Commissioning of Electron Monte Carlo Dose Calculation Algorithm for Rectangular Shaped Cutouts

8-9-2021

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Allied Health Sciences

Azizur R Molla

2020/2021

Abstract

Introduction

The Monte Carlo method is the statistical approach for calculation of dose distribution and provides the most accurate calculation. The previous research found that electron Monte Carlo (eMC) calculated cutout factors had overall good agreement with measured values. However, this work was limited to a select number of cutouts, electron energies and treatment geometries. The purpose was to evaluate the accuracy of cutout factors determined by Varian Electron Monte Carlo (eMC, Eclipse V15.6) and Radformations ClearCalc (CC, V1.7.6) when compared to measured values for a wider variety of cutout shapes, electron energies and treatment geometries. This study performed to evaluate the eMC with commissioning purpose for a clinical electron treatment.

Methods

To accomplish commissioning of eMC, the cutout factors measured were compared to the values determined by eMC and CC. A total of 825 cutout factors from 55 different electron blocks were evaluated. The calculation by eMC and CC as well as physical measurements at Varian Trilogy was performed with various rectangular shaped cutouts according to the combination of energies (6MeV, 9MeV, 12MeV, 16MeV, 20MeV), SSDs (100cm. 105cm, 110cm). Two ion chambers were used for measurements. The micro chamber was employed for small fields to overcome the limitation of the detector size of the Farmer chamber. To avoid partial volume effects, the chamber was placed parallel to the longitudinal side of the field. The ion chambers were placed in solid water and additional solid water was added so that the chamber was at a depth of 1.2 cm for 6 MeV and 2.5 cm for 9-20 MeV.

Results

To evaluate the eMC, the cutout factors measured were compared with the ones calculated by eMC and CC respectively. The differences were within 5% above 4x4 field sizes regardless of energies or SSDs, and most of them were within 3%. The greater discrepancies were observed at the smaller side of field size less than 4 cm. 5 out of 825 cutout factors had more than 5% discrepancies. The mean percent differences between the measured cutout factors and the cutout factors calculated by eMC and CC for a specific SSD were within 3.5% regardless of the SSD or electron energy. When the cutout factors measured or calculated at a specific SSD were compared to other SSDs, larger discrepancies were observed for smaller field sizes and for lower electron energies. As field size and electron energy increased, the discrepancy in cutout factors measured or calculated at one SSD versus the other were reduced.

Conclusion

The results supported the clinical practice of using eMC to calculate the cutout factors for those cases with 4% uncertainties in our clinical machine. For the field size that has the smallest dimension of 3 or 4cm, COFs are required to manually measure to reduce the uncertainties. The analysis of the mean percent differences between measurement and calculations of eMC or CC by SSDs provides evidence that eMC effectively calculated the MUs taking the correct inverse square law effect factors in our clinic machine. The discrepancies by energies occurred since the measurement was conducted at the same depth of 2.5cm with 9 to 20MeVe while eMC and CC were calculated at Dmax. Further study is required to verify the discrepancies with less uncertainties by the energy as measuring the dose at Dmax for each energy. The application of eMC is expected to increase efficiency as saving the planning time, and manpower but also it would make it possible to have precise, accurate planning for the present and future.