Monte carlo dose calculations in conventional thorax fields for 60Co photons.

Purpose: Our purpose was to apply the MCNP4C Monte Carlo (MC) code for dose calculations in the thorax region and compare the results with those of measurements and a conventional treatment planning system (TPS).

Materials And Methods: We modeled a Theratron 780E 60Co unit and benchmarked our modeling with percent depth doses (PDDs), beam profiles, and output factors measured in a water phantom. For PDDs and beam profiles, the differences between measurements and MC calculations were less than 1% and 2%, respectively. We used an anatomic thorax phantom for evaluation of a conventional TPS and our MC calculation results.

Results: In comparing the results of calculations and measurements for our thorax geometries, the errors of conventional and MC methods were 20% and 2%, respectively. For the anterior mediastinal field and large thorax field the accuracy of the conventional method was acceptable, but for small fields of lateral thorax irradiation, the error of the conventional method was as high as to 20%. In all MC calculations, discrepancy from the measurements was less than 2%.

Conclusion: Our results showed that the MCNP4C MC code could be used in dose calculations in treatment planning for 60Co photon irradiation. In addition, the application of the MC method for dose calculations in radiotherapy with 60Co photons was recommended.