Technical Note: Empirical altitude correction factors for well chamber measurements of permanent prostate and breast seed implant sources.

Purpose: Previous studies in the literature have measured an altitude effect for low-energy brachytherapy seeds; a correction factor applied in addition to P to account for the breakdown of Bragg-Gray cavity theory at low energies in well-type ionization chambers. In clinical practice, many centers use altitude correction factors that are not seed-model-specific. The purpose of this work is to present altitude correction factors for several seed models without documented factors in the literature.

Methods: An in-house constructed pressure vessel was used with a well-type ionization chamber to measure the air-kerma strength of the IsoAid Advantage (Pd-103), Theragenics AgX100 (I-125), and Nucletron selectSeed (I-125) at a pressure range representative of those encountered worldwide. The TheraSeed 200 (Pd-103) was also measured for comparison to the originally published correction factor for validation of the experimental process. When correction factors derived in this work were within experimental uncertainties of those published, no new correction factors were proposed.

Results: The three seed models measured herein all demonstrated a similar response to change in pressure as previously documented in the literature with the HDR 1000 Plus well-type ionization chamber. Correction factors of the functional form PA=k1(P[torr])k2, consistent with those previously published, were found to be appropriate for these seed models. A new correction factor is proposed for the Theragenics AgX100 and Nucletron selectSeed (k  = 0.0417, k  = 0.479). The IsoAid Advantage, however, agreed to within uncertainty with the published altitude correction factor for the TheraSeed 200; thus the application of the same correction factor is appropriate (k  = 0.0241, k  = 0.562).

Conclusions: This work presents altitude correction factors for three permanent implant brachytherapy seed models in clinical use. This will allow clinics to utilize model-specific factors, reducing systematic errors in their air-kerma strength verifications.