A tissue-mimicking prostate phantom for 980 nm laser interstitial thermal therapy.

Purpose: To develop a phantom with optical and thermal properties matched to human prostate. This phantom will provide a platform for the development and characterization of 980 nm laser interstitial thermal therapy (LITT) systems. Methods: A polyacrylamide gel was doped with Naphthol Green B, Intralipid, and Bovine Serum Albumin (BSA). The necessary concentration of each ingredient was determined by measuring the optical properties via fluence measurements and light diffusion theory. LITT was then performed under the same conditions as a previous clinical trial in which temperature was monitored via a thermal probe. The thermal data and induced coagulation zone were compared to clinical data to illustrate the similarity between the phantom and patient. LITT was also performed under magnetic resonance thermometry (MRT). Results: The requisite concentrations of Naphthol Green B, Intralipid and BSA were found to be 0.144% (w/v), 8.06% (v/v) and 31.4% (v/v) respectively. In the native state, the absorption coefficient and reduced scattering coefficient ( µs' ) were found to be 0.66 ± 0.06 cm-1 and 8.27 ± 0.50 cm-1 respectively, with µs' increasing to 17.63 ± 1.41 cm-1 after coagulation. The thermal response of the phantom was similar to that observed clinically with maximum thermal probe measurements of 64.2 °C and 66.9 °C respectively. The shape of the induced coagulation zone was qualitatively and quantitatively similar to the MRT zone of elevated temperature and the coagulation zone observed clinically. Conclusions: A phantom which simulates optical and thermal response to 980 nm LITT was constructed and demonstrated to be similar to human prostate.

The law of unintended consequences in action: increase in incidence of hypokalemia with improved adequacy of dialysis.

In 1996, we raised our peritoneal dialysis (PD) dose to meet new DOQI adequacy targets. Concurrently, we noted an increase in the frequency of K+ levels below 3.5 mEq/L. A continuous quality improvement (CQI) project was initiated to quantify the impact of increasing dialysis dose on the prevalence of hypokalemia in our unit. Measurements of serum K+, blood urea nitrogen (BUN), creatinine, residual renal function, and the number and type of clinical interventions required to maintain eukalemia were abstracted from the charts of 62 patients enrolled in our program for more than 6 months and having more than two adequacy data points. In the seven consecutive 6-month periods from January 1996 to June 1999, dialysis dose progressively increased while median serum K+ decreased, and the percentage of patients requiring either diet counselling or K+ supplementation rose from 9% to 42%. We conclude that the increased clearance of K+ that occurs with increasing dialysis dose may lead to significant hypokalemia in a large proportion of PD patients dialyzed to DOQI adequacy targets. Maintenance of eukalemia in this population often requires increased K+ intake and or oral supplementation. Further studies are needed to ascertain whether the prevalence of hypokalemia is sufficient to warrant routine addition of K+ to PD dialysis solutions.