Electromagnetic Transponders Indicate Size Fluctuation during the Course of External Beam Radiation Therapy

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

Real-time image guidance promises the delivery of more accurate and effective radiation therapy. Tracking electromagnetic transponders placed directly in the prostate and adjusting patient position during a treatment fraction is increasingly popular. Beyond following the prostate position during the course of radiation therapy, characteristics of how the prostate itself changes may be an overlooked source of target inaccuracy. These fluctuations could change target volumes, the dosimetry of the tumor volume, and doses to critical surrounding normal tissue.

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Materials/Methods 

22 men had 3 electromagnetic transponders implanted transperineally with ultrasound guidance at least 1 week prior to obtaining a radiation planning CT scan. The men received image-guided radiation therapy with daily cone beam CT setup positioning and continual transponder monitoring. Three time points were measured: the first day of radiation treatment (day 0), seven days later (day 7), and the last day of radiation therapy (mean = 54.3 days, median = 55 days). The transponders form a triangle, and the length of each leg and the perimeter was measured with the Calypso® proprietary system at the start of each treatment. Changes in the CT prostate volume were then determined by scaling the original volume by the cube of the ratio of the triangle perimeters relative to day 0.

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Results 

The study group had a mean age of 72.2 years, and almost all presented with early stage prostate cancer (94% T1b-T2a). There was an increase in the mean vector spread (distance from one transponder to another) by day 7. This correlated to a mean increase in prostate volume of 2.3% comparing day 0 to 7. However, by the end of treatment, the prostate volume decreased by a mean of 12.6%. The differences in vector lengths, perimeter and volume were significantly different from one another between the two time points (p < 0.001). Furthermore, initial prostate size did not predict the amount of increase or decrease in the prostate size throughout the course of therapy in any of the time intervals.

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Conclusions 

Throughout the course of therapy, significant changes occurred in the prostate size as measured by intertransponder distances. The transponders provided evidence of swelling of the prostate during the first seven days of radiation therapy and then significantly shrinkage by the last day. These changes may be due to the radiation therapy, ADT, or both, but regardless of how interfraction volume changes are interpreted, this could lead to increased normal tissue dose (in the case of prostate shrinkage). Only with better understanding of all aspects of prostate behavior during treatment, including volume fluctuation, in addition to its movement, can real-time imaging technology perform to its fullest potential.