SU-E-T-179: MVCB Pelvic Surface Dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Accurate skin dose estimation is crucial for radiotherapy planning.
• Megavoltage Conebeam CT (MV CBCT) is increasingly used for pelvic treatments.
• Clinically available dosimetry methods require validation for MV CBCT geometries.

Purpose of the Study:

• To compare the accuracy of commonly used clinical dosimetry methods for estimating skin dose in pelvic MV CBCT.
• To evaluate the performance of film, ion chamber, diode, and treatment planning system calculations.
• To identify potential improvements for skin dose assessment in MV CBCT.

Main Methods:

• Utilized a Siemens Oncor linac for 6 MV conebeam CT simulations.
• Employed XV-2 film, parallel plate ion chamber, and PDM diode for dose measurements.
• Compared measurements with Pinnacle 8.0m treatment planning system calculations.
• Used Optically Stimulated Luminescence dosimeters for surface dose distribution verification.

Main Results:

• All methods showed expected lateral dose asymmetry due to conebeam geometry.
• Diode measurements generally agreed with Pinnacle calculations at 3-5 mm depth.
• A ~5% higher diode dose was observed anteriorly, attributed to diode response and beam geometry.
• Skin dose at ~2 mm depth was found to be lower than calculated values.

Conclusions:

• Clinically available dosimeters can estimate pelvic skin dose in MV CBCT geometry.
• Corrections for measurement depth and beam geometry enhance dosimetry accuracy.
• Improved dosimetry supports safer and more effective radiotherapy treatments.