Tissue tolerances

[jezyk]

I'm trying to grasp two things here. Please don't bash me for a seemingly stupid question.

1. For second and possibly successive cancers, if a tissue has already been irradiated to it's tolerance in the first cancer treatments how to you treat that same tissue? Let's say a brainstem has recieved 54 Gy. Say in about 5 years, if another tumor is found in the same area, how much dose should it be getting?

2. For hypofractionated SBRT, how are OAR tolerances set?

Again, I apologize if these questions seem dumb but I'd really appreciate your insight.

Thanks

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[Palex80]

1. Reirradiation is quite a grey area. We don't have lots of data on reirradiation for most of the normal tissue. There is some data on spinal cord tolerance, but we know little about bowel for example. It's difficult to answer your question, but I presume that you can retreat the brainstem with something around 36-40 Gy after 54 Gy if 5 years have passed. I am just extrapolating data from spinal cord though...

2. OAR-tolerances in the SBRT setting has been addressed for some tissues in the Quantec-Series Papers. Look up the Radiation Oncology Wikibook for the links.

 

[Neuronix]

2. For hypofractionated SBRT, how are OAR tolerances set?

As a rough guide, see:

http://www.semradonc.com/article/S1053-4296(08)00030-1/fulltext

These are referred to as the "Timmerman Tables". However keep in mind that most of these numbers are based on limited or no data.


Alternatively, there are several RTOG SBRT trials that many use as constraints even off trial. A good summary table is here:

http://www.jacmp.org/index.php/jacmp/article/view/3368/2212

 

[Gfunk6]

As mentioned by the two posters above, SBRT dose tolerances are more-or-less standardized. However, keep in mind that SBRT dose tolerance data was created mostly by extrapolation rather than empiric data (like Emami's original tables).

Re-treatment is a thornier issue. As Palex noted above, one must strongly take into consideration (a) what was previously irradiated, (b) the total dose, and (c) the timeframe between original treatment and proposed re-treatment. There is a fair volume of data for breast re-treatment indicating that if you re-treat a breast > 5 years later, then you can probably go to 45 Gy with a relatively low (< 5-10%) of > Grade 3 complications. The former chair of UCSF (Ted Phillips) used to say that, "after one year, half the rads are gone." About a good a principle as any in this situation!