# Gray to mrem

• Medical
Does anybody know how to convert Gray's to mrem?

http://en.wikipedia.org/wiki/TomoTherapy

My wife is thinking of undergoing a treatment using the above machine.
The "Plan" is a total of 38.5 Gy in 10 sessions over 5 days.

Since this machine uses electrons, the weighting factor is 1.
So, 38.5*115 = 4,400 Rad = 4,400 Rem.
or 440 Rem per session (440,000 mRem/session).

For anybody knowledgeable of radiation, this is a lot of exposure.

Am I making an error?

Unfortunately, the people at the University Hospital (a Professor)
don't seem able to readily convert from Gy to Rem.
Medical Doctors can be scarey.

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tiny-tim
Homework Helper
Hi Xnn!

The gray (Gy) measures absorbed radiation per mass.

1 rad is the old cgs unit, equal to 0.01 Gy … see http://en.wikipedia.org/wiki/Gray_(unit)" [Broken]

1 sievert (Sv) = w gray, where the weighting factor w is 1 for electrons, so in this case 1 Sv = 1 Gy … see http://en.wikipedia.org/wiki/Sievert" [Broken]

But there is also a weighting factor for tissue type, ranging from 0.01 to 0.2.

100 rem = 1 Sv … see http://en.wikipedia.org/wiki/Roentgen_equivalent_man" [Broken]

So 38.5 Gy = 38.5 Sv = 3850 rem, but I don't know whether you have to reduce that by multiplying by the tissue type weighting factor.

(Compare dose examples at http://en.wikipedia.org/wiki/Sievert#Dose_examples")

I hope your wife's treatment is successful.

Last edited by a moderator:
Thanks tiny-tim;

I was missing the tissue weighing factor...

It's a treatment for Breast Cancer, so that would include
both skin and breast tissue.

Weighing factors according to Wikipedia:

skin 0.01
breast 0.05

However, Wikipedia also claims 1 Gy = 115Rad

So, skin dose would be 4.4 Rem per session (44 Rem total) and
breast tissue dose would be 22 Rem per session (220 Rem total)

This exceeds the 10CFR Part 20 occupational dose dose limit of 50 Rem annually to the organ, but not the skin. Of course, for medical purposes there are no limits that I'm aware of but at least it's in the ball park.

The Doctor did tell me that dosing the heart is more of a concern than the breast since it can cause heart damage. However, in 10CFR Part 20 land, there is no such distinction.

No weighing factors for heart, but it's 0.12 for lung and bone marrow.

tiny-tim
Homework Helper
Hi Xnn!

As a physicist, I applaud your attempt to understand the physics involved.

But I don't see how it affects the decision whether to have the treatment.

Every radiation treatment has side-effects …

the question is whether the possible benefits outweigh the possible damage.

A more appropriate question to ask, or to research on the internet (and I'm sorry if this is upsetting, but perhaps it's better that you should ask this, rather than your wife), is: what are the survival rates for similar treatment (both here and at alternative hospitals)?

my bad; it's the Novac7 that uses electrons.

Anyhow, the average survival rate for breast cancer patients with a similar diagnosis as my wife are excellent (Invasive Ductal Carcinoma Stage 1, Grade 2 ER/PR positive, clear margins without lymph node involvement). Radiation is considered standard treatment after breast conserving surgery as it lowers the 10 year risk of local recurrence from between 20-30% down to between 5-10%; so the decision has pretty much been made.

Oncotype DX Genomic testing has shown the average rate of distance recurrence for my wife is 9% (95% CL: 6% -11%) with 5 years of tamoxifen.