Proton beams are characterized by their rapid fall-off at the distal end of the Bragg peak and sharp lateral penumbra, depending on energy, depth, and delivery. These physical characteristics give proton beam therapy (PBT) a better dose distribution than X-ray irradiation, and PBT is now deemed a feasible and effective treatment modality that provides curative high-dose irradiation to the tumor volume without increasing normal tissue toxicity.
There are several published data about the outcomes of proton beam therapy for head and neck cancer from our institution. On the other hand, 91 patients who satisfied both criteria, definitive or postoperative PBT (> 50GyE) from January 1999 through December 2008, and more than 1 year follow-up, were traced to check the late toxicity. The median observation period was 57.5 months (range 12.4∼162.7), and the median time to onset of Grade 2 or greater late toxicity except cataract was 39.2 months (range 2.7∼99.8 months). Grade 4 visual loss occurred in 5 patients.
We consider that a relatively short observation period will result in the underestimation of late toxicity.
In the present study, we found many events which would not usually be encountered without long-term follow-up, and an adequate understanding of the toxicity profile of PBT in these patients thus requires long-term follow-up.