Design and characterization of an economical (192)Ir hemi-brain small animal irradiator

PURPOSE

To describe the design and dosimetric characterization of a simple and economical small animal irradiator.

MATERIALS AND METHODS

A high dose rate (HDR) (192)Ir brachytherapy source from a commercially available afterloader was used with a 1.3 cm thick tungsten collimator to provide sharp beam penumbra suitable for hemi-brain irradiation of mice. The unit was equipped with continuous gas anesthesia to allow robust animal immobilization. Dosimetric characterization of the device was performed with Gafchromic film measurements.

RESULTS

The tungsten collimator provided a sharp penumbra suitable for hemi-brain irradiation, and dose rates on the order of 200 cGy/minute were achieved. The sharpness of the penumbra attainable with this device compares favorably to those measured experimentally for 6 MV photons, and 6 and 20 MeV electron beams from a linear accelerator, and was comparable to those measured for a 300 kVp orthovoltage beam and a Monte Carlo simulated 90 MeV proton beam.

CONCLUSIONS

Due to its simplicity and low cost, the apparatus described is an attractive alternative for small animal irradiation experiments requiring steep dose gradients.

A study on the dose distributions in various materials from an Ir-192 HDR brachytherapy source

Dose distributions of ¹⁹²Ir HDR brachytherapy in phantoms simulating water, bone, lung tissue, water-lung and bone-lung interfaces using the Monte Carlo codes EGS4, FLUKA and MCNP4C are reported. Experiments were designed to gather point dose measurements to verify the Monte Carlo results using Gafchromic film, radiophotoluminescent glass dosimeter, solid water, bone, and lung phantom. The results for radial dose functions and anisotropy functions in solid water phantom were consistent with previously reported data (Williamson and Li). The radial dose functions in bone were affected more by depth than those in water. Dose differences between homogeneous solid water phantoms and solid water-lung interfaces ranged from 0.6% to 14.4%. The range between homogeneous bone phantoms and bone-lung interfaces was 4.1% to 15.7%. These results support the understanding in dose distribution differences in water, bone, lung, and their interfaces. Our conclusion is that clinical parameters did not provide dose calculation accuracy for different materials, thus suggesting that dose calculation of HDR treatment planning systems should take into account material density to improve overall treatment quality.

Numerical dosimetric reconstruction of a radiological accident in South America in April 2009

A severe irradiation accident involving a victim occurred in April 2009 in South America. The victim has found a (192)Ir source fallen from a gammagraphy device and has put it in the left pocket of his pants. Very quickly, an erythema and a blister appeared on the left leg of the victim involving hospitalisation. Following the request of the IAEA assistance, the Ionizing Radiation Dosimetry Laboratory of IRSN was asked to perform a numerical dosimetric reconstruction. A personalised voxel phantom of the victim has been constructed thanks to the Simulation of External Source Accident with Medical images tool developed by the laboratory, and a calculation of the dose with the MCNPX computer code allowed to determine the boundary of the necrotic dose at 25 Gy. On the basis of these calculations, the physicians have performed exeresis of the necrotic region on the left leg on 4 May 2009. Associated with mesenchymal stem cell injection, the leg of the victim was healthy on December 2009.

Toward endobronchial Ir-192 high-dose-rate brachytherapy therapeutic optimization

A number of patients with lung cancer receive either palliative or curative high-dose-rate (HDR) endobronchial brachytherapy. Up to a third of patients treated with endobronchial HDR die from hemoptysis. Rather than accept hemoptysis as an expected potential consequence of HDR, we have calculated the radial dose distribution for an Ir-192 HDR source, rigorously examined the dose and prescription points recommended by the American Brachytherapy Society (ABS), and performed a radiobiological-based analysis. The radial dose rate of a commercially available Ir-192 source was calculated with a Monte Carlo simulation. Based on the linear quadratic model, the estimated palliative, curative and blood vessel rupture radii from the center of an Ir-192 source were obtained for the ABS recommendations and a series of customized HDR prescriptions. The estimated radius at risk for blood vessel perforation for the ABS recommendations ranges from 7 to 9 mm. An optimized prescription may in some situations reduce this radius to 4 mm. The estimated blood perforation radius is generally smaller than the palliative radius. Optimized and individualized endobronchial HDR prescriptions are currently feasible based on our current understanding of tumor and normal tissue radiobiology. Individualized prescriptions could minimize complications such as fatal hemoptysis without sacrificing efficacy. Fiducial stents, HDR catheter centering or spacers and the use of CT imaging to better assess the relationship between the catheter and blood vessels promise to be useful strategies for increasing the therapeutic index of this treatment modality. Prospective trials employing treatment optimization algorithms are needed.

Radiochemotherapy and transplantation allow long-term survival for nonresectable hilar cholangiocarcinoma

Results of liver transplantation in the treatment of cholangiocarcinoma have been poor as a result of the high incidence of locoregional dissemination and tumor recurrence. This study evaluates the effect of neoadjuvant chemoradiation therapy combined with orthotopic liver transplantation in a carefully selected group of patients with hilar cholangiocarcinoma. Seventeen patients were included in the study. The neoadjuvant protocol included 6,000 cgy biliary brachy-therapy delivered through percutaneous transhepatic catheters and intravenous infusion of 5-fluorouracil (300mg/m2/day) until transplantation. Five of the 17 patients demonstrated tumor progression precluding transplantation. One patient died of sepsis on the waiting list. Eleven patients underwent liver transplantation, a median of 3.4 months (range = 1-26 months) after diagnosis. Five of the 11 (45%) are alive without evidence of tumor recurrence with a median follow up of 7.5 years (range = 2.8-14.5 years). Six deaths occurred in the transplanted patients. Tumor recurrence was responsible for two deaths at 10 and 18months, respectively, after transplantation. Three mortalities resulted from bacterial or fungal peritonitis and sepsis. One patient underwent re-transplantation for chronic rejection and died from graft failure resulting from hepatic artery thrombosis 16 months after diagnosis without evidence of tumor recurrence. Complications of transhepatic catheter placement included bile duct perforation (n = 4) and biliary-portal vein fistula (n = 1). All these patients died of tumor recurrence or sepsis. Cholangiocarcinoma should not be considered an absolute exclusion criteria for orthotopic liver transplantation. Long-term, tumor-free survival was achieved in 45% of the transplanted patients. Complications of biliary catheter placement for brachytherapy were associated with poor outcome.