Technical note: relation between the Spencer-Attix-Nahum cavity integral and absorbed dose

Area of Science:

• Medical Physics
• Radiation Dosimetry
• High Energy Photon Beams

Background:

• The Spencer-Attix-Nahum (SAN) cavity integral is crucial for determining absorbed dose using detectors in high energy photon beams.
• Understanding the precise relationship between absorbed dose and the SAN cavity integral is essential for its substitution.
• Cavity theory explains dose conversion from detector sensitive volume to the medium without the detector.

Purpose of the Study:

• To review the formalism of the Spencer-Attix-Nahum (SAN) cavity integral.
• To numerically determine the relationship between absorbed dose and the SAN cavity integral using Monte Carlo methods.
• To validate the substitution of absorbed dose by the SAN cavity integral in high energy photon dosimetry.

Main Methods:

• Reviewed the two-term formalism of the SAN cavity integral (fluence and track-end terms).
• Emphasized the commonality with restricted cema.
• Employed Monte Carlo (MC) dose calculation using FLURZ and DOSRZ codes from the EGSnrc system.
• Improved approximations within the FLURZ code.

Main Results:

• An exact formulation of the relationship between the SAN cavity integral and absorbed dose was achieved.
• Monte Carlo calculations numerically reproduced this relationship.
• High agreement was observed between MC-calculated SAN cavity integral and absorbed dose, independent of cutoff values.

Conclusions:

• The Spencer-Attix-Nahum (SAN) cavity integral can be reliably substituted for absorbed dose in high energy photon beams.
• This finding validates and extends the applications of cavity theory in radiation dosimetry.
• The study provides a precise formulation and numerical confirmation of this dosimetric relationship.