amount of radiation

[koch]

I was just told "my tumor received 12.5 Gy to its margin" during gamma knife.  Can anyone tell me whether this is high, low, or average?  I'm not sure exactly the size of my tumor but it was "small" ie maybe 10mm. 

[Jim Scott]

Koch ~

I'm not a doctor or an expert in radiation but I recall my radiation oncologist telling me that I received a total of 27 gy over my 26 FSR sessions to destroy the DNA on a re-sectioned AN that was approximately 2.5 cm at the time it was radiated.  He stated it was the least amount of radiation that would be effective on the tumor.  I suffered no ill effects from the radiation and it did the job, with the AN showing necrosis and shrinkage within a year of being radiated.

On that basis, I would venture a guess that 12.5 gy is a low amount of radiation.  However, a call to your doctor would likely render a more credible answer to your question.

Jim

[koch]

Thanks for this info, Jim

[mk]

This amount is standard for GK. Historically doses as high as 16 Gy were used, with detrimental effects on surrounding structures. In modern practice the amount ranges usually from 12-13 Gy, although 11 Gy is sometimes used for larger tumors, to minimize the risk of swelling. The dosage corresponding to the centre of the tumor is about 2 times this.

Marianna

[free2be]

FWW, according to my radiologist, I had a total of 18 Gy during my fractionated CyberKnife a month ago.

Connie

[koch]

My radiologist told me when designing the GK treatment that a compromise had to be struck between too little radiation which might not stun the tumor and too much which would damage the inner ear (cochlea) against which the tumor lay.  So it's interesting to learn that the amount I did receive is standard.

[ppearl214]

For GK, that is my understanding for single dose treatment (within range of what is average for GK).  My CK, I received 30Gy (6 Gy over 5 days).  When I was treated, I had not heard of 30Gy for CK based on homework I had done (I had heard 18Gy-21Gy as "standard" for CK).  Now hearing more and more CK'ers receiving 30Gy (6Gy over 5 days) to help with hearing preservation goal.

Phyl

[koch]

I am surprised to hear that a larger total dose would be given in order to avoid hearing damage.  That sound counter-intuitive to me.  Could you say more please?

[Tumbleweed]

Hi, everybody:

Here 's the reason why people seemingly got (but didn't actually get) a larger dose of radiation with CK (and with Jim's FSR treatments) than with GK: the number of Gy for each fractionated treatment can't simply be added up to arrive at the total. For example, I had three CK treatments of 6 Gy each, but the biologically equivalent dose was only 11.5 Gy, according to Dr. Chang (my doctor at Stanford). You would think that 6 Gy + 6 Gy+ 6 Gy = 18 Gy total, but because it is fractionated it is biologically equivalent to receiving only 11.5 Gy at once (in one dose).

What's with the biologically equivalent lingo? The answer informs the basis for fractionated radiation treatments' success with better preserving hearing. The hearing nerve is more resilient to radiation than the AN, and it recovers somewhat better than the AN over the 24-hour period immediately following the radiation treatment. By giving three or more treatments spaced 24 hours apart (as with CK), the hearing nerve has a chance to recover from the radiation but the tumor doesn't. The result is that three treatments of 6 Gy each has the same biological effect as a single dose of 11.5 Gy.

By the way, CK and GK (and I'm pretty sure all forms of radiation treatment, for that matter) make no attempt to avoid cranial nerves that are in the way of radiation delivery to the tumor. So, if a tumor is wrapped partially or completely around a cranial nerve, the nerve is going to get zapped along with the tumor. With CK, however, the radiation dose is more uniform than with GK. CK delivers only 15% higher dose at the center of its radiation cloud compared to at its periphery, while GK delivers fully double the amount at the center compared to at its periphery. So, since the dose needs to be optimized at the periphery of the tumor in order to ensure the treatment's success throughout the tumor (i.e., it would be stupid -- malpractice, perhaps -- to purposefully deliver too low of a dose to any part of the tumor), a cranial nerve that is at the center of the area planned for radiation treatment will receive a much lower dose of radiation with CK than with GK.

Best wishes to all,
TW

[mk]

As TW mentioned in his very informative message, the radiation dose delivered at the centre of the tumor with GK is twice that in its periphery. So, a typical 12-13 Gy dose translates to 24-26 Gy of radiation at the centre. Which is not far off from the 27-30 Gy that other people have mentioned in FSR treatments.

Marianna

[koch]

It's really interesting to me that TW has been so much better informed, presumably by his doctor, than certainly I was by my internet research and my doctors.  Thanks for putting it out there for the rest of us.

[Tumbleweed]

It's really interesting to me that TW has been so much better informed, presumably by his doctor, than certainly I was by my internet research and my doctors. 

Part of the reason is because I asked my doctors a lot of questions. The other reason is that my doctors (Chang and Gibbs) were both very patient with answering those questions. Dr. Chang, in particular, is an amazingly charitable man. He has never rushed me through any consultation or failed to answer my emails promptly. I try not to abuse his accessibility -- and I've had no compelling reason to contact him for many months now, because I'm doing so well. And basically I've asked so many questions at this point that I've, ummm, run out of questions! 

I also did around 200 hours of research on the Internet in the first month after I was diagnosed. That led me here, to this forum, which has been a life raft in an angry sea.

Best wishes,
TW

[free2be]

Boy, TW, I'm impressed. I think you asked a few more questions than I did, and I asked a lot; I did 100s of hours of research too. But, I do agree, Dr. Chang is very good at answering questions, it's just you do need know what you want to ask. In fact, he e-mailed me recently to ask if things were settling down; so, now I owe him an e-mail.

Connie

[Tumbleweed]

Gotta love Dr. Chang. How many doctors do we know that take the initiative to email their patients to see how they are doing?!

He's the best, IMO.

Best wishes,
TW

[JLR]

This forum has been so amazing.  When I had a chat with my doctors yesterday prior to the actual procedure, I asked the amount of radiation and was very pleased to hear a total of 18 gy. Keep in mind before joining this forum I had absolutely no idea what that meant. The forum is quite a learning tool.