Mammogram and diagnostic X-rays–evidence of protective Bystander, Adaptive Response (AR) radio-protection and AR retention at high dose levels

Abstract

Purpose: The recently published dose response data by Dr Redpath's research group for low energy (30 kVp) mammography X-rays, displaying Adaptive Response (AR) radio-protective behavior, is significant for millions of American women that undergo annual breast cancer screening. We here, using the recently developed Microdose Model that encompasses the Bystander Effect (BE) and AR behavior, examine the data for BE, AR and high radiation domination by the priming radiations high dose Direct Damage.

Results: The dose response is divided into three regions, Bystander Effect Region, Adaptive Response Region and Direct Damage Region (with possible retention of the AR protection). The Bystander Effect Region is below the microdose Specific Energy deposition for single photon induced charged particle traversals through the cell nucleus (the microdose Specific Energy Deposition per Traversal value = <z₁> = 0.638 cGy per Hit). Strong evidence is shown that a protective BE of about 50% occurs at a very low dose of 0.054 cGy, the BE is depleted reverting the response back to nearly the zero dose control value at 0.27 cGy, a 42% AR protection then is developed at 1.08 cGy and then the Direct Damage increasingly begins to dominate in the range from 5.4–21.6 cGy. Using the precise Method of Maximum Likelihood Estimator (MLE), the high dose Direct Damage Region is examined. We show that to the dose of 21.6 cGy the AR protection is retained in spite of the significant Direct Damage. We apply the same MLE analysis to the Redpath data for ¹³⁷Cs gammas and find that the AR protection is completely dissipated at high Direct Damage inducing doses of 100 cGy.

Conclusions: The model shows that a protective BE of about 50% occurs at a low factor of 12 below single tracks traversals where less than 10% of the cell nuclei have been hit. Poisson distributed single tracks activates the 42% AR protection. The AR protection is retained at high dose but one needs to understand why ¹³⁷Cs does not. Other Redpath group AR data sets for ¹³⁷Cs, 232 MeV protons, and brachytherapy ¹²⁵I photons did not reveal BE since the lowest data points were above the <z₁> for the radiations, but diagnostic X-rays do.

Keywords:

• Breast cancer
• Bystander effects
• Adaptive Response
• microdosimetry