We conducted a prospective study to determine the minimal dose of total body irradiation (TBI) sufficient to achieve sustained engraftment when it is used in combination with 3 cycles of 30 mg/kg of antithymocyte globulin (ATG) and 4 cycles of 50 mg/kg of cyclophosphamide (CY).

Of the HLA-matched recipients prepared with CY/ ATG/TBI, all 20 who received 3 x 200 or 2 x 200 cGy of TBI achieved engraftment, and 10 are alive.

Overall, the highest probability of survival (73%) was observed among patients who underwent transplantation within 1 year of diagnosis, compared with patients who underwent transplantation after a longer period of disease. In addition, younger patients (aged greater than 20 years) were more likely to survive than older patients (aged less than 20 years). Thus, for patients with an HLA-matched, unrelated donor, a TBI dose of 200 cGy (in combination with CY/ATG) was sufficient to allow for engraftment without inducing prohibitive toxicity.

To take advantage of this, total body irradiation is generally given over several fractions, 2 to 3 times a day for 2 to 5 days. This is to allow the normal tissue cells such as lung to repair and tolerate the treatment while also increasing the chances that the abnormal cells will be affected and killed.

TBI without bone marrow transplant through multiple exposures at lower doses (e.g., 10 to 30 rad), known as "fractionated radiation," to achieve cumulative total body doses of 150 to 300 rad. TBI has again been used to treat certain widely disseminated, radioresistant carcinomas at doses as high as 1,575 rad in conjunction with effective bone marrow transplantation, which became routinely available in the late 1970s

As my taking somewhat out of context from the above expertise; I've come to understand that 200 rad or 200 cGy is sufficient dosage for many bone marrow transplant patients. 300 rad or 300 cGy seems to have become another accepted alternative dosage along with medication adjustments, though I've read that even greater levels of TBI have been applied.

Generally speaking, of TBI without involving bone marrow transplant;
Above 200 rem = 5% obtain death within 6 weeks.
Above 350 rem = 50% obtain death within 6 weeks.
Above 500 rem = 99% death within 4 weeks.
Above 550 rem = 100% death within 3 weeks.

Trust me, it goes seriously down hill from anything above 600 rem (6 gray), as in terms of days before certain extinction of most forms of life as we know it. Thus red-lining at 200 rem ott to be the absolute maximum accumulation.

This new found understanding of TBI is becoming fairly clear of what your chances are if you should receive a dosage of more than 200 rem, especially if that's an absorbson or accumulation within a few days and not over a two year stint; it's not terribly good to say the least. Though I do not ever recall hearing that even one of our infamous Apollo astronauts received any radiation treatments, nor of any bone marrow transplants whatsoever (were they good or what, as not even a hair fell out?).

Although this nominal risk factor of 5% death expected within 6 weeks at 200 rem seems morbidly interesting but, considering that the to/from and of any VL2 stationkeeping mission dosage tolerance will be that of a slow build from zero to perhaps 100 rem in the first year, this could represent of those estimated 5% fatalities might stretch that 6 week timeline out to 2+ years worth, especially if the VL2 mission crew stashes away some of their own marrow prior to leaving Earth, where at the very least or worst, that's if they're still sufficiently alive and kicking upon return to Earth, at which point they'll have a better than good chance at recovery. So, until I learn more, I'll be considering this accumulated dosage of 200 rem (2 Sv) over 2 years as being risky but certainly not of an absolute death sentence, especially if certain medications can manage to reinforce those damaged T-cells and DNA wherever it counts, then of receiving your own bone marrow injections ott to greatly improve your chances.

Over a two year period, 200 rem is a dosage accumulation of 0.274 rem/day, a mostly survivable level which I believe can be accommodated, especially if you have banked bone marrow, if not further improved upon by employing multi-layered shielding, as augmented onto the existing ISS or of one like it. Either of shielding ISS entirely or by way providing 100 g/cm2 pods as personal coffins ott to further cut that dosage by fairly good 10:1 safety margin, achieving an average of 27 mrem/day would certainly eliminate the need for receiving those bone marrow injections, unless EVAs get involved, as then there's simply little likelihood of avoiding an unacceptable TBI dosage, as even limited shuttle/ISS EVAs can take a month(s) off the career exposure limits.

I'm thinking (always a bad sign); what if we managed to stuff 20 death defying individuals onboard ISS and, even if expecting one of them to kick the bucket before returning, this is actually not all that bad, as on Earth those same 20 individuals over an active two year period of exposure to the everyday threats (such as commuting) will likely lose out by 10%, or that's 2 out of the bunch that will likely have died before their time.

I'm not super terrific at math but, seems losing one beats two any day of the week.

Of course, packing 20 (already insane) individuals off to never never land in a sardine can is hardly a rational option, as I don't think we could supply enough pot on top of the Prozac to enable a civilized mission, at least not with any realistic hope of returning anyone in one peace.

Lets say we conservatively accommodate 6 paying passengers and of 4 crew members (well trained in the arts of self defense). A packed ISS mad house of 10 totally wired individuals, each having to some degree a death wish, otherwise willing to risk it all for the benefit of mankind and possibly even lizardkind, 6 of those individuals willing to pitch in a hundred million each for their first chance at insulting someone from another world.

But folks, somehow we first have to get ourselves there. That's where the idea of using ISS could become just the ticket, or in this instance the bucket of bolts that's already in the sky. All that ISS needs is a modest (25 MW to 50 MW) nuclear pumped Xenon or lithium EMP rocket engine or an electrified array of such, plus a hundred or so tonnes of Xenon/Lithium, adding onboard a few additional hundred tonnes of shielding and perhaps the rest comprised of beer, vodka and pizza.

Sponsor wise, I don't believe there'll be any shortage. In fact, I believe this mission could be turning a tidy profit long before it gets under way.

I'd also be interested in hearing about other rocket engine powered craft alternatives (perhaps the faster the better), including of what it's going to take in order to get the spent crew and those crazed passengers back from VL2. I've been assuming a great deal more energy per tonne, as what if the return mission craft were but 100 t, it seems we may be in need of those solid fuel rockets plus additional alternatives such as a 1-t nuclear pumped propulsion (that's 100 times greater in proportion to the ISS 0.1-t thruster to mass ratio), where the outbound (return-home) mission could still require twice the amount of inbound time per equal distance, thus also twice the radiation exposure, unless the majority of the craft were involved in shielding those already radiated crew and guest passenger members.

Other ongoing research pages related to Venus L2 (VL2) and ISS:
https://guthvenus.tripod.com/vl2-rocket.htm
and
https://guthvenus.tripod.com/vl2-radiation.htm
and
https://guthvenus.tripod.com/vl2-iss-03.htm
and
https://guthvenus.tripod.com/vl2-iss-03.htm

Perhaps somewhere on this internet there's a qualified radiation wizard, plus a rocket propulsion and space travel expert or perchance a software rocketship simulator that'll permit this level of insanity, of applying various engines to ISS, sort of a Microsoft "rocket flight Simulator" that allows a student to construct and fly his own custom spaceship or airplane, into the ground if need be (that's a RESET!).