Re: Sr-89, Radiotherapy & cord compression.

[carol marcus <csmarcus@ucla.edu>]

At 08:37 AM 2/8/00 -0600, you wrote:
>>From: "Carolan, Martin" <CarolanM@IAHS.NSW.GOV.AU>
>>
>>Strontium 89 chloride is commonly used for bone metastases pain palliation
>>in prostate cancer patients.
>>
>>Can some one please explain the following apparent paradox:
>>
>>1.One of the contra-indications on the strontium package inserts and in the
>>literature is impending spinal cord compression.
>>
>>2. A common treatment for spinal cord compression is either surgery and
>>radiotherapy or radiotherapy alone. In some centres only a small number of
>>large fractions are given (eg. 2 x 8Gy etc).
>>
>>Why is external beam used to control SCC (usually with pretty good results)
>>but a radiation dose from Sr89 is contraindicated? Is it just because the
>>use of Sr89 precludes or complicates the use of external beam or surgery if
>>a compression develops shortly after the Sr89 dose? Or alternatively is it
>>because the Sr89 dose could provoke a SCC? If so how/why?
>>
>>There is obvously a dose rate difference. Some publications record tumour
>>absorbed doses of up to 231 cGy/MBq for Sr89 (Breen et al 1992). For a
>>standard 148 MBq (4 mCi)administered activity this gives about 34.2 Gy to
>>the tumour. Much of the literature cites doses significantly less than this.
>>The biological half life of the Sr89 in the tumour is known to be greater
>>than the physical half life of 50.5 days (Lewington 1996). If we assume a
>>half life of 50 days and a total dose of 34.2 Gy we get an inital dose rate
>>of about 20 cGy/hour. Therefore the Sr89 dose over the first 24 hours is
>>less than a single 8 Gy dose given on a linac during the course of a couple
>>of minutes.
>>
>>If the Sr89 contra indication is on the basis of provoking SCC how does this
>>happen with a dose and dose rate much less than the dose sometimes given on
>>a linac to treat SCC?  Is it just the steroids given with the external beam
>>that make all the difference?  Can anyone tell me what I am overlooking or
>>misunderstanding?
>>
>>Martin Carolan, Senior Physicist
>>Wollongong Hospital
>>Wollongong NSW, AUSTRALIA.
>>email: carolanm@iahs.nsw.gov.au
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Dear Martin and Radsafers:

I think it means that external beam therapy is the treatment of choice
because a large radiation dose can be given quickly.  "Quickly" is the
operative concern here.  Impending spinal cord compression is a medical
emergency that gets the radiotherapist in at night or on weekends.
Physicians should not use Metastron or Quadramet INSTEAD OF external beam
radiation in this situation. Using either of these drugs would probably just
confuse the dosimetry, and serve little purpose.

The best use of these drugs is early, when mets are small, because these
drugs go to where the tumor erodes bone.  In order to do much damage to the
tumor, the diameter of the tumor must be no larger than the range of the
beta particle.  Otherwise, you just "shave off the edge", but it grows back
because there was tumor remaining.  External beam radiation therapy, of
course, goes wherever the beam is aimed, and as long as the port is
adequate, THE WHOLE tumor gets irradiated.  With impending spinal cord
compression, one wants to zap ALL the tumor.  However, after the external
beam radiotherapy is over, it may be quite advantageous to use Quadramet or
Metastron, because if you have that big a met to cause SCC, you probably
have lots of other smaller mets to bone, and that is where the drugs are
superior to external beam therapy.  You can't do external beam to the whole
skeleton without knocking out all the bone marrow, so targeted therapy is
best in this situation.

Ciao, Carol

Carol S. Marcus, Ph.D., M.D.
<csmarcus@ucla.edu>

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