SU-E-T-194: Evaluation and Simulation of Shallow Depth Skin Dose from Couch Top in Radiotherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Patient skin dose is clinically significant when the treatment couch-top contacts the skin.
• Radiation beams passing through the couch can alter dose distribution in the build-up region.

Purpose of the Study:

• To investigate the effect of a Varian carbon fiber exact couch-top on shallow depth dose.
• To evaluate the couch's contribution to patient skin dose in radiation therapy.

Main Methods:

• Dose measurements in the build-up region using a calibrated PTW chamber for 6MV and 18MV photon beams.
• Simulations in the Eclipse treatment planning system with adjusted couch CT numbers to match measurements.

Main Results:

• Surface dose increased with decreasing field size, with dose increases of 35-53% (6MV) and 50-113% (18MV) for field sizes from 20x20cm to 2x2cm.
• At 0mm depth, the dose difference due to the couch top reached 300%.
• The couch's effect on build-up dose extended to 1cm (6MV) and 2cm (18MV); optimal CT number is -200 to -300 HU.

Conclusions:

• Couch simulation in Eclipse improves dosimetric accuracy for couch-contacted skin dose.
• Accurate dose calculation requires consideration of surface definition, heterogeneity, resolution, and algorithm.