This is an unsurprising truth from Proton Beam Therapy and Treatment for Localized Prostate Cancer: If You Build It, They Will Come, by David S. Aaronson et al.:
No prostate cancer treatment has been proven superior to the others. There are, however, substantial differences in cost, which are becoming more important to society and are a focus of health care reform in the United States. While there are theoretical advantages to proton beam therapy from a radiation physics standpoint, no study yet has demonstrated its superiority to modern photon-based therapy in terms of either oncologic or quality of life outcomes. To our knowledge, we show for the first time that the availability of a technology, in this instance a proton beam facility, in one’s HRR is associated with a higher likelihood of receiving proton beam therapy compared with those living in an HRR where this technology is not available.
Proximity matters, even when there’s no proven increase in effectiveness and despite the cost.