickness of flat compensator filter.

Figure 5

Comparison chart of theoretical and measured doses at 18 MV energy, 10 cm depth, 15 × 15 cm² field dimensions, for (a) 1 cm thickness, (b) 5 cm thickness of flat compensator filter.

Figure 6

Comparison chart of theoretical and measured doses at 10 cm depth and 15 × 15 cm² field dimensions, at energies of (a) 6 MV, (b) 10 MV, and (c) 18 MV in block-piled-up compensator filter.