Is it appropriate to use photon energies above 15 MV for prostate SBRT?

We have abandoned energies of 15MV or greater in our entire clinical enterprise. We do so much IMRT that the risk of neutron production with higher energies is not worth it. Furthermore, multi field IMRT or SBRT eliminate the need for the high energies. Our standard machine configuration is 6MV & 10MV xrays.

In the IMRT era, energies > 6MVX are moot in my opinion. In addition to the neutron contamination issue already raised above, energies > 10 MVX also have a wider lateral penumbra versus 6 MVX, and will thus not perform as well in the formation of extremely steep IMRT dose gradients. We have the exact same energy pairing as mentioned by @Dr. First Last (6 MVX, 10MVX), but that said, the 10 MVX is more likely to be used for NON-IMRT cases . . e.g. - Good old fashioned AP/PA, 3DCRT breast in large women, etc.

The question is whether 15 MV or above would improve dosimetry with IMRT for prostate cancer in comparison to 6MV. Dosimetric studies have shown similar coverage and critical organ DVH with low or high energy for most part. That being said, once in a while you run across a case where based on patient size and volume treating 6MV is not able to achieve acceptable dosimetry. Then I have used 18 Mv for pelvic malignancies accepting high neutron contamination.

With great humility I would like to respectfully disagree with the several Experts as it pertains to use of high energy in prostate cancer IMRT. My desire in placing this challenge is to learn from you, and not an attempt to be contrary for contrary’s sake. I have reviewed several papers that have attempted to model the risk of second malignancy (SM) associated with high energy photons with IMRT. As expected, all find neutron contamination that is not limited to “in field”. But then there is an unexplained jump to the conclusion on SM risk without satisfactory data or explanation. The best paper I have read on this modeling subject is a rigorous Monte Carlo calculation done by Stephen Kry,et al at MD Anderson in this reference - Radio...

In the 3D era, high photon energies meant a lower Dmax and better dose homogeneity, at the expense of a wider penumbra and a wider dose build up at the edges of fields. In the IMRT era, the main determinant of penumbra width and Dmax, is the multi leaf Collimator thickness. A tomotherapy unit with 2 mm leaves and 6mv will give you better plans than a machine with 10 mm leaves, and 15 mv.

So the answer is use a machine with thinner leaves as opposed to higher energy to get the nicest plans, and as a bonus there is less worry about neutron contamination

Everyone is worried about Neutron contamination, but where in any clinical literature is that relevant? we all feel better and theoretically it is true, but many, many patients were treated with higher energy therapy over decades, and none the worse off from absorbed or scattered neutrons.

Some patients require higher energies due to large seperations, and we should not feel guilty or feel that we are “accepting neutron contamination”, if higher energies are necessary, use them!

It is simply not necessary to use 18MV photons to achieve satisfactory dosimetry in prostate SBRT. I've not seen any dosimetric comparison showing any semblance of superiority that would make the increased neutron production worthwhile.