Understanding Standard Fractionation and SRS

[rad0nc]

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I am just an intern and have not had any of the rad bio. I am trying to understand how to to add stereotactic radiation therapy with standard fractionation to determine the total dose delivered. I am trying to determine the dose delivered to organs at risk in patients who have had multiple treatment courses. For example, if a patient had brain mets that were initially treated with 30Gy in 10 fractions and I found the total dose delivered to a point on the brainstem, how can I add the radiation delivered to that same point with SRS with 15Gy in 3 fractions.

If the point got 20Gy in 30 fractions from the standard fractionation and 12 Gy in 3 fractions from the SRS, I can not simply say that it got 32 Gy, correct?

 

[Gfunk6]

When you reach residency, you will learn about BED or biologically equivalent dose. You can use BED calculations to figure out equivalency of a treatment if it were delivered using standard fractionation (1.8 - 2.0 Gy). The formula also consists of a component called "alpha/beta" which is a parameter you can use to specific late (spinal cord) vs early (tumor) responding tissue.

As a caveat, this formula breaks down with extreme hypofractionation like SRS (e.g. the dose per fraction exceeds 8 Gy).

Here is a link to an online BED calculator.

 

[TarHeelRadOnc]

You have to be really careful when combining various dose and fractionations; even at rad bio, boards, etc. what I have really learned is that there is no perfect way to do it. 30/10 whole brain is close to 45 in standard fx and as GFunk said adding SRS to using BED is tough because our models break down as we hypofractionate.

There are a few studies looking at this specific issue. BED formula breaks down in one well-done study with fractional doses above 6.2Gy. Even if prescription dose is >6Gy/fx (as in SRS), if the region of interest receives < 6Gy/fx the BED model should hold up reasonably well.

According to BED formula, 30/10 equivalent to 32.5Gy in 2Gy fractions for tumor control (a/b 10Gy) and between 36Gy and 37.5Gy for late effects (a/b 2-3Gy). 45Gy is a generous estimate.

 

[rad0nc]

Thank you all for the replies and the calculator. I can't find the paper showing that the formula breaks down with fractional doses above 6.2Gy. Would you please point me in the right direction?

 

[TarHeelRadOnc]

Here you go

PMID: 18262098

 

[rad0nc]

Here you go

PMID: 18262098

Thanks! 👍

 

[rad0nc]

Does anyone know where you can find a table of values for the quasi-threshold dose (Dq), &#945;, &#946;, and Do for different normal tissues. I am attempting to calculate the BED utilizing the equation developed by Park et al, but I am seeing variables that are all over the place.

 

[MadamCurie]

Does anyone know where you can find a table of values for the quasi-threshold dose (Dq), &#945;, &#946;, and Do for different normal tissues. I am attempting to calculate the BED utilizing the equation developed by Park et al, but I am seeing variables that are all over the place.

At the risk of being a stickler for detail, the target theory model (i.e., n, Dq and D0) was kicked to the curb in the late 1970's in favor of the &#945;/&#946; model, so technically, it shouldn't be used at all anymore. (It tends to hang around though, if only for the sake of historical continuity and comparative purposes.)

And it especially should not be used mixed-and-matched with parameters and concepts of the &#945;/&#946; model, because that is like comparing apples and oranges.

That said, you're still going to have a hard time finding Dq and D0 values for normal tissues; the values that are out there are at least 35 years old if not more, are by their very nature highly variable as you've already pointed out, and most were determined for rodent tissues and tumors (or else using cells in vitro, which aren't necessarily a good comparison).

As for human normal tissues (and tumors), almost all the information we have is post-1980, so it is &#945;/&#946; ratios that are quoted. Nor are individual values for &#945; and &#946; available...you could always use "representative values" (e.g., assume &#945; &#8776; 0.35 Gy^-1), but again, these won't be very accurate.


Meanwhile, speaking of being a stickler for detail, BED stands for Biologically EFFECTIVE Dose (not equivalent dose). BED's are theoretical doses pegged to tissues with particular &#945;/&#946; ratios, and as such are not "equivalent" to anything.