Influence of CT contrast agents on dose calculations in a 3D treatment planning system.

In treatment planning for conformal radiotherapy, it is possible to attain high accuracy in contouring the outline of the target volume and organs at risk by giving contrast agents (CAs) during the CT scan. In order to calculate the dose from the CT scans, Hounsfield units (HUs) are converted into the parameters of a standard set of tissues with given atomic composition and density. Due to the high atomic number of contrast media, high HU values are obtained during CT scanning. The Helax treatment planning system, for instance, erroneously takes them for high density tissue. This misinterpretation results in high absorption of high-energy photon beams and thus affects the dose calculation significantly. A typical bolus diameter of 3 cm and HU values of 1,400 cause an overdose of up to 7.4% and 5.4% for 6 MV and 25 MV photon beams, respectively. However, since the CA concentration and its expansion are rather low the effect on dose calculation in treatment planning is negligible.

192IR endovascular brachytherapy for avoidance of intimal hyperplasia after percutaneous transluminal angioplasty and stent implantation in peripheral vessels: 6 years of experience.

PURPOSE: Percutaneous transluminal angioplasty (PTA) with or without stent implantation is the accepted standard in the therapy of occlusive arterial disease. Despite improvements in the technique and medical equipment, there is still a restenosis rate of up to 40%. A high-dose-rate afterloading technique to avoid vascular stenosis or occlusion after PTA and subsequent stent implantation caused by intimal hyperplasia is presented with long-term results. METHODS AND MATERIALS: Intravascular brachytherapy with a 10-Ci 192Ir source was performed in cases of recurrent vascular occlusion or stenosis which appeared within 6 months after a previous PTA. After recanalization by PTA and stent implantation, a 9-Fr ReKa catheter was positioned within the stent to center the applicator with its tip 2 cm below the stent. This catheter served as a guide for a 5-Fr flexible applicator. After this procedure the isodose was calculated and a 12-Gy to 3-mm source distance was applied. The procedure was followed by 72 h of heparinization. RESULTS: From May 1990 to June 1996, 28 patients (21 male and seven female) were treated with endovascular brachytherapy. All patients had a clinically relevant restenosis or reocclusion of the arteria femoralis. Follow-up time ranged from 1 to 71 months. Twenty-eight patients had a sufficient follow-up time; 25 of these patients were examined. Twenty-one patients had treated vessel segments; four patients had no flow in the treated area. Two patients moved away with unknown addresses, and one patient died without any follow-up examination. Radiation-associated side effects were not notable. CONCLUSION: Intraluminal brachytherapy with 192Ir is a safe and useful procedure to avoid endovascular hyperplasia after transluminal percutaneous angioplasty.