While this article started on the news of the proposed building of another proton therapy in the Chicagoland area, it later on dove into what it is, and the current question on whether patients do benefit from it better than conventional, less-costly treatment.
I think the last line kinda says it all. If it were of the same cost as conventional treatment, it would have been built everywhere. But as of yet, no study on the cost versus benefit has been conducted. There certainly is no question that proton therapy causes less damage to surrounding tissues/cells, but how this translates to a patient's overall well-being is still unknown.