Cardiovascular and Cerebrovascular Systems (Sections 2.5 and 4.6)
(1) The model was based on findings obtained for fractionated exposures, and used for extrapolation to chronic exposure scenarios. However, such approach should not be taken to judge whether the same holds true for acute, fractionated and chronic exposure scenarios (Paragraph 666, Appendix B).
The NTCP model described in Appendix B assumes that cells or tissues hit by radiation undergo death or severe functional impairment. This model may be applicable at the dose per fraction used in therapeutic fractionated exposure, and at 0.3 Gy (1/10 of the alpha/beta value) or around. However, this model is not applicable at low doses and at very low dose rates, where cell death does not occur at the frequency that is extrapolated from findings obtained in high dose-rate situations. It is thus unacceptable to generalize the NTCP model-based hypothesis that dose rate effects do not occur at 0.3 Gy or below by extrapolation to very low dose rate situations. Accordingly, its application to public and occupational exposure situations is inappropriate.
(2) Mechanisms whereby low-dose and low-dose-rate irradiation induces circulatory disease should be explained (Paragraphs 220-229, 669).
Conclusions of the same threshold value for chronic irradiation assume the accumulativeness of radiation effects. To understand the validity of such conclusions, its proposed supportive mechanisms should be specified, albeit limited biological evidence.
(3) Descriptions of anatomical features and the modeling should also be included for vascular system (Paragraphs 172-175, Appendix B).
This draft contains descriptions of anatomical features and the modeling mainly for heart, but lacks those for vascular system although atherosclerosis is a vascular disease considered important at low doses.
(4) The ERR/Gy is applied at 0.5 Gy assuming the linear dose-response relationship, but this approach is inappropriate in that the model does not guarantee such linearity (Paragraphs 663, 664, Appendix B).
The NTCP model is a non-linear model, and the validity of its application as a linear model is not discussed.