SBRT: the optimal dose

While excellent outcomes of stereotactic body radiation therapy (SBRT) have been reported since it was first used to treat prostate cancer in 2003, the delivered doses have ranged from 35 Gy in five treatments to 40 Gy in five treatments. We saw in a University of Texas Southwest (UTSW) study (see this link) that toxicity escalates when doses are greater than 45 Gy.

Memorial Sloan-Kettering Cancer Center (MSKCC) designed a clinical trial (described here) among low- and intermediate-risk men. They started with about 35 men treated at 32.5 Gy and checked for dose-limiting toxicity. When most reached 6 months of follow-up, and if fewer than 10 percent had dose-limiting toxicity, they increased the dose to the next group of 35 men by 2.5 Gy in five treatments. In all, they had 136 patients who were followed up as follows:

  • Average (median) follow-up was
    • 5.9 years for men dosed at 32.5 Gy
    • 5.4 years for men dosed at 35.0 Gy
    • 4.1 years for men dosed at 37.5 Gy
    • 3.5 years for men dosed at 40.0 Gy

Their toxicity and oncological outcomes are reported here and shown in the table below:

Other than the one urinary stricture, there were no acute or late-term grade 3 (serious) toxicities.

Because follow-up decreased with increasing dose, it is unclear whether the zero biochemical failure rates for doses of 37.5 Gy and 40.0 Gy will be sustained, but, in other studies, almost all SBRT failures had occurred within 5 years. The positive biopsy rates will probably continue to decline with longer follow-up because the non-viable cancer cells can take up to 5 years to clear out. The dose level of 32.5 Gy is too low because of its unacceptable oncological results.

A dose of 40.0 Gy in five treatments has very acceptable toxicity and excellent cancer control. It would be reasonable to use doses as low as 37.5 Gy in patients with insignificant amounts of low-grade cancer (who would usually be excellent candidates for active surveillance). Based on the UTSW study, it would be reasonable to escalate the dose as high as 45 Gy in patients judged to have radioresistant cancers.

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

6 Responses

  1. I’ve had 30 Gy in three treatments, three times: femur, scapula and one rib. No side effects

  2. Dear Bob:

    The use of SBRT in the situation you are defining (i.e., oligometastatic disease) is very different from its use in the study described above. There would be not significant expectation of side effects in your situation (unless your were being given doses far higher than 30 Gy in three fractions of 10 Gy each.

  3. I should have made clear that this was for primary (prostate) treatment.

  4. In my opinion: Per multiple experts a 40 Gy/5 fraction dose is reckless and irresponsible with Cyberknife and an acceptable dose with Varian TrueBeam.

  5. I’ve had all my 10 Gy per session of SBRT to bone metastases via Varian Trubeam and my radiation oncologist has a Cyberknife machine as well.

  6. JJ:

    I have no idea who these “experts” are, but all of the 309 patients in the prospective trial discussed in this commentary were treated with CyberKnife with excellent oncological and toxicity outcomes.

    My radiation oncologist (Christopher King. MD) started with CyberKnife at Stanford, but switched to Truebeam with RapidArc at UCLA. He likes the homogeneity of dose he gets. Others argue that heterogeneity is preferable, so high volume experts like Don Fuller in San Diego and Alexander Gottschalk at UCSF use CyberKnife at even higher dose levels per fraction with intentional heterogeneity and report good outcomes. My personal opinion is that it does not matter much.

    The other advantage to RapidArc and other VMAT LINACs is higher throughput. The facility can treat more patients and make more revenue. It is also nice for the patient to have a 5- to 10-minute session (with a full bladder) with VMAT than a 30- to 60-minute session with CyberKnife.

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