Radiation dose outside of the prostate indicates likelihood of treatment failure
MedWire News: The likelihood of radiotherapy-treated prostate cancer patients experiencing biological or chemical failure is linked to how much radiation reaches beyond the primary target area, Dutch study results show.
Men who do not experience failure are those most likely to have received higher doses of radiotherapy to areas where regional cancer could be expected.
“We have found strong indications that incidental dose outside the primary target affects the treatment outcome for patients at a high risk for regional cancer spread,” report Marnix Witte, from The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital in Amsterdam, and colleagues.
The team selected 352 intermediate- (n=258) and high-risk (n=94) patients treated in a previous trial with either 68 or 78 Gy radiotherapy at one of two hospitals.
They constructed two three-dimensional image mappings of the average dose distributions based on prostate and rectum delineations, and examined whether the dose received outside of the target area correlated with biochemical or clinical failure among the cohort.
Both biochemical (the prostate-specific antigen nadir plus 2 ng/ml) and clinical (loco-regional or distant progression or the start of salvage therapy) failure was measured at 4 years after treatment.
The main dose differences, of more than 7Gy, were observed in the presacral region among intermediate-risk patients, and in the obturatorial region for high-risk patients.
Furthermore, univariate analysis showed (and multivariate analysis confirmed) that for each 10 Gy dose received in the obturatorial and presacral regions, biochemical and clinical failure was reduced by 17% and 28%, respectively.
The researchers then identified a point on all maps, arbitrarily chosen for its significant dose differences, and observed that patients who received the highest dose (above 50 Gy) to this specific area had significantly longer times to failure than those who received the lowest (under 40 Gy) dose.
Overall, a significantly better 7-year freedom from failure rate was seen in all patients treated in the higher dose arm of the original trial, at 56% versus 45%.
Radiation volumes have decreased significantly in previous years to reduce toxicity. However, “there is a concern that with the radiation becoming more and more focused to the prostate, subclinical disease around the prostate could be missed,” Witte and team write in the International Journal of Radiation Oncology Biology Physics.
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By Sarah Guy