The new U.S. measurement standard for the air-kerma strength from low-energy photon-emitting brachytherapy seed sources is formally described in detail. This instrument-based standard was implemented on 1 January 1999, with its salient features and the implications of differences with the previous standard given only through a series of informal communications. The Wide-Angle Free-Air Chamber (WAFAC) is specially designed to realize air kerma from a single-seed source emitting photons with energies up to about 40 keV, and is now used to measure the wide variety of seeds used in prostate-cancer therapy that has appeared in the last few years. For the two 125I seed models that have been subject to both the old and new standards, the new standard reduces the air-kerma strength by 10.3 %. This change is mainly due to the removal of the influence on the measurement of the Ti K x rays produced in the source encapsulation, a component with no clinical significance.

Characterization measurements and calculations were performed on a new medical seed developed by IsoRay Inc. in Richland, Washington, that utilizes the short-lived isotope 131Cs. This model has recently received FDA 510(k) clearance. The objective of this work was to characterize the dosimetric properties of the new seed according to the AAPM Task Group 43 recommendations. Cesium-131 is a low-energy x-ray emitter, with the most prominent peaks in the 29 keV to 34 keV region. The intended application is brachytherapy for treating cancers in prostate, breast, head and neck, lung, and pancreas. The evaluations performed included air-kerma strength, radial dose function, anisotropy in phantom, half-life, energy spectra, and internal activity. The results indicate the CS-1 seeds have a dose-rate constant of 0.915 cGy hr(-1) U(-1) in water, dose penetration characteristics similar to 125I and 103Pd, anisotropy function values on the order of 0.71 at short distances and small angles, and an average anisotropy factor of 0.964. The overall dosimetric characteristics are similar to 125I and 103Pd seeds with the exception of half-life, which is 9.7 days, as compared to 17 days for 103Pd and 60 days for 125I. The shorter half-life may offer significant advantages in biological effectiveness.

A ne w t ype of 3-bod y calorim ete r for meas ul-in g absorb ed dose produced by ionizing radiation is described in de tail. All three bodi es ri se in te mperature during irradi ation, and the heat absorbed by th e ce ntral core is meas ured by sta ndard mea ns. Only th e centra l co re is heated during electrical calibration, but th e in c reased heat lo sses a re co mpe nsated by measuring mos t of the hea t lost to the s urro undin g jac ke t a nd a uto mati cal ly ad din g it to the heat retain ed by the co re. Th e th;rd body is a mass ive, the rmall y-Roatin g shi eld , whose prese nce red uces th e heat losses durin g irra diation , with a c onsequent in c rease in se nsitivit y and stability. A mathe matical desc ription of th e calorime te r beh avio r is prese nted , along with a di sc ll ssion of co ntro l a nd operation tec hn ique. In parti c ul a r, it is show n h ow thi s 3-bod y calorim eter ca n be calibra ted as a I -body calorim e te r, with la rge hea t losses, or as a 2-bod y calorim e te r, in th e quasi-adi abati c mode. This ca lorime ter design de c reases th e effec ts of thermal gradi e nts a nd a t th e same tim e provides th e mea ns to test for th ese e ffe cts. The results of these tes ts show th at for th is parti cul ar mod el , sys te mati c e rrors ca used by th er mal gradi e nts, durin g electri cal measureme nts, a re no la rge r than 0.1 perce nt. Errors in co mparing a n electri ca l run with an irradiati o n may be so mewh a t large r because of differe nt te mperature gradi e nts within the sys te m. It is also point ed out th a t th e ge ne ral design of thi s calo rim ete r is not re stricted to meas urin g a bso rbed dose but ca n be appli ed to cal orim etry in ge neral. K ey word s: Absorbed dose; calorim eter; heat-loss-co mpe nsati on; the rm al grad ie nts.

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The ab sorbed dose in graphite fro m a cobalt·60 gamma·ray source was measured with a s pheric al air·filled cavity ionization chamb er and with two s ph eri cal calorim eters. The ins trum ent s were constructed from hig h·purity graphite . The curre nt per unit mass of air and the ab sorbed pow er per unit mass of graphite were determined with un certainties of 0.40 and 0.17 perce nt respectively. Whe n the two res ults are combined the value of Walr • ~m is found to be 33.72 electron volts in graphite per ion pair in air, with an uncerta int y of ± 0.1 4 electron volts.