The results suggest SABR could be a way to delay the advancement of metastatic prostate cancer without using hormone therapy (androgen deprivation therapy, or ADT), which can cause erectile dysfunction, loss of bone density and muscle mass, fatigue, weight gain, and breast tissue growth in men, wrote a team led by senior author Dr. Phuoc Tran, PhD, of Johns Hopkins University in Baltimore.
"Many men are interested in avoiding the unpleasant adverse effects and potential health risks of ADT for as long as is reasonable," the group noted.
About 1.3 million men are diagnosed with prostate cancer each year, and about 20% of them have metastatic disease, Tran and colleagues wrote. This study is phase II of the Observation Versus Stereotactic Ablative Radiation for Oligometastatic Prostate Cancer (ORIOLE) trial, which compares SABR's effectiveness with a "wait and watch" approach. Initial results of the trial were presented at the 2019 annual meeting of the American Society for Radiation Oncology.
The current phase included 54 men with recurrent cases of oligometastatic prostate cancer (that is, disease that has spread from the primary tumor to one to three sites in the body). Of these, 36 underwent SABR and 18 were observed.
Tran's group found that, within six months, seven out of 36 men (19%) treated with SABR experienced disease progression, compared with 11 out of 18 men (61%) who underwent observation alone. Risk of new cancers at six months was lower in men who were treated with SABR compared with those under observation (16% versus 63%). None of the men in either group reported meaningful side effects or pain.
Men who underwent the SABR treatment also showed an increase in T cells, which suggests that the therapy stimulated an immune system response, according to Tran and colleagues. The results also suggest that "SABR treatment may remove or affect signals that promote the development of micrometastases in recurrent oligometastatic prostate cancer, rather than just 'resetting' the clock on the disease until metastases grow large again," Tran said in a statement released by Johns Hopkins.
"It has been a longstanding question, especially important now in the era of immunotherapy, whether any type of radiation, and SABR specifically, can stimulate the immune system," he said. "Our trial offers the best data to date to suggest that SABR can cause a systemic immune response."
It's possible that SABR could be used with other immunotherapies to treat recurrent prostate cancer. But more research is needed, according to the authors.
"Although SABR alone may or may not be sufficient as curative management, the combination of SABR with systemic therapies may provide the multipronged attack required to cure this disease," the group concluded.
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