Dose escalation of SBRT in treatment of localized prostate cancer

Most published data on the use of stereotactic body radiation therapy (SBRT; CyberKnife radiation therapy) has been based on  a total radiation dose of 32 to 38 Gy delivered in a series of four or five sessions — as compared to the delivery of more like 75 to 80 Gy using intensity-modulated radiation (IMRT) and other forms of targeted external beam radiation therapy (EBRT).

A new article by Boike et al. in the May issue of the Journal of Clinical Oncology has now reported data from a dose escalation study of SBRT using doses of up to 50 Gy to treat men with localized prostate cancer.

In this study, Boike and colleagues treated three groups of patients, all of whom met the following eligibility criteria:

  • Gleason score 2 to 6 with a PSA level ≤ 20 ng/ml or Gleason score 7 with a PSA ≤ 15 ng/ml
  • Clinical stage ≤ T2b
  • Prostate volume ≤ 60 cm3
  • American Urological Association (AUA) symptom score ≤ 15.

Prior to treatment, all patients were required to have an enema and placement of a protective rectal balloon.

The key results of the study are reported as follows:

  • Three groups of 15 patients (45 patients in total) each received SBRT in five fractions.
    • Group A received a total of 45 Gy.
    • Group B received a total of 47.5 Gy.
    • Group C received a total of 50 Gy.
  • The average (median) follow-up to date is
    • 30 months (range, 3 to 36 months) for patients in Group A
    • 18 months (range, 0 to 30 months) for patients in Group B
    • 12 months (range, 3 to 18 months) for patients in Group C
  • For all patients
    • Post-treatment PSA control is 100 percent based on the Phoenix criteria for definition of biochemical failure (nadir + 2 ng/ml).
    • Gastrointestinal toxicity of grade ≥ 2 and grade ≥ 3 occurred in 18 and 2 percent of patients, respectively.
    • Genitourinary toxicity of grade ≥ 2 and grade ≥ 3 occurred in 31 and 4 percent of patients, respectively.
  • Mean AUA symptom scores increased significantly from baseline in the patients in Group B (P = 0.002) compared to patient in Groups A and C.
  • Mean AUA symptom scores among patients in Groups A and C returned to baseline post-treatment.
  • Rectal quality-of-life scores (assessed using the Expanded Prostate Cancer Index Composite) fell from baseline up to 12 months but trended back to baseline levels at 18 months.

The authors conclude that doses of SBRT up to 50 Gy can be administered without the occurrence of dose-limiting toxicity (defined as significant occurrence of gastrointestinal or genitourinary toxicities of grade ≥ 3). Initiated as a Phase I clinical trial, the multi-center Phase II component of this trial is currently recruiting patients who will be treated to 50 Gy in five fractions to further evaluate the clinical effectiveness and safety of this form of treatment.

Further commentary on this study, with statements from the senior author of the paper, can be found on the MachinesLikeUs web site.

2 Responses

  1. Why are higher doses already being tested when the early data is claiming such good results at lower doses?

  2. Perhaps they think they can do better? I cannot read minds … only published data! :O)

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