Late rectal toxicity after prostate RT depends on dose distribution
MedWire News: The incidence of late rectal toxicity after prostate radiotherapy (RT) could be reduced by considering the entire dose distribution to the rectum, researchers report.
Sarah Gulliford (Institute of Cancer Research and Royal Marsden National Health Service Foundation Trust, Sutton, UK) and colleagues comment that “considering only ]the maximal or high-dose region of the rectal dose distribution from a prostate RT plan might not minimize the risk of patients experiencing late rectal toxicity.
“It is recommended that appropriate rectal dose–volume constraints using the currently available data should be introduced into routine treatment planning for prostate RT.”
Gulliford et al analyzed data from 388 patients who underwent prostate RT. They aimed to calculate odds ratios (ORs) for seven rectal toxicity endpoints when the volume of rectum receiving a given dose of radiation increased by 5%.
The rectal toxicity endpoints were rectal bleeding, proctitis, sphincter control (subjective), stool frequency (subjective), sphincter control (management), loose stools, and rectal urgency. The doses considered were 20 Gy, 30 Gy, 40 Gy, 50 Gy, 60 Gy, 65 Gy and 70 Gy.
The researchers report in the International Journal of Radiation Oncology Biology Physics that a statistically significant dose–volume response was observed for six of the seven endpoints for at least one of the dose levels tested in the range 30–70 Gy.
The ORs for both grade 1/2 rectal bleeding and grade 1/2 proctitis increased progressively from 20 Gy to 70 Gy with a 5% increase in volume of rectum receiving radiation dose. For rectal bleeding, statistical significance was reached at 40 Gy at an OR of 1.08,, increasing to an OR of 1.25 at 70 Gy. A similar pattern was seen for proctitis.
In contrast, grade 1/2 stool frequency and sphincter control showed little correlation with an increase in volume. A 5% increase in volume with a50-Gy and 60-Gy dose led to a statistically significant increase in stool frequency toxicity , whereas 60 Gy was the only statistically significant dose level associated with an increased risk for sphincter control issues.
Patient-reported endpoints for rectal urgency and loose stools indicated a consistently strong dose–volume response for doses of 30–40 Gy through 60 Gy.
Gulliford et al comment: “The results we have presented have indicated that the entire dose distribution to the rectum should be considered carefully during treatment planning.”
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By James Taylor