HDR brachytherapy tandem and ovoid titanium applicator safety assessment in 3T MRI

Earl Nixon; Yusung Kim; William R Kearney; Joseph M Modrick; Geraldine M Jacobson; Bhatia K Sudershan; John E Bayouth

doi:10.1016/j.brachy.2008.02.134

Purpose: CT imaging is useful for volumetric high-dose-rate (HDR) brachytherapy (BT) treatment planning for gynecological (GYN) cancer. However, CT is limited in visualization of the tumor volume. MRI allows differentiation between tumor and the uterus and cervix. According to the vendor the titanium tandem and ovoid (TO) applicator is MRI compatible up to 1.5 Tesla (T), but has not been assessed at 3.0 T. As part of the implementation of MRI-based treatment planning using 3.0 T MRI for HDR BT at our institution, the treatment applicator was assessed for safe use. Methods and Materials: A protocol similar to the ASTM International method for assessing radiofrequency (RF) heating, magnetic displacement and induced torque was performed. A Titanium TO Fletcher-Suit-Delclos style applicator set (Varian) was placed in a phantom. The phantom consisted of a solution (22L distilled water, 0.8g/L NaCl, 5.85g/L Polyacrylic Acid) in a plastic container (liquid dimensions: 67.8 cm × 40.1 cm × 11 cm). The MRI used in the study was a clinical Siemens Trio 3.0 T with Syngo MR 2004A software. A body array coil was used. A high specific absorption rate (SAR) inducing protocol consisted of the following parameters: axial spin echo, RF of 123.23 Mhz, 20-mm slice thickness, TE = 8ms, TR = 538 ms, 32 averages, 400 mm Field of View (FOV), 1 slice, 106 180° flip angle pulses/ second, 781 Hz bandwidth and 57 echoes. An MRI compatible temperature probe was used (SA Instruments). The temperature was recorded at points within the phantom and temperature change recorded. The setup was exposed to the MRI protocol for 20 minutes 56 seconds. Temperature was measured before, during, and after the procedure. Control measurements were repeated in the phantom without the TO present. Displacement measurements were performed by suspending the applicator by a string within the magnetic field. Results: The solution temperature was stable at 22.1°C. Under the testing conditions used above the rise in temperature of the phantom when the MRI sequence ended without the TO was 1.1°C, and with the TO applicator 1.4°C was measured. The phantom solution was made of material that does not allow bulk transport of convection currents, has a heat capacity similar to water, and similar conductivity to that of tissue. A rise in temperature was not observed in a water phantom alone. No displacement or induced torque was observed. Conclusions: An MRI protocol that would maximize RF induced heat to the applicator was used to assess temperature changes that may be harmful when the applicator is placed within the patient for imaging. Under the conditions described above, the applicator did not appear to increase in temperature by more than 0.3°C above control.

HDR brachytherapy tandem and ovoid titanium applicator safety assessment in 3T MRI

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