Absolute dosimetry in a d(14 MeV) + Be fast neutron beam

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiological Dosimetry

Background:

• The Universitätsklinikum Essen has utilized a d(14 MeV) + Be fast neutron beam for patient treatment since 1978.
• Accurate absorbed dose determination is critical for effective radiation therapy.
• Existing treatment planning systems may have inaccuracies in dose calculation for neutron beams.

Purpose of the Study:

• To perform precise dosimetric studies of a d(14 MeV) + Be fast neutron beam.
• To calibrate absorbed dose-to-water directly using a transportable water calorimeter.
• To assess the accuracy of the current treatment planning system in dose calculation for neutron therapy.

Main Methods:

• Utilized a transportable water calorimeter developed at the Physikalisch-Technische Bundesanstalt for direct absorbed dose-to-water calibration.
• Employed the twin detector method to separately determine photon and neutron doses.
• Transferred calibration from a cylindrical phantom to a larger water phantom using a calibrated ionization chamber.
• Combined experimental data with Monte Carlo calculations for comprehensive analysis.

Main Results:

• Achieved a relative uncertainty of 2.6% (1 SD) in absorbed dose-to-water determination.
• Demonstrated that the treatment planning system underestimates the physical dose to tissue by 9%.
• Identified the need to increase prescribed doses by 9% to maintain constant physical dose and achieve desired biological endpoints.

Conclusions:

• Direct absorbed dose-to-water determination by calorimetry provides a more accurate measure for fast neutron therapy.
• The findings necessitate adjustments in clinical dose prescription protocols to compensate for treatment planning system underestimation.
• Improved dosimetry enhances the precision and efficacy of fast neutron radiotherapy.