GAF film dosimetry of a tandem positioned beta-emitting intravascular brachytherapy source train

Monte Carlo dose calculations using MCNP4C and EGSnrc/BEAMnrc codes to study the energy dependence of the radiochromic film response to beta-emitting sources

The energy dependence of the radiochromic film (RCF) response to beta-emitting sources was studied by dose theoretical calculations, employing the MCNP4C and EGSnrc/BEAMnrc Monte Carlo codes. Irradiations with virtual monochromatic electron sources, electron and photon clinical beams, a (32)P intravascular brachytherapy (IVB) source and other beta-emitting radioisotopes ((188)Re, (90)Y, (90)Sr/(90)Y,(32)P) were simulated. The MD-55-2 and HS radiochromic films (RCFs) were considered, in a planar or cylindrical irradiation geometry, with water or polystyrene as the surrounding medium. For virtual monochromatic sources, a monotonic decrease with energy of the dose absorbed to the film, with respect to that absorbed to the surrounding medium, was evidenced. Considering the IVB (32)P source and the MD-55-2 in a cylindrical geometry, the calibration with a 6 MeV electron beam would yield dose underestimations from 14 to 23%, increasing the source-to-film radial distance from 1 to 6 mm. For the planar beta-emitting sources in water, calibrations with photon or electron clinical beams would yield dose underestimations between 5 and 12%. Calibrating the RCF with (90)Sr/(90)Y, the MD-55-2 would yield dose underestimations between 3 and 5% for (32)P and discrepancies within +/-2% for (188)Re and (90)Y, whereas for the HS the dose underestimation would reach 4% with (188)Re and 6% with (32)P.

[Investigations of beta-dosimetry at two different sources for the cardiovascular brachytherapy]

The intracoronary brachytherapy is used at the Hamburg University Hospital as a method to treat in-stent restenosis. Two different radiochromic film types were applied to obtain dosimetric information of the beta-sources used (32P and 90Sr/90Y). First, these films were analyzed for their suitability for dosimetry. Within the investigated dose range (MD-55-2: 0 to 33 Gy, HD-810: 0 to 105 Gy), both films showed a linear behavior between the dose and the optical density (OD). Because radiochromic films are subject to time-based changes in OD, a method for colour stabilization was investigated (RCS-method). This method allowed to greatly shorten the time between irradiation and evaluation from 24 hours (time necessary for the film to reach a quasi-stable status) to 2.5 hours. Colour-stabilized films can also be stored for a long time and reanalyzed with almost the same results. Within the limits of the measurements error, both film types showed an energy independent response. Within the dose profiles, analyses of the two source types resulted in differences of 13.5% (32P) and 21% (90Sr/90Y). These inhomogenities are consistent with the fabrication tolerances given by the manufactures.

Dosimetric consequences of manual pullback procedure for coronary artery radiotherapy with 90Sr/90Y beta-source

PURPOSE

This work presents a quantitative dosimetric analysis of the Novoste (90)Sr/(90)Y beta-source cardiovascular brachytherapy treatments using a manual pullback technique for patients with in-stent restenosis.

METHODS AND MATERIALS

Based on our previous measurements, a model was developed to estimate the dose in the middle of the junction region for tandem irradiation expressed as fraction of prescription dose (FPD) and dosimetric overlap length (DOL) receiving more/less than a threshold dose. The overlap/gap size was measured using the digital cine images recorded during treatment and then FPD and DOL were quantified.

RESULTS

Statistical analysis of 55 patients showed that the overlap size and the FPD at 2 mm radial distance were in range of 0 to 23 mm and 13-200% of prescription dose (Rx), respectively. Four gaps out of 76 pullback cases were found, but their size was at most 5 mm.

CONCLUSION

Use of a 5 mm overlap avoided underdosed regions in the vast majority of the cases. These results are the first step towards an analysis of the clinical outcome of these patients.

[Accuracy of measurement of radiochromic film density for high-energy X-rays]

Radiochromic film (RC-film) is of great interest as a film-type dosimeter for radiation oncology applications. We present a two-dimensional image-based evaluation of the measurement accuracy of a commercial RC-film product (Gafchromic MD-55-2 film, ISP TECHNOLOGIES, Inc.) by using a commercial Laser Densitometer (Model 1710, Computerized Medical Systems, Inc.) as an optical density imaging system. The coefficient of variation of the density (pixel-value) in one sample was approximately 3% to 11% at 3 Gy or less, and 3% or less at 4 to 60 Gy. Although the coefficient of variation between three samples at the same dose was about 14% at 1 Gy, it decreased as the dose increased, reaching several percent. In 1 to 6 Gy samples, geometric imaging artifacts [interference (moire) patterns] were observed, and it was found that scan-sampling pitch influenced the accuracy of measurement of the density of the sample. To improve the accuracy of density measurement, sufficient knowledge about characteristic features of the density measuring system is essential.