Clinical Experience on Localization and Real-Time Tracking of the Prostate During External Radiotherapy using Calypso^® 4D Localization System

Article Outline

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

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Purpose/Objective(s) 

To report our clinical experience on localization and real-time tracking of prostate motion during external beam radiation therapy using a wireless electronmagnetic target positioning and continuous monitoring system. (Calypso Medical Technologies, Inc, Seattle, WA)

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Materials/Methods 

To date, 32 patients have had three Beacon^® electromagnetic transponders implanted and 16 of these patients have been treated with intensity modulated radiation therapy (IMRT). The accuracy of localization using the electromagnetic system was verified by comparing with on-board X-ray imaging of the Beacon transponders for selected fractions. Patient positioning using this electromagnetic system was compared to setup using skin marks alone. And continuous prostate monitoring over the full course of IMRT treatment was performed for these 16 patients. Treatment time varied from 7 to 15 minutes and the fraction number varied from 35 to 40.

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Results 

Good localization agreement (<2 mm) was observed between the electromagnetic system and the on-board X-ray imaging system in non-simultaneous comparisons. The difference between skin marks vs. the electromagnetic system alignment was found to be >5 mm in 15.4%, 26.8% and 53.4% of fractions and >8 mm in 4.5%, 16.4% and 32.4% of fractions in left-right, cranial-caudal and anterior-posterior directions, respectively. Prostate motion is unpredictable and varies from continuous excursion to suddenly jumping during the treatment. Prostate displacement in the left-right direction is within ±2 mm for >99% of the treatment time for all patients. The displacement in the cranial-caudal direction is within ±2 mm 86.8% of the time, and within ±4 mm 99.5% of the time. The displacement in anterior-posterior direction is within ±2 mm 77.4% of the time, and within ±4 mm 94.1% of the time. However, prostate motion is patient dependent with some patients having >4 mm displacement for >33% of the treatment time while others have no displacement >4 mm during the full course of treatment.

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Conclusions 

The Calypso electromagnetic localization system is clinically efficient for patient localization and real-time motion monitoring of the prostate gland. This data suggests that all the patients would have benefited from the electromagnetic localization compared with skin mark based localization and 6 of 16 patients would have benefited from continuous, real-time target tracking and threshold-based intervention.