For diagnostic radiographic studies the concept of "safe period" is a myth. It is in any case by an order lower than the doses of ionizing radiation which can cause damage to cells. Damage threshold is 250 roentgen equivalent once. And, for example, CT whole body in step 1 sm patient receives about 1 roentgen equivalent.

Indeed, no safe periods with diagnostic exposures. The reason is the character of exposure, its stochastic. The repair of eventual damages is performed in hours.

Safe periods are needed in radiation therapy to give normal issues time for repair and regeneration.

 Thank you for your useful contributions (B.A Mopo3 and Hanno Krieger) please can we have some supportive document/articles/book chapters on this subject to help us in our ongoing studies please

Here is a book chapter that may be of help

"It is in any case by an order lower than the doses of ionizing radiation which can cause damage to cells. Damage threshold is 250 roentgen equivalent once. And, for example, CT whole body in step 1 sm patient receives about 1 roentgen equivalent".

I don't understand this. Are you claiming CT doses are below some kind of threshold for DNA damage to occur? Please clarify.

Richard W Harbron: Do you think that the damage threshold does not exist? I would love to debate with you about this quite well known and unsubstantiated ideas into practice. However, please note that the problem was at the level of the patients and not on the chemical level (DNA).

You can ask me any question on [email protected]

A short remark to threshold for DNA modifications. For low doses you have no threshold, you have a curve to describe probability for DNA defects. But isolated DNA (chemical level) is not the same as DNA in living organisms which will repear imediately or delete the cells with severe defects by apoptosis.

Thresholds will be found for tissue reactions, that means obvious clinical symptoms (erythema, epilation, fibrosis, stenosis etc) for higher doses (starting at 1-2 Gy level). In radiology or diagnostical nuclear medicine you apply doses in the mGy region. Here you cannot expect such clinical effects. 

If you want to read about the official philosophy please read ICRP 103.

I don't think there is a low dose threshold for either cell killing or cell mutation. I think that it's perfectly possible for a single x-ray to cause sufficient damage to a DNA molecule to result in a double strand break leading to cell death or mutation. Whether that implies there is no threshold for cancer induction, that's a different matter, although my personal opinion is that there is no threshold for cancer induction either.

To add to Hanno's statements:

The linear, no-threshold model for risk is for stochastic effects, not deterministic effects. Deterministic effects are diminished by fractionation. Large, therapeutic doses must be fractionated to protect the patient.

Diagnostic doses are below deterministic thresholds. Risk, based on cancer incidence, is indicated by the linear, no-threshold model (LNT) for doses below the deterministic thesholds. The ICRP includes a DDRF (dose and dose rate effectiveness factor) in the LNT. The DDRF could be used if diagnostic doses are fractionated or spaced out in time, i.e., a lower dose rate. This is not the intent of the DDRF. There is no basis to assume or calculate a safe period for diagnostic doses.

The LNT is a regulatory risk model and does not have evidential support. One may opine that there is no threshold for hereditary or carcinogenic effects, but this, also, has no evidentiary support.

I wouldn't say there is 'no evidentiary support' Joseph. Recent findings of increased risk of solid cancer (Richardson 2015) and leukaemia (Leuraud 2015) among nuclear workers, or leukaemia related to background radiation (Kendall 2013, Spycher 2015) or radioactive pollution (Krestinina 2013) are consistent with a 'no-threshold' model. Sure, there are studies that suggest otherwise - the Kendall study also found reduced risks of non-leukaemia cancers for moderately elevated background radiation, but that's a long way from 'no evidentiary support'. Or have I misinterpreted your comments?

In CT and x-ray diagnostic procedures the initial excitation events are brief ( ms to s) and then they end. The biologically relevant interactions are mediated by ions and radicals and they occur within a us to ms time frame.  The radiation used appears to be insufficient to overwhelm normal cellular defence mechanisms. (That is, we are unable to detect cellular injury.) As HK indicates, system restoration occurs over hours. As to risk from this event, that at the core of a lot of debate. In some unknown percentage, the molecular repair will be unsuccessful... and an unknown but small percentage of these will have future issues. Those are risk factors associated with the initial exposure and there is plenty of observational data to suggest that this risk is very low. So, if by "safe" you mean a return to pre-exposure risk levels, the risk associated with a second (hours or days later) identical diagnostic exposure will be indistinguishable from that of the initial condition. However, the patient carries forward the risk from both events.

As indicated, radiation therapy and elevated background radiation are different.  

Thanks to all your comments and suggestion, please more supporting documents on the topic of discuss.

Hi Geofery Luntsi, I followed the discussion and was surprised about how much repetition I saw.

Hanno Krieger has written many books related to the topic  -in German. This discussion may give him the idea and incentive to transfer his work to English.

Supporting documents that you are looking for can be found in large numbers. Specific questions are addressed in the scientific literature, for example, in the journal "Radiology". Of course, you or your institution may have to have the necessary subscriptions.

Here I offer a text that is freely available from the International Atomic Energy Agency. It contains text that directly addresses the issues of the present discussion that you have stirred up with your question. It provides the definitions of the relevant quantities and it also contains suggestions for further reading:

http://www-pub.iaea.org/MTCD/publications/PDF/TCS-42_web.pdf

I hope that this will be interesting and helpful for all who follow the present "safe period" discussion.

@Alfred Müller,

Good tip for reading.Regards HK

@ Alfred Muller, Thank you so much sir for your kind suggestions and very useful link it was very helpful sir. actually so many repetitions sir. 

if it is necessary, you may examine your patient only first ten days after menstruation.

The dose depends on the types of radiographic examinations, but take chest X-rays for instance. It roughly gives 0.2 mGy/test, which is expected to induce one DNA double strand break in 125 cells (c.f., one spontaneous DSB in 20 cells). If this test is repeated 5 times within a day, the total dose becomes 1 mGy. DSB induced by 1 mGy remains unrepaired for many days, which is eliminated upon proliferation ( https://www.ncbi.nlm.nih.gov/pubmed/12679524 , PDF freely downloadable). So, whether or not DNA damage induced is repaired depending on various conditions such as proliferation status as highlighted above.

A series of radiobiological experiments in cultured cells (in vitro) and animal models (in vivo) have shown that the low dose "priming" irradiation activates DNA repair or other protective machinery, which renders cells or animals more resistant to the subsequent high dose "challenge" irradiation. This phenomenon is called (radio-)adaptive response, and an interval in time between priming irradiation and challenge irradiation can somewhat be considered as "safe period". However, such adaptive response cannot be assumed for radiation protection purposes, as is the case for radiation hormesis.

Yes, in deed. There is no particular threshold in diagnostic imaging for damage to occur. However, we definitely have safety rules during pregnancy and menstruation period. The above discussion was very helpful. Thank you very much.

@Surendra: Why during menstruation? Never heard that before.