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PubMed1984

Relative biological effectiveness of 125I sources in a murine brachytherapy model.

The relative biological effectiveness of 125I and 192Ir has been determined in a murine brachytherapy model that uses a clonogenic cell assay as the end point. Removable 125I or 192Ir sources were implanted at right angles to the surface of RIF-1 tumors grown in the flanks of C3H/He mice. After irradiation for 1-5 days, mice were sacrificed and isodosed annuli of irradiated tumor tissue were sampled for the clonogenic cell assay. The slopes and intercepts of the two radiation survival curves for implanted sources with activities of 10 to 50 Gy for 125I (dose rate = 39.8 +/- 4.3 cGy/hr) and 192Ir (dose rate = 42.3 +/- 2.7 cGy/hr) were identical; the relative biological effectiveness was 1.

Animals↗
PubMed2004

A phenomenological model for the relative biological effectiveness in therapeutic proton beams.

To study the effects of a variable relative biological effectiveness (RBE) in inverse treatment planning for proton therapy, fast methods for three-dimensional RBE calculations are required. We therefore propose a simple phenomenological model for the RBE in therapeutic proton beams. It describes the RBE as a function of the dose, the linear energy transfer (LET) and tissue specific parameters. Published experimental results for the dependence of the parameters alpha and beta from the linear-quadratic model on the dose averaged LET were evaluated. Using a linear function for alpha(LET) in the relevant LET region below 30 keV per micrometre and a constant beta, a simple formula for the RBE could be derived. The new model was able to reproduce the basic dependences of RBE on dose and LET, and the RBE values agreed well with experimental results. The model was also applied to spread-out Bragg peaks (SOBP), where the main effects of a variable RBE are an increase of the RBE along the SOBP plateau, and a shift in depth of the distal falloff. The new method allows fast RBE estimations and has therefore potential applications in iterative treatment planning for proton therapy.

Animals↗
PubMed1982

[Relative biological effectiveness (RBE) of low-dose californium-252].

Relative biological effectiveness (RBE) of 252Cf, with respect to 192Ir, has been determined at the low dose rates commonly used in interstitial and intracavitary therapy. The biological criterion was growth reduction in Vicia faba bean roots. Two varieties of Vicia faba were used. For Vicia faba Sutton's seeds, an RBE of 5.7 to 6.6 was obtained for 252Cf Dn + gamma doses of 0.5 to 0.2 Gy respectively and at a Dn + gamma dose rate of 0.11 Gy-1. The gamma contribution D gamma/Dn + gamma at the level of the root tipes was 0.35 and the derived RBE of the neutron emission of 252Cf was then 8.2 to 9.7. For Vicia faba Be1B and in the same irradiation conditions, an RBE of 5.1 to 6.2 was obtained for the total (n + gamma) 252Cf emission and for Dn + gamma doses of 0.4 to 0.2 Gy respectively. These values lead to an RBE of 7.4 to 9.0 for the neutron emission of 252Cf. For Vicia faba BelB, but for another source arrangement (Dn + gamma dose rate of 0.13 Gy . h-1 for 252Cf), an RBE of 5.6 to 7.5 was obtained for the total (n + gamma) emission of 252Cf and for Dn + gamma doses of 0.4 to 0.1 Gy respectively. The gamma contribution (D gamma/Dn + gamma) at the level of the root tips was 0.42, and the derived RBE of the neutron emission of 252Cf was then 8.9-12.3.

Californium↗
PubMed2005

Dose rate dependence of the relative biological effectiveness of 103Pd for continuous low dose rate irradiation of BA1112 rhabdomyosarcoma cells in vitro relative to acute exposures.

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 (103)Pd 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.

Animals↗