Experimental and Monte Carlo evaluation of Eclipse treatment planning system for lung dose calculations

Abstract

Background

In this study the accuracy of a pencil beam based treatment planning system (TPS) was evaluated for lung dose calculations by comparison with measurement and the Monte Carlo (MC) method.

Aim

In the current study we assessed the performance of the Eclipse treatment planning system in the thorax region by ionization chamber measurements and Monte Carlo calculations. We examined two analytic methods: modified Batho (MB) and equivalent tissue-air ratio “ETAR” methods for thorax region irradiations. For Monte Carlo calculations in the thorax phantom, we modelled a Varian Clinac 2100EX linac. After benchmarking our model with water phantom measurements we used this model for thorax phantom calculations.

Materials/Methods

8 and 15 MV photon beams of Varian 21EX linac were used for irradiations. Using MANP4C Monte Carlo code, the geometry of the linac head was simulated. After commissioning “MC” beam models, lung doses were calculated by the Monte Carlo (MC) method. Irradiation cases were: (1) posterior fields of single lung with field sizes of 4×4 and 10×10cm² (2) lateral fields of thorax with 4×4 and 10×10cm² field sizes.

Results

TPS calculations involving ETAR and MB methods were in close agreement with Monte Carlo results and measurements for a 10×10 cm² field size at both energies. For a field size of 4×4cm² the maximum differences in local dose between TPS calculations and measurement were +33% (MB) and +28% (ETAR). Also, they ignored lung dose reduction due to lateral electronic equilibrium for small field size. Similar results would be expected for other TPSs implementing these algorithms. MC calculations were in excellent agreement with measurement, showing local differences of no more than 2% for all measured points.

Conclusions

Our study findings showed great differences between both analytical methods and measurements for 4×4cm² field sizes for points in the lung. Our study recommends using the MC method for small-field lung dose calculations.