Hypofractionated proton beam radiation therapy

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

PBRT has come under fire because of its high cost and the lack of proven benefit compared to photon-based 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)
  • 79.2 RBE in 44 treatments (33 patients)

(RBE stands for “relative biological effectiveness” of the total dose of radiotherapy administered. Note that this trial has been designed to enroll 192 patients in total and won’t be completed until 2018.)

After a median 18 months of follow-up:

  • Urinary, rectal, and sexual function scores were not different at 3, 6, 12, 18, or 24 months after treatment.
  • At 12 months, the American Urological Association 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-year 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 previously mentioned, prospective patients may want to investigate the following (some trials include ADT for higher-risk patients):

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

Editorial note: This commentary was written for The “New” Prostate Cancer InfoLink by Allen Edel.

One Response

  1. It is worth noting that all of the trials referred to above include both low-risk and very low-risk patients within their eligibility criteria.

    While treatment of men with low- and very low-risk forms of prostate cancer (with PBRT or any other form of invasive therapy) is certainly an option, I would note that there is good reason to believe that a very high percentage of such patients may gain no clinical benefit whatsoever from immediate invasive treatment of any type. Thus, any supposed benefit from treatment with PBRT among this subgroup of patients in the abovementioned trials would be questionable at best since there is no comparator group of men being managed on a simple monitoring protocol.

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