Taha, there are a number of dose/fraction schemes that are used for prostate conformal radiotherapy, generally all deliver about 1.8-2Gy per fraction. Back in the "old" days it was a four field shielded box arrangement (ant, post, L&R lats) 66Gy in 33 fractions. The beams were weighted to somewhat reduce what the rectum was receiving overall. 66Gy was about the highest dose that "managed" short and long term side effects. As the dose was increased there were trials which indicated an increase in long term rectal complications. One method of dealing with this was to spread the dose around more by increasing the number of beams and the simple 4 field became 6 fields which used oblique lats to try and reduce the dose delivered to the rectum. 78Gy in 39 fractions I think is about the limit used in 3DCRT. There are some hypo-fractionation schemes which delivered dose of 2.5Gy and up per fraction, I can recall 50Gy in 16 fractions, even 60Gy or 63Gy in 20 or 21 fractions respectively. There have been a number of trials in the US, UK, Netherlands and Canada (to name a few). So the reason for my long winded intro is just on dose/fraction grounds there isn't one simple answer to your question. With the old 4 field box arrangement there was a sizable proportion of the 50% isodose line an example is illustrated http://oftankonyv.reak.bme.hu/tiki-download_file.php?fileId=572&display this is 50% of 63Gy. I had hoped that I would find a few examples easily on google images of 3DCRT techs for prostate unfortunately everything seems to be IMRT, hopefully someone else will have example on hand to discuss this further
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