Men who were treated with SABR were less likely to have increases in prostate-specific antigen (PSA) levels and also lived longer without disease progression than men who didn't undergo the treatment. What's more, blood tests of the men treated with SABR indicated higher levels of T cells, hinting that radiation therapy spurred an immune response in the body, according to lead study author Dr. Ryan Phillips, PhD, of Johns Hopkins University.
Previously, men with oligometastatic prostate cancer -- disease that has been treated but then returned to a limited number of sites around the body -- were thought to be incurable. But recent studies have found that high-dose targeted SABR could help these men.
In the current research, dubbed the Observation Versus Stereotactic Ablative Radiation for Oligometastatic Prostate Cancer (ORIOLE) study, Phillips and colleagues looked at the use of SABR in men with oligometastatic prostate cancer to see if it would improve their outcomes; 36 men were randomized to receive SABR, as well as undergo blood tests and PET scans with a prostate-specific membrane antigen (PSMA) radiopharmaceutical at baseline and six months. A control group included 18 men. The researchers followed up the outcomes at six months, with treatment success considered to be a reduction in PSA levels.
At the primary end point, they found that only 19% of the men treated with SABR had rising PSA levels, compared with 61% of those in the observation arm (p = 0.005). Over half the men in the SABR group were free of disease progression over a year after treatment, while median progression-free survival for men in the control group was 5.8 months.
But Phillips' team made an interesting discovery beyond the findings of progression-free survival: Measurable changes occurred in the level of T cells in the SABR arm that were not present in the control group, and the difference was statistically significant. The group believes the effect could be due to an immune response to dying cancer cells targeted by radiation therapy.
In another intriguing secondary finding, the ORIOLE researchers found that only 16% of patients whose baseline PSMA-PET scans showed no additional untreated lesions developed new metastatic lesions at six months, versus 63% of those whose baseline PSMA-PET scan indicated at least one additional lesion. The researchers believe this finding indicates that PSMA-PET can be used to help radiation oncologists prevent disease progression and new metastases.
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