Ravinder Nath

During the past decade, permanent radioactive source implantation of the prostate has become the standard of care for selected prostate cancer patients, and the techniques for implantation have evolved in many different forms. Although most implants use or sources, clinical use of sources has also recently been introduced. These sources produce different dose distributions and irradiate the tumors at different dose rates. Ultrasound was used originally to guide the planning and implantation of sources in the tumor. More recently, CT and/or MR are used routinely in many clinics for dose evaluation and planning. Several investigators reported that the tumor volumes and target volumes delineated from ultrasound, CT, and MR can vary substantially because of the inherent differences in these imaging modalities. It has also been reported that these volumes depend critically on the time of imaging after the implant. Many clinics, in particular those using intraoperative implantation, perform imaging only on the day of the implant. Because the effects of edema caused by surgical trauma can vary from one patient to another and resolve at different rates, the timing of imaging for dosimetry evaluation can have a profound effect on the dose reported (to have been delivered), i.e., for the same implant (same dose delivered), CT at different timing can yield different doses reported. Also, many different loading patterns and margins around the tumor volumes have been used, and these may lead to variations in the dose delivered. In this report, the current literature on these issues is reviewed, and the impact of these issues on the radiobiological response is estimated. The radiobiological models for the biological equivalent dose (BED) are reviewed. Starting with the BED model for acute single doses, the models for fractionated doses, continuous low‐dose‐rate irradiation, and both homogeneous and inhomogeneous dose distributions, as well as tumor cure probability models, are reviewed. Based on these developments in literature, the AAPM recommends guidelines for dose prescription from a physics perspective for routine patient treatment, clinical trials, and for treatment planning software developers. The authors continue to follow the current recommendations on using and as the primary quantities, with more specific guidelines on the use of the imaging modalities and the timing of the imaging. The AAPM recommends that the postimplant evaluation should be performed at the optimum time for specific radionuclides. In addition, they encourage the use of a radiobiological model with a specific set of parameters to facilitate relative comparisons of treatment plans reported by different institutions using different loading patterns or radionuclides.

The methodology of brachytherapy source dosimetry with Gafchromic MD 55-2 film (ISP Technologies, Inc.) is examined with an emphasis on the nonlinearity of the optical density-dose relation within the dynamic dose range, the radial distance-dependent measurement uncertainty, and the format of data presentation. The specific source chosen for this study was a Checkmate (Cordis Corporation) intravascular brachytherapy system. The two-dimensional dose distribution around the source was characterized by a comprehensive analysis of measurement uncertainties. A comparative analysis of the dosimetric data from the vendor and from the scientific literature showed a substantial consistency of the information available for the Checkmate source. Our two-dimensional dosimetric data for the Checkmate source trains is presented in the form of measured along and away dose tables.

Nath, R., Bongiorni, P., Chen, Z., Gragnano, J. and Rockwell, S. Relative Biological Effectiveness of 103Pd and 125I Photons for Continuous Low-Dose-Rate Irradiation of Chinese Hamster Cells. Radiat. Res. 163, 501–509 (2005).Monolayers of Chinese hamster lung cells (CCL-16) in a polystyrene phantom were irradiated in vitro by 103Pd and 125I sources at dose rates of 6 to 72 cGy/h. Cell survival curves for acute high-dose-rate irradiation (over 30 Gy/h) were also measured using nearly monoenergetic X-ray beams which were designed to simulate the mean energies of photons emitted by 125I and 103Pd and also using a clinical 250 kVp X-ray beam. A profound dose-rate effect is observed over the dose-rate range of 6 to 20 cGy/h. An inverse dose-rate effect was observed for both radionuclides, with its onset occurring at a dose rate of about 20–30 cGy/h. The average RBE of 103Pd relative to 125I was determined to be 1.45 ± 0.07, 1.41 ± 0.07, 0.70 ± 0.07 and 1.49 ± 0.07 at dose rates of 6.9, 12.6, 19.0 and 26.7 cGy/h, respectively. Because 103Pd implants are generally prescribed at a higher initial dose rate (21 cGy/h) than the corresponding 125I implants (7 cGy/h), the effects of both dose rate and photon energy on biological response must be considered together. For the CCL-16 cells, the RBE of 103Pd at 19.0 cGy/h relative to that of 125I at 6.9 cGy/h was estimated to be 2.3 ± 0.5.

Purpose: To measure the relative biological effectiveness (RBE) of continuous low dose rate irradiation (CLDRI) using ¹⁰³Pd sources relative to acute high dose rate irradiations (AHDRI) from a 250 kVp x-ray beam and an x-ray beam having an equivalent mono-energetic photon energy equal to the average energy of the ¹⁰³Pd source for BA1112 rhabdomyosarcoma cells. Materials and methods: A customized ¹⁰³Pd irradiator was built to provide CLDRI using ¹⁰³Pd at different dose rates relevant to clinical interstitial brachytherapy to BA1112 rhabdomyosarcoma cells growing in exponential phase in culture. A special x-ray beam that simulates the photon energies emitted by the ¹⁰³Pd source was also developed to provide acute high dose rate irradiation at those energies. Cell survival curves from different irradiation conditions were measured. The RBE with respect to AHDRI using standard 250 kVp x-rays was determined from the doses required to achieve a cell surviving faction of 0.01. Results: For acute irradiation, the RBE of the x-rays simulating ¹⁰³Pd was 1.24 relative to 250 kVp x-rays. A profound dose rate effect was observed at low dose rates in the range of 6.8 – 14.4 cGy/h that are typical of permanent interstitial brachytherapy. At cell-surviving fraction of 0.01, the RBE of CLDRI at 6.8 and 14.4 cGy/h using ¹⁰³Pd sources was reduced by a factor of 3 and 2, respectively, relative to the acute exposure. This observation is in good agreement with recent in vivo tumor cure studies performed on BA1112 tumor. Conclusion: The relative biological effectiveness of the photons emitted by ¹⁰³Pd depends on both the linear energy transfer (LET) of the low energy photons and the dose rate of the irradiation. The higher LET of ¹⁰³Pd photons is biologically more effective in killing BA1112 tumor cells compared to conventional 250 kVp x-rays when both are delivered at the same dose rate. But the gain in RBE that results from the higher LET can be quickly negated by the reduced dose rate of the irradiation.

The dosimetric characteristics were measured for two types of I 125 low-energy photon-emitting sources by using a wide and highly sensitive thermoluminescent (TL) sheet film, which was developed for two-dimensional dose distribution measurements. The TL film is made of Teflon homogeneously mixed with small powders of thermoluminescence ( Ba S O 4 :Eu doped). Various dosimetric parameters (i.e., radial dose function, 2D and 1D anisotropy functions) of model 6711 and 6702 I 125 sources were obtained at various distances from the source surfaces to 15 mm . These parameters obtained with TL sheet were compared with the data recommended in the updated AAPM TG-43 report. The radial dose functions measured with TL sheet are in agreement with those established data of model 6711 I 125 seed and model 6702 I 125 seed at most of the distances within 5% and 7%, respectively. All the measuredanisotropy functions showed symmetry about the longitudinal source axis. The anisotropy of dose distributions was clearly present in the immediate vicinity of the source edges. The measured 2D anisotropy function values at 1 cm are in reasonably good agreement with the recommended values. The differences at two points in the 1D anisotropy functions measured with TL sheet and the established data at 1 cm from source center were 0.7% and 1.9% for model 6711 and 6702 I 125 sources, respectively; the differences at 0.5 cm were 1.5% and 1.7% for model 6711 and 6702 I 125 sources, respectively. The relative dosimetric characteristics in the vicinity of actual interstitial brachytherapy sources containing I 125 have been experimentally determined by using the TL sheet as a 2D dosimeter.

The dose distributions in the bifurcated vessels treated with a catheter-based delivery system are complicated by the geometry of bifurcation consisting of a main and a branch vessel at different angles, and it is difficult to generate satisfactory dose distributions. We hypothesize that increasing the number of dwell positions can result in better dose distributions. An optimization method based on the simulated annealing was developed to demonstrate the validity of this idea. In this method, the source in the branch treatment was allowed to take up to five dwell positions. A cost function was constructed to deliver the prescription dose to the planning targets with penalties for both overdosing and underdosing. By using the optimization algorithm, it was found that for 90 degrees to 60 degrees bifurcated vessels an optimized single dwell position treatment scheme can be as effective as the schemes that include up to five dwell positions. As the bifurcation angle becomes smaller than 60 degrees, the dose distributions generated with the single dwell position scheme become less satisfactory than the more complicated treatment schemes with multiple dwell positions. By using a three-dwell-position treatment scheme for the 192Ir source, the overdosing can be kept under 166% even at a bifurcation angle of 20 degrees.