The German POC study from 2018 states single fraction doses are 0.5 - 1.0 Gy to total doses of 2-8 Gy are being used.
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The European (mostly Germany) literature in which the prescription practices are described is relatively few. Many used kV therapies. Those that described their MV prescription practices almost entirely prescribe to isocenter. I have only found 1 publication prescribing to a 3D volume.
60 cgy is one of the more common single fraction doses evaluated. I don't know what developments led to the choice of 60 cgy -- maybe it was an "educated guess". If dose is prescribed to the isocenter, that correlates with a lower dose covering the entirety of the joint. If I prescribe 50 cgy to a volume (the joint) the isocenter dose is between 60 to 100 cgy.
My insurers will only pay for 5 fractions. Therefore, I have limited my prescription to 5 fractions which delivers a total dose to the isocenter meeting or exceeding 300 cgy as described by DEGRO.
Regarding dose/fraction, several in vitro studies have shown more beneficial effects on immunomodulators with doses between 30 - 70 cgy compared with higher doses. The most effective dose/fraction (and total dose) are unknown, as far as I can tell. But, it appears to me that the lower the dose/fraction the greater the anti-inflammatory effect. What should the total dose be? I don't know, but suspect the lower the better there also. There are many patients who report significant pain relief within 1 or 2 fractions. Makes me wonder if those patients really need to complete the remainder of their treatments.
I started out treating 5 x 60 cgy. For the past couple years I have been treating with 5 x 50 cgy to a volume (i.e., the joint). I have treated hundreds of joints with this regimen since and haven't noticed a detriment to response rate.
Fortunately, or unfortunately depending on how you look at it, my arthritis practice has taken off as patients return to their PCP's/Ortho's and they see the results. I had to limit the number of arthritis patients I see in a week as it was crowding out my ability to take care of oncology patients. I have arthritis consults commonly booked out 3+ months now. But, it doesn't seem to bother them as they have already been told by their orthopod that it will be 5-6 mos before they can have surgery for their joint anyway. I am currently treating the spouse of one of my top referrers.
When I started this years ago I did not believe it would work. I thought the Europeans were quacks! I am amazed by what I have seen and wish I had started doing this sooner as it can have a tremendous positive impact on a patient's life.