Proton Hypofractionation

We have recently seen Level 1 evidence that IMRT hypofractionation (fewer, more intense treatments) is no worse than conventional fractionation. The same may hold true for proton therapy.

Proton therapy has come under fire because of its high cost and lack of proven benefit compared to photon IMRT. We are, therefore, interested in changes to the treatment protocol that may reduce costs and increase patient convenience, as long as efficacy and safety are not compromised. Vargas et al. reported the interim patient-evaluated quality-of-life scores of a small randomized pilot trial (NCT01230866) to determine whether proton therapy can be completed in just five treatments (similar to SBRT). Low risk patients were randomized to receive either:

• ·      38 RBE in 5 treatments (49 patients), or
• ·      79.2 RBE in 44 treatments (33 patients)

After 18 months median follow up:

• ·      Urinary, rectal, and sexual function scores were not different at 3, 6, 12, 18 or 24 months after treatment.
• ·      At 12 months, American Urological Association (AUA) Symptom Index Score was low, but slightly worse (8/35) for the hypofractionated therapy than for the conventionally fractionated therapy (5/35).
• ·      Scores remained low and equivalent for both groups in all other time periods.
• ·      There was no grade 3 or higher toxicity at any time in either group.

Kim et al. reported on a trial among 83 patients treated with five different fractionation schedules ranging from 60 CGE in 20 fractions to 35 CGE in 5 fractions. There was no significant difference in 4-yr biochemical failure for any of the treatment schedules within any risk group. Toxicity was low in all groups.

The low toxicity is certainly encouraging, and larger scale trials seem warranted based on this. In addition to the ongoing trial of the 5-treatment protocol, prospective patients may want to investigate the following (some include ADT for higher risk patients):

• ·      Loma Linda and Provision Center for Proton Therapy in Tennessee have ongoing clinical trials, (NCT00831623) and (NCT02198222), respectively, of a 20-treatment protocol .
• ·      MD Anderson is testing a 15-treatment protocol (NCT01950351).
• ·      The University of Florida is testing a mild hypofractionation schedule (NCT01368055).