Water equivalent path length measurement in proton radiotherapy using time resolved diode dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Proton radiotherapy requires precise verification of the water equivalent path length (WEPL) to ensure accurate dose delivery.
• In vivo dosimetry methods are crucial for real-time verification of treatment parameters before patient exposure.

Purpose of the Study:

• To validate the use of time-resolved in vivo diode dosimetry for pre-treatment verification of WEPL in proton radiotherapy.
• To assess the accuracy and feasibility of this method for adjusting proton beam energy.

Main Methods:

• A passively scattered, range-modulated proton beam was used with a diode and fast current-to-voltage amplifier.
• Diode output was recorded at various depths in a water tank, observing bursts of proton pulses.
• The root-mean-square duration of these bursts was computed and fitted with a cubic polynomial for calibration.

Main Results:

• The method accurately determined unknown depths within 0.3 mm root-mean-square.
• A calibration curve derived from the polynomial allowed for precise depth determination.

Conclusions:

• Diode dosimetry, potentially integrated with devices like rectal balloons, can determine in vivo WEPL with submillimeter accuracy prior to proton therapy.
• This allows for beam energy adjustments, with minimal associated dose (approx. 0.2% of a single fraction dose).