Year

2007

Degree Name

Master of Science (Research)

Department

School of Engineering Physics, Faculty of Science

Abstract

Independent monitor unit (MU) calculations are a vital part of radiotherapy treatment planning quality assurance. In the case of complex treatment planning methods, such as intensity modulated radiotherapy (IMRT), traditional independent monitor unit calculations using tables of beam data and manual calculations are inadequate. Recently, computer programs have been developed that can perform independent monitor unit calculations for IMRT treatment plans using scatter summation methods. One such program is RadCalc, produced by Lifeline Software Inc. The purpose of this project was to test RadCalc, and determine whether it is suitable for routine use in IMRT treatment planning quality assurance.

Once the software was installed, beam data measured on the treatment linear accelerator (linac) was imported into RadCalc, to be used in MU calculations. RadCalc was tested for data integrity to ensure that the correct data was accessed for its calculations. The interface between RadCalc and the treatment planning system, Pinnacle3, was set up so that treatment plan data could be imported directly from Pinnacle3 into RadCalc. Test plans were imported into RadCalc to ensure the Pinnacle3-RadCalc interface was working correctly.

Test plans were created with open, blocked, segmented and IMRT fields, and delivered to a phantom on the linac to test RadCalc’s block correction algorithm. Doses were measured using a thimble ionisation chamber, and compared to the doses calculated by RadCalc and Pinnacle3. The agreement between RadCalc and measured doses for most situations was comparable to the agreement between Pinnacle3 and measured doses. However, a systematic difference between RadCalc and measured dose was shown to occur for asymmetric fields. In addition to this, an increase in the level of blocking of the calculation point for segmented and IMRT fields appeared to increase the difference between RadCalc and measured dose.

Thirty-two patient IMRT plans at the Illawarra Cancer Care Centre (ICCC) were verified by reproducing the plan using a phantom CT dataset, and then delivering the fields to the phantom and measuring the delivered dose. This data was compared to the doses calculated by RadCalc and Pinnacle3. The doses calculated by RadCalc and Pinnacle3 for the plans created on patient CT datasets were also compared. In analysing the data, a systematic difference between RadCalc and measured dose was detected. Improved agreement was achieved by adjusting the MLC transmission parameter in RadCalc. The average percentage difference per field for the phantom plans between RadCalc and measured dose was 0.1% with a standard deviation 5.3%, while the average percentage difference between Pinnacle3 and measured dose was -0.2% with a standard deviation of 4.2%. The average percentage difference for total plan dose for the phantom plans between RadCalc and measured dose was 0.0% with a standard deviation 1.7%, while the average percentage difference between Pinnacle3 and measured dose was -0.3% with a standard deviation of 1.1%. For the patient plans, the average percentage difference per field between RadCalc and Pinnacle3 was 0.8% with a standard deviation of 5.6%, while the average percentage difference per plan was 1.1% with a standard deviation of 1.1%.

The final recommendation is that RadCalc is accurate enough for routine IMRT treatment planning quality assurance. A physical measurement should accompany the RadCalc check to verify the transfer of data to the record and verify system and the dose delivery process.

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