An Evaluation of PTV Margins for Prostate SBRT
Abstract
Purpose
Prostate SBRT requires an expansion on the CTV to create the PTV. There is discussion over how large this expansion should be to adequately account for patient motion during treatment. A 3mm symmetric expansion of the CTV is beneficial for healthy tissue sparing but could lead to underdosing the target if patient movement is significant. Conversely, some institutions believe a 5mm margin is superior and ensures patient movement does not interfere with the treatment. This evaluation was done to compare 3mm and 5mm margins, considering patient motion during treatment and dosimetric differences.
Methods
Three fiducials were placed in the prostate prior to treatment. CBCTs were taken before and after each fraction of treatment, and patient movement was evaluated by measuring the fiducial shift between the images. To also understand the effect on plan quality, ten patients were planned with both 3mm and 5mm margins. Dose to the target and OARs were compared between plans to understand the dosimetric effect of expanding the PTV margin.
Results
It was determined that a PTV margin of 5mm would be acceptable for 95% of analyzed fractions. A PTV margin of 3mm would be acceptable for 85% of fractions. Overall, OAR dose increased with the 5mm margin compared to the 3mm margin. The bladder wall, rectal wall, penile bulb, and urethra did not meet constraints when planned with a 5mm margin in every plan.
Conclusion
To better account for patient movement during treatment, a 5mm margin is advised. To make this change in the planning process, more time and consideration would need to be given in the optimization to reduce OAR dose, with specific care given to the bladder wall, rectal wall, and the urethra structures.