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

Objective. In high energy photon beams, the Spencer-Attix-Nahum (SAN) cavity integral can play an important role for the determination of absorbed dose in a medium of interest using a detector. This is due to the fact that the SAN cavity integral may serve as a substitute for absorbed dose. In order to justify this substitution, the relation between absorbed dose and the SAN cavity integral must be precisely known.Approach. The formalism for the SAN cavity integral consisting of two terms, a fluence term and a track-end term, was reviewed. The commonality to the quantity of restricted cema was emphasized. Since the SAN cavity integral is considered a dosimetrical quantity, the tool of Monte Carlo (MC) dose calculation is adopted to obtain numerical values for both, absorbed dose and SAN cavity integral. The codes FLURZ and DOSRZ from the EGSnrc system were used for that along with improving certain approximations made in FLURZ.Main Results. The relation between the SAN cavity integral and absorbed dose was exactly formulated and numerically reproduced by the MC calculation. Using a comprehensible example it is confirmed that there is a very high level of agreement between MC calculated SAN cavity integral and MC calculated absorbed dose, and that this applies independently from the choice of cutoff values used in the MC calculation.Significance. The theory which is dealing with the conversion of the absorbed dose in the sensitive volume of a detector into the dose without the detector in place is generally known as cavity theory. The well-founded substitution of absorbed dose by the SAN cavity integral can provide useful additional applications of this theory.