A radiobiological investigation on dose and dose rate for permanent implant brachytherapy of breast using I125 or P103d sources.

PURPOSE: The present report addresses the question of what could be the appropriate dose and dose rate for I125 and P103d permanent seed implants for breast cancer as monotherapy for early stage breast cancer. This is addressed by employing a radiobiological methodology, which is based on the linear quadratic model, to identify a biologically effective dose (BED) to the prescription point of the brachytherapy implant, which would produce equivalent cell killing (or same cell survival) when compared to a specified external radiotherapy scheme.

METHODS: In the present analysis, the tumor and normal tissue BED ratios of brachytherapy and external radiotherapy are examined for different combinations of tumor proliferation constant (K), α/β ratios, initial dose rate (R0), and reference external radiotherapy scheme (50 or 60 Gy in 2 Gy per fraction). The results of the radiobiological analysis are compared against other reports and clinical protocols in order to examine possible opportunities of improvement.

RESULTS: The analysis indicates that physical doses of approximately 100-110 Gy delivered with an initial dose rate of around 0.05Gyh-1 and 78-80 Gy delivered at 0.135Gyh-1 for I125 and P103d permanent implants, respectively, are equivalent to 50 Gy external beam radiotherapy (EBRT) in 2 Gy per fraction. Similarly, for physical doses of approximately 115-127 Gy delivered with an initial dose rate of around 0.059Gyh-1 and 92 Gy delivered at 0.157Gyh-1 for I125 and P103d, respectively, are equivalent to 60 Gy EBRT in 2 Gy per fraction. It is shown that the initial dose rate required to produce isoeffective tumor response with 50 or 60 Gy EBRT in 2 Gy per fraction increases as the repopulation factor K increases, even though repopulation is also considered in EBRT. Also, the initial dose rate increases as the value of the α/β ratio decreases. The impact of the different α/β ratios on the ratio of the tumor BEDs is significantly large for both the I125 and P103d implants with the deviation between the α/β=10.0Gy ratios and those using the 4.0 and 3.5 Gy values ranging between 18% and 22% in most of the cases.

CONCLUSIONS: For the cases of I125 and P103d, the equivalent physical doses to 50 Gy EBRT in 2 Gy per fraction are associated with an overdosage of the involved normal tissue in the range of 4%-16% and an underdosage by 10%-15% for a BED for normal tissue, using an α/β value of 3.0 Gy (BEDNT,3Gy) of 100 Gy. These values are lower by 10%-20% than the published value of 124 Gy for I125 and by about 13% when compared to the published isoeffective dose of 90 Gy for P103d. Similarly, the equivalent physical doses to 60 Gy EBRT in 2 Gy per fraction are associated with an overdosage of the involved normal tissue by 10%-20% and an underdosage by 4%-10% for BEDNT,3Gy of 110 Gy.