2462 Real-Time Prostate Localization Using Superconducting Quantum Interference Devices

J. E. McGary, B. S. Teh

Radiotherapy, Baylor College of Medicine, Houston, TX

Purpose/Objective: To demonstrate the physical properties underlying the concept of radio frequency identification and remotelocalization in a heuristic manner to understand the operational parameters needed for real-time clinical applications. Thisapproach is to demonstrate the benefit of using superconducting quantum interference device (SQUID) magnetometers fordetecting low-level magnetic fields that can be used for calculating the prostate position. We will present the theoreticalfoundation for this technology and illustrate the governing equations and circuits required to create a real-time basedlocalization system.

Materials/Methods: The localization system consists of (1) a dipole antenna, (2) an inductive electronic circuit, and (3)superconducting quantum interference device (SQUID) magnetometers. The dipole antenna is designed to power the inductiveelectronic circuit to create a passive system in place of using batteries. The electronic circuit consists of a microchip and aninductor-capacitor circuit for both passive charging and discharging needed for operation. The inductor is designed as dualpurpose, to receive and transmit signals by creating an inductor current and the associated magnetic field. For tumorlocalization, the implantable microchip restricts the size of the inductor and reduces the magnetic field strength per current.Given these small field strengths, the detection system relies on SQUID magnetometers to increase flexibility into the clinicaldesign.

Results: To design an implantable, passive transmitter for prostate localization, the implant size constrains the magnetic fieldstrength to relatively small values for either charge build up or current discharge during signal transmission. This is due to boththe inductor size and the real-time constraint where positioning information is defined in less than one second. To reduce themagnetic field strength attenuation through tissue, low frequency operation is desirable for both transmission and chargingshown using Poynting vector calculations. Operating at low frequencies decreases field attenuation, however, the charge up timeincreases as well as the time interval for acquiring the prostate location. Therefore, the circuit parameters and detection systemmust be optimized. With regard to the dipole antenna, the loop radius should be greater than the length of the transmitterdistance to maximize power transfer, which influences the clinical design and implementation. One possible mode of operationis to continuously charge and discharge the transmitter circuit in half or full duplex modes to provide power and location withoutinterruption. Another mode is to sequentially power the transmitter and then discharge the capacitor to create an alternatingmagnetic field. For duplex modes, the minimum induction field needed to power the transmitter is reduced using resonant circuitdesign for both the dipole antenna and the transmitter inductor circuit. In addition, calculations show that range between theantenna source and transmitter depend on the minimum field strength and the microchip power specifications. Estimates of thecircuit parameters, field strength, and range are calculated for both operational modes. For sequential operation, the charge timeis calculated according to varying circuit parameters to determine feasibility in terms of creating a detectable magnetic fieldstrength with fractions of a second for real-time prostate localization.

Conclusions: Superconducting quantum device (SQUID) magnetometers create flexibility into the clinical design for real-time,three-dimensional prostate location by increasing the detection sensitivity required for this application. Magnetic field strengthis limited by the implant size, passive power transfer, and real-time design constraints imposed by the localization system. Withthese design constraints, the duplex modes are unable to provide sufficient magnetic field strengths for detection given that themutual inductance depends upon distance and orientation. Therefore, the sequential operating mode offers the best compromisewith respect to field strength and data acquisition frequency.

2463 Dosimetric Comparison of Daily Prostate Alignment Utilizing Skin Marks, Ultrasound, and In-Room CT

J. C. O’Daniel, L. Dong, M. Bonnen, R. de Crevoisier, H. Wang, R. Cheung, A. Lee, R. Mohan, D. Kuban

Division of Radiation Oncology, M.D. Anderson Cancer Center, Houston, TX

Purpose/Objective: With the availability of in-room CT-guided radiotherapy, positional uncertainties caused by both setuperrors and internal organ motion may be minimized. However, it is still unclear what would be the quantitative dosimetricbenefits from using such procedure in daily target localization. In this study we will examine the dosimetric impact of daily CTalignment for prostate cancer radiotherapy.

Materials/Methods: Utilizing the CT-on-rails, patients can be scanned on the treatment couch in the treatment positionimmediately prior to radiotherapy. In an IRB-approved study, fifteen prostate patients were scanned 3 times each week duringtheir 8-week treatments for an average of 24 scans per patient. Three cases were selected for this introductory study, two withsmall prostate center-of-volume (COV) variations relative to bony reference (Pat01 & Pat12) and one with large variations

S617Proceedings of the 46th Annual ASTRO Meeting