Improved stoichiometric model for megavoltage computed tomography number conversion and dose calculation within the TomoTherapy delivery system

Abstract

Purpose

This study investigated a novel stoichiometric computed tomography (CT) number calibration (SCC) model for megavoltage CT (MVCT). This model was specifically designed to convert CT numbers to mass densities (MDs) for dose calculations when using the TomoTherapy MVCT radiotherapy delivery system.

Materials and methods

The MVCT-SCC model extended the conventional SCC model originally developed for kilovoltage CT by incorporating additional parameters to account for pair production and water-equivalent CT number corrections, which are specific to MVCT applications. The accuracy of the model was evaluated through a benchmark test that involved comparing its performance to those of three conventional models using a Catphan700 phantom equipped with 10 density plugs and air. The MVCT numbers derived from the MVCT-SCC and conventional models were assessed by comparing them to the actual measured values using the root mean square error (RMSE) in Hounsfield units (HU). Additionally, potential percentage estimated mean dose error (PMDE) discrepancies were quantified based on the tolerances specified in the CT-MD conversion table and the observed RMSE values.

Results

The RMSE for the MVCT numbers calculated with the MVCT-SCC model was significantly lower, at 10.8 HU, compared to values of >35.3 HU for the conventional models. Furthermore, the PMDE when using the MVCT-SCC model was within 1 %, which was significantly better than the differences of >2 % observed with the conventional models.

Conclusion

The results of the benchmark test confirmed that the MVCT-SCC model significantly enhanced the accuracy of the calculated HU numbers and dose estimates compared to conventional models.