Validation of Treatment Couch Top Modelling by Monaco Treatment Planning System

Abstract : Radiotherapy techniques such as Image-Guided Radiotherapy, Intensity-Modulated Radiotherapy, and Volumetric-Modulated Arc Radiotherapy require the delivery of radiation doses from posterior angles through the treatment couch top. As a result, the beam is attenuated, leading to dosimetric errors that compromise treatment outcomes and patient safety. Therefore, the treatment couch top must be properly modelled in the treatment planning system. The primary objective of this study is to validate the accuracy of the treatment couch top modelling in the Monaco Treatment Planning System and its implications for clinical practice. A 30 x 30 cm solid water phantom with a 0.6 cc Farmer-type ionization chamber positioned centrally in the phantom at depths 3 cm, 5 cm and 10 cm was utilized for attenuation measurements. The setups were irradiated with 100 MU using a SAD technique with a 6 and 15MV Synergy linear accelerator at gantry angles ranging from 1800 to ±1100 with an increment of 50, plus the respective parallel opposed beams. The beam field sizes ranged from 5 cm × 5 cm to 25 cm × 25 cm, with an increment of 5 cm. The CT data of the setups obtained by the Siemens CT machine were imported into the TPS and measurements performed on the LINAC machine were replicated with the TPS. Calculated doses of the TPS having the couch top inserted were compared to their measured counterparts using percentage dose deviation(%DDev). The maximum %DDev recorded was 2.82% and 2.92% for 6 MV and 15 MV beams, respectively. The couch attenuation varies inversely with the beam energy and field size, with maximum attenuation of 4.576% for 6 MV and 4.136% for 15 MV measurements taken at 10 cm depth with a 5 cm x 5 cm field size at gantry angle 1250. This couch effect is affected by phantom thickness. Beam angle orientation for maximum attenuation shifted from 1250 to 1100 when depth was varied from 10 cm to 5 cm and 3 cm. Attenuation values measured at 5 cm depth were relatively higher, maximum attenuation of 6.178% measured at this depth. The couch effect became less pronounced at 3 cm than at both 10 cm and 5 cm. Monaco TPS accurately simulated the couch effects, with similar observations made from the calculated attenuation and maximum %DDev of 2.92%. Caution needs to be taken during treatment planning to avoid the worst-case scenario of maximum beam attenuation depending on the tumour depth.