BERKELEY, CA (UroToday.com) - This study had a surprising negative result.[1] We expected the prostates of obese men to move more frequently and farther than those of non-obese men during radiation treatments.
As external beam radiation doses for prostate cancer have escalated, so has the need for accurate targeting, not only when the patient is setup each day, but also throughout the course of treatment. One solution is to implant the prostate gland with radiofrequency transducers (Calypso Medical Technologies, Seattle, WA USA) that may be tracked in real time.
The locations of three electromagnetic tracking markers implanted in the prostates of 66 men were monitored at 10 Hz during the entire course of radiation therapy. There were 45 nonobese patients and 21 patients with body mass index (BMI) ≥ 30. Of the three cardinal planes, motion was least in the left-right lateral direction and about twice as great in the superior-inferior and anterior-posterior directions. Therapists would realign the prostate to the radiation isocenter if the displacement in any direction exceeded 4 mm. The net vector displacement of the prostate was slightly less in obese men, 1.85 ± 0.52 mm, compared to 1.97 ± 0.45 mm in non-obese men, but the difference was not significant (p = 0.326).
This paper also indicates how a challenging question from a reviewer can greatly improve the work. The question asked how our statistical analysis accounted for the correlation between measured displacements. To answer this we went beyond our Fourier analysis of the oscillatory motion induced in the prostate by respiration to include Poincaré analysis of the sequencing.[2] In any time series of measurements, the value at one point is not independent, but will be highly dependent, on the value of the preceding point. Our Poincaré analysis plotted the vector displacement at time t against the displacement at the next time point, t+1. Such a plot allows calculation and separation of the short-term velocity — primarily due to respiration — from the long-term velocity due to rectal and bladder filling. Neither the short-term nor long-term distribution of velocities was significantly different between obese and non-obese patients.
References:
- Butler WM, Morris MN, Merrick GS, et al. Effect of body mass index on intrafraction prostate displacement monitored by real-time electromagnetic tracking. Int J Radiat Oncol Biol Phys 2012;84:e173-179.
- Piskorski J, Guzik P. Asymmetric properties of long-term and total heart rate variability. Med Biol Eng Comput 2011;49:1289-1297.
Written by:
Wayne M. Butler, PhD and Gregory S. Merrick, MD as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Schiffler Cancer Center, Wheeling Hospital, Wheeling, West Virginia
Wheeling Jesuit University, Wheeling, West Virginia
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