Sr-89, Radiotherapy & cord compression.

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

>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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