The evaluation of an algorithmic model, created for the image guided radiotherapy treatment couch for integration into the Pinnacle Treatment Planning System

Abstract

Introduction. It has been demonstrated that the radiotherapy treatment couch top affects the radiation beams traversing the couch top. These effects include a reduction in the dose at depth, and an increase in the surface dose. This research study involved the investigation of these effects at the study site, and the development and evaluation of an algorithmic couch model for the Varian Exact IGRT (Varian Medical Systems) couch top for incorporation into the Pinnacle3 treatment planning system (Philips Medical Systems). The aim was for the predicted doses at depth on the Pinnacle treatment planning system to be within 3% of the measured doses on the treatment unit. Materials and methods. The radiological properties of the treatment couch top were measured at the treatment unit with a 20 cm cylindrical phantom for the dose at depth, and a 30 x 30 cm square phantom for the surface doses. The PTW PinPoint and Farmer 0.6 cm3 detectors were used for the dose at depth, and nanoDotTM dosimeters for surface doses. The measurements were done for both 6 MV and 18 MV, as well as for various gantry angles, field sizes and different sections of the couch top. These measured doses at depth were then used to develop the couch model. Additional treatment plans with more complex geometries were used to verify the generalisability of the couch model algorithm. Furthermore, analysis was done to determine how the Pinnacle predicted doses compared with the measured doses if the couch model was ignored by Pinnacle. Results. The amount of attenuation of the dose by the couch ranged from 1.9% to 4.3% for 6 MV, and from 1.0% to 2.6% for 18 MV. The verification plans resulted in a maximum percentage difference between the Pinnacle predicted doses at depth and the measured doses for these plans of 2.03%. The measured surface doses due to the couch for 6 MV ranged from 86.45% to 98.39%, compared to 36.19% to 47.06% for open beams. For 18 MV it ranged from 60.53% to 83.45%, compared to 23.33% to 36.64% for open beams. All surface doses were reported as percentages of the beam dose at Dmax. If Pinnacle ignored the couch model, the percentage differences between the predicted doses at depth and the measured doses ranged from 1.85% to 4.14% for 6 MV, and from 0.93% to 2.43% for 18 MV. Pinnacle underestimated the surface doses by almost 50% for 6 MV and more than 50% for 18 MV if the couch model was ignored. Discussion. It was shown that the beam energy, field size, gantry angle, and couch section all had an impact on the effect of the treatment couch on the dose at depth, as well as the surface dose. It was also shown that if Pinnacle ignored the couch model then Pinnacle overestimated the doses at depth, and underestimated the surface doses, for all beams traversing the couch.

Conclusion. The study concurred that the treatment couch has an effect on the dose at depth and on the surface dose. It was determined that a couch model algorithm was required, and resulted in an improvement of the dose calculation accuracy of the Pinnacle predicted doses at depth to within 2.03% of the measured doses and an improvement in the predicted surface doses to within 10% of the measured surface doses.