111 Open magnetic resonance imaging using titanium-zirconium needles: Improved accuracy for interstitialbrachytherapy implants ?

Materials and methods: Both radiograph and CT reconstructions were performed for a phantom and also for a series of 22 patients with interstitial breast implants. After appropriate transformation of the applicator coordinate systems as calculated by the two reconstruction methods, the geometrical distances between the source coordinates from the two reconstructions were determined and analyzed. Dose distributions and total reference air kerma (TRAK) resulting from the two reconstruction methods were compared geometrically and numerically. Results: The phantom study confirmed that the shifts in CT axial direction correspond to 1/2the slice distance for CT increments down to 5mm. For smaller slice distances, reconstruction uncertainties may occur when identical marker positions on the dummy chains used for reconstruction appear in several subsequent CT slices. This can happen when metallic dummies cause artefacts. In CT studies with small slice distances, e.g. spiral studies with a slice distance/thickness of 3mm, the artefacts will show up in more than one slice. In the patient study histograms were calculated for each patient showing the distribution of dwell position shifts in the three coordinates, and the total shift. The histogram means for the individual patients ranged between 1.7-4.9mm, with standard deviations of 0.7-2.9mm. The overall mean was 3.2mm with a standard deviation of 1.4mm. However these values are preliminary, and may need to be corrected in the final results for better coordinate transforms. For the considered slice distances/thicknesses (incremental CT: 5mm/5mm, spiral CT: 3mm/3mm, and 5mm/5mm) no distinction was possible between CT slice resolution and shifts caused by patient motion. For breast implants, breathing was the most prominent source of reconstruction uncertainty, however other motions such as different arm positions were detectable. A serious miscalculation of the source outdrive length can occur in unfavourable cases when patient movement causes large displacements of subsequent tracking points used for reconstruction. The catheter is then incorrectly reconstructed in kinky shape. Source outdrive length calculated along the undulating catheter results in longer outdrives than calculated along the correct smooth path. This can lead the user to choose incorrect applicator loadings delivering dose to anatomical sites different than the site on which it appears in the CT. In an extreme patient case, the length of tissue volumes covered by activity differed by 5mm when applying an identical catheter loading to the dose distribution with miscalculated outdrive and the corresponding film-based distribution. Due to the different implant volumes, the difference in TRAK was 9%. Conclusions: Similar to the practice in external beam therapy, target volume definition and dose adjustments to risk structures in CT-based treatment planning of brachytherapy implants must yield security margins which take into respect the geometrical uncertainties introduced by the reconstruction. Analysis of CT reconstruction may provide guidelines as how to define appropriate geometrical and dosimetric margins for the brachytherapy planning target volume. Algorithms to correct for motion-induced catheter displacements must be developed, and implemented in brachytherapy planning systems. Before definitely changing to CT-based brachytherapy reconstruction, any user should acquire sufficient experience in the possible reconstruction errors. 111 OPEN MAGNETIC RESONANCE IMAGING USING TITANIUM-ZIRCONIUM NEEDLES: IMPROVED ACCURACY FOR INTERST1TIALBRACHYTHERAPYIMPLANTS ?

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Youri Popowski, MD*, Emile Hiltbrand, PhD#, Dominique Joliat, dipl. Ing#, Michel Rouzaud*, ScD *Radiation Oncology Division, #Diagnostic Radiology Division, University Hospital of Geneva, 1211 Geneva 14, Switzerland Purpose: To evaluate the benefit of using an open magnetic resonance (MR) machine and new MR-compatible needles to improve the accuracy of brachytherapy implants in pelvic tumors. Patients, material a n d methods: The open MR machine, foreseen for interventional procedures, allows direct visualization of the pelvic structures that are to be implanted. For that purpose, we have developed MR- and CT-compatible titanium-zirconium (Ti-Zr) brachytherapy needles that allow implantations to be carried out under the magnetic field. In order to test the technical feasibility of this new approach, stainless steel (SS) and Ti-Zr needles were first compared in a tissue-equivalent phantom. In a second step, two patients implanted with Ti-Zr needles in the brachytherapy operating room were scanned in the open MR machine. In a third phase, four patients were implanted directly under open MR control. Results: The artefacts induced by both materials were significantly different, strongly favoring the Ti-ZR needles. The implantation in both first patients confirmed the excellent quality of the pictures obtained with the needles in vivo and showed suboptimal implant geometry in both patients. In the next 4 patients, the tumor could be punctured with excellent accuracy and the adjacent structures could be easily avoided. Conclusion: We conclude that open MR using MR-compatible needles is a very promising tool in brachytherapy, especially for pelvic tumors. 112 VIRTUAL BRACHYTHERAPY USING MULTIPLE NON-CONFLUENT PENCIL BEAMS: A NEW PARADIGM IN THE MANAGEMENT OF ADVANCED CANCERS Mohammed Mohiuddin, M.D., Mahesh Kudrimoti, M.D.; William F. Regine, M.D., Ali S. Meigooni, Ph.D.; and Mansoor Ahmed, Ph.D. Department of Radiation Medicine, University of Kentucky, Lexington, KY Background: In the early 1940's, the concept of using spatially fractionated radiation (SFR) to treat advanced cancers was well established with orthovoltage radiation to overcome the problems of skin tolerance in an effort to deliver higher dose at depth. SFR has been proven to be safe and effective in delivering large cumulative doses (> 100 Gy) of ortho-voltage irradiation. Objective: We have evaluated this concept using an array of multiple non-confluent pencil beams to simulate high dose brachytherapy dose distributions using megavoltage photon beams. Method: 120 patients with advanced cancers (tumor sizes > 8 cm) were treated with HISFR using a specially designed 50:50 GRID (open to closed area). A single fraction of 1000-2000 cGy (median 1500cGy) was delivered to dmax using 6 MV photons. 27 end-staged patients received HISFR therapy alone for palliation. In 93 patients HISFR was combined with conventional radiation (dose range 2000-7500cGy) and in 11 patients treatment was followed by surgery. Treatment sites included abdomen and pelvis (36), head and neck (40), thorax (13), extremities (19) and other sites (12). Results: An overall response was observed in 103/120 (85%) patients with a complete response (CR) in 18/120 (15%). Most patients had dramatic regression of their bulky tumor masses. The highest response rates (CR 27.5%, PR 65%) were for head and neck sites. No significant Grade 3 or higher late tissue toxicity was observed in spite of these high single fraction doses. In the 11 patients who received definitive treatment, clinical CR

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