Measurement of density and calcium in human atherosclerotic plaque and implications for arterial brachytherapy

Purpose. To measure density of arterial plaque specimens for purposes of improving calculation of intravascular radiation dose.Methods and Materials. In the described technique, the mass of the specimen submerged in water is compared with its mass in air. Thirty-three plaque specimens harvested from cadavers and subsequently histologically classified (18 coronary, 15 noncoronary) were subjected to density measurement, and were also assayed for calcium using inductively coupled plasma optical emission spectroscopy (ICPOES). A dose point kernel (DPK) computer model extended to heterogeneous media is used to determine delivered dose to tissues for stents labeled with ³²P, ¹⁰³Pd, and ¹³¹Cs, based on measured density values.Results. Plaque specimens identified histologically as noncalcified (non-class VII) had an average density of 1.22 ± 0.03 g/cm³ (n = 19). Plaque specimens identified as calcified (Class VII) had an average density of 1.45 ± 0.06 g/cm³ (n = 13). Density of calcified portions of plaque may be even higher because plaque specimens are heterogeneous. Plaque density was found to be correlated with calcium weight percentage (R² = 0.67) and histologic percent area calcification (R² = 0.58). Significant variations in calculated dose were found according to isotope, plaque density, and plaque thickness. The assumption of an “all water density” dose model overestimates dose to tissues. For 1-mm thick calcified (class VII) plaque, computed dose to tissues (via DPK model) are decreased by 29%, 34%, and 15%, for ³²P, ¹⁰³Pd, and ¹³¹Cs stents, respectively, compared with an “all water density” assumption model, when density is taken into account. Similar decreases are expected for catheter-based brachytherapy systems using beta or low energy (< 100 keV) gamma sources.Conclusions. This work has importance for radioactive stents and catheter-based brachytherapy due to dependence of dose on density at distances between 0.1 mm and 3 mm away from the radiation source. This dependence is important for both beta- and gamma-based systems.