T Podder

In vivo motion and force measurement of surgical needle intervention during prostate brachytherapy

In this paper, we present needle insertion forces and motion trajectories measured during actual brachytherapy needle insertion while implanting radioactive seeds in the prostate glands of 20 different patients. The needle motion was captured using ultrasound images and a 6 degree-of-freedom electromagnetic-based position sensor. Needle velocity was computed from the position information and the corresponding time stamps. From in vivo data we found the maximum needle insertion forces to be about 15.6 and 8.9N for 17gauge (1.47mm) and 18gauge (1.27mm) needles, respectively. Part of this difference in insertion forces is due to the needle size difference (17G and 18G) and the other part is due to the difference in tissue properties that are specific to the individual patient. Some transverse forces were observed, which are attributed to several factors such as tissue heterogeneity, organ movement, human factors in surgery, and the interaction between the template and the needle. However, theses insertion forces are significantly responsible for needle deviation from the desired trajectory and target movement. Therefore, a proper selection of needle and modulated velocity (translational and rotational) may reduce the tissue deformation and target movement by reducing insertion forces and thereby improve the seed delivery accuracy. The knowledge gleaned from this study promises to be useful for not only designing mechanical/robotic systems but also developing a predictive deformation model of the prostate and real-time adaptive controlling of the needle.

SU‐EE‐A2‐05: Tissue Ablation by Medium Intensity Focused Ultrasound for Breast Cancer Treatment: Preliminary Study

Purpose: Focused ultrasonicradiation is known for its ability to induce thermal effects for therapy deep in tissue without surgical intervention. To design a medium intensity focused ultrasound (MIFU) system with distributed ultrasound transducers for breast cancer therapy, physical properties of the acoustic field must be fully characterized. Method and Materials: Custom made ultrasound transducers and driving system were calibrated with 1MHz sine wave input (1mW power). Calibrated gain of RF amplifier was utilized for ultrasound power control. Numerical simulation of the ultrasoundradiation field was carried out with Rayleigh‐Sommerfold integral. The mathematical model for simulation was verified by measuring the acoustic output with a hydrophone. Heat distribution model was established based on the simulated ultrasound field. The heating experiment was implemented under different control strategies (duty cycle, pulse repetition) with tissue‐mimicking phantoms (12W each transducer) and animal tissue phantoms (20W each transducer).Results: The output power for MIFU can be controlled accurately with calibration of ultrasound driving system. The focal length of the transducer was found to be 6.9cm with resonance frequency 1MHz. At −6dB focal zone, the beam width was 0.3cm and the focal zone depth was about 3cm. The mathematical model of the ultrasound field was quite comparable with the measured results. In the simulation of ultrasound fields generated by two orthogonal transducers, the area of −6dB focal zone was 6mm×6mm. The mathematical model of thermal field distribution was verified with heating experiment. The temperature of the target point rose up to 65°C from ambient temperature within 3 minutes of sonication. Significant lesion was visible in the tissue ablation experiment. Conclusion: From the mathematical model and experimental results, it appears that MIFU can potentially be used for soft tissue ablation such as treating breast cancer with better skin sparing. Acknowledgement: Supported by NCI‐R33‐CA107860.

SU‐FF‐T‐382: Special Dosimetric/Measurement Considerations in Commissioning a Novel Integrated MiniMLC Linear Accelerator

Purpose: To present an overview of commissioning a novel integrated miniMLC linear accelerator system, including the complete steps in final clinical implementation. Special precautions due to the collimator design are identified. Reference datasets for electrons are included. Method and Materials: Elekta BeamModulator has 40 pairs of 4mm leaves. There are no movable backup diaphragms and the field is defined only by the open leaves. Smaller field sizes and more precise leaf positioning (<1mm) are specified. The film scanning system was verified to within 0.2mm accuracy. Microchambers were used for in‐water scanning. Brass and graphite miniphantoms were constructed for head‐scatter factor measurements of fields from 0.8cm×0.8cm to 16cm×16cm. A complete set of scan and point measurement data was collected for photons and electrons. Beams were modeled in XIO and PrecisePlan and an independent calculation program (RADCALC). Before actual clinical implementation, periodic QA baselines were established, and site specific IMRT plans and QA measurements were performed on phantoms. Results and Conclusion: An extensive and comprehensive program was employed in commissioning. Beam data collection and calibration were internally verified by at least two independent measurements and checked against standard datasets. Treatment planning system modeling followed the guidelines of TG53. When compromises had to be made, the best fits were chosen for situations mimicking IMRT segments (4cm×4cm and 4.8cm×4.8cm). QA measurements of 3D conformal plans and IMRT plans achieved the following agreement statistics: 3mm DTA, 3% difference, produced pass rate of 97.8% average (2.6%STD). Dose point measurements with chambers agreed to plan values within 3.6%. After comparisons between 3D dose, independent monitor unit(MU) and manual calculations; Radcalc and XIO independent MU calculation programs were deemed unusable (the discrepancy reaching 5.4%), due to incorrect modeling of head‐scatter factors for this collimator. Additionally, measured electron cone factors varied up to 13% from standard Elekta linacs.

TU‐D‐BRB‐09: Comparison of a Novel Curvilinear Approach to Conventional Rectilinear Approach for Prostate Seed Implant

Purpose: To evaluate dosimetric benefit of curvilinear distribution of seeds for low‐dose‐rate (LDR) prostate brachytherapy with I‐125 isotopes.Materials and Methods:In this study, ten LDR prostate brachytherapy cases used clinically were randomly selected as reference rectilinear cases. These cases were planned intra‐operatively with VariSeed version7.1 using I‐125 seeds (STM‐1251 model). All the cases were replanned by using curved‐needles keeping the same individual seeds activity and volume receiving 100% of prescribed dose 145Gy (V100). Various parameters such as number of total needle, number of total seeds (or total activity), and dose coverage of prostate (D90, V150, V200), urethra (D30, D10) and rectum (D5, V100 in cc) were compared. Results: Average reductions in required number of needles and seeds (or activity) were 30.1% and 10.6%, respectively for curvilinear approach. In case of prostate, average D90 reduced by 7.9% (203.9Gy in rectilinear, 187.5Gy in curvilinear); average V150 and V200 coverage reduced by 17.9% (66.4% in curvilinear) and 32.6% (29.4% in curvilinear), respectively. For urethra, average D30 reduced by 10% (from 205.3Gy in rectilinear to 184.7Gy in curvilinear) and D10 reduced by 9.4% (from 210.8Gy in rectilinear to 190.0Gy in curvilinear). In case of rectum, average reduction in V100 was 71.6% (from 0.91cc in rectilinear to 0.21cc in curvilinear) and reduction in D5 was 17.7% (from 160.4Gy in rectilinear to 131.2Gy in curvilinear). Discussion and Conclusions: We observed considerable reduction in all studied parameters for curvilinear approach. Prostate dose homogeneity (V150, V200) improved while urethral dose is reduced, which will potentially result in better treatment. Large reduction in rectal dose would potentially reduce rectal toxicity and complications. Reduction in number of needles would minimize edema and thereby would improve accuracy of seed delivery and total dose distribution. This study indicates that the curvilinear approach is dosimetrically superior to conventional rectilinear technique.

SU‐FF‐J‐26: Multimodal Sono‐Contrast NIR Spectroscopy System for Breast Cancer Diagnosis

Purpose: To present a multimodal imaging system that combines three modalities, optical spectroscopy,ultrasonography and acoustic radiation force (ARF) for improved diagnosis of breast cancer based on noninvasive interrogation of vasculature. Method and Materials: The scanhead incorporates two 1MHz focused transducers aligned orthogonally to deliver ARF in a 2.4cm region. A commercial ultrasound probe is positioned to image the focal plane. The ultrasound probe and focused transducers have two degrees‐of‐freedom, so that the focal zone can be scanned in tissue depth‐wise and laterally. Three force sensors are mounted on the breast compression plate to ensure consistent, slight compression. A fiber array (6 sources and 12 detectors) connects to the concave bottom plate of the scanhead. Sources are illuminated by two laser diodes (680 and 830nm) through a multi‐channel optical switch. Diffuse reflectance signals are collected by 12 detectors, amplified by avalanche photodiodes, and transmitted through a multi‐channel data acquisition card for analysis. The scanhead is mounted on a 360° ring gantry, which can rotate around the breast and move towards or away from the patient. A software system was implemented in Visual C++ to perform real‐time tasks, including acquisition of optical and force signals, control of optical switch and function generator, and data display. Results: The maximum exposure to laser was controlled within 0.2W⋅cm−2 (ANSI Z136.1). The ARF fields were maintained below FDA diagnostic limits (0.72W⋅cm−2) for manipulating blood flow and oxyhemoglobin concentrations at the time and location of detection, causing measurable differences in the dynamic behavior of tissue blood supply as reported by optical spectroscopy. This technique was demonstrated to be highly diagnostic in a murine tumor model. Conclusions: A multimodal system incorporating ARF, optical spectroscopy and ultrasonography has been developed to characterize breast cancers. Pilot clinical study is being carried out. Acknowledgement: Work supported by NCI Grant CA107860.

SU‐FF‐J‐161: Feasibility Analysis On Converting Conventional Orthovoltage Biological Irradiator to a Micro‐Beam Array for Small Animal/cell Irradiation

Purpose: To evaluate the feasibility and restrictions in converting an X‐ray orthovoltage biological irradiator, PXI‐X‐RAD‐320, to a micro‐beam array using Geant4 Monte Carlo Simulation. Two important aspects of the beam, the peak‐to‐valley ratio (PVR) and beam flux, were evaluated quantitatively. Method and Materials: X‐ray tube used in the irradiator (MXR‐321) was simulated to generate the angular dependent X‐ray spectrum. With this spectrum, X‐ray beams impinged onto a plate with micro‐grids (micro‐beam plate) or a control plate (normal‐beam plate), with photons collected by detectors underneath at spatial resolution of 0.02mm. The impact of such factors as source‐to‐plate distance, plate‐to‐detector distance, with/without blocking, and the diameter of blocking on the beam profiles and flux was evaluated. Kodak EDR2 films were analyzed by VIDAR‐Dosimetrypro‐Advantage scanner.Results: PVR increased and beam flux decreased with decreasing plate‐to‐detector distance. For instance, keeping the source‐to‐plate distance at 345mm from source, a decrease of the plate‐to‐detector distance from 5cm to 1mm led to PVR increase from 1.3 to 687.3 and beam flux increase from 1.16% to 1.27%. The relative low resolution of scanner caused micro‐beam doses being averaged within vicinity area. It was showed that a PVR of 1.916 got from film was consistent with 1.843 from average of the simulation data for 1mm plate‐to‐detector distance. As the source‐to‐plate distance became larger at constant plate‐to‐detector distance, PVR increased whereas beam flux decreased. In situations where a larger plate‐to‐target distance is desired, a blocking with small orifice must be used. While providing an outstanding PVR at orifice diameter of 0.65mm, this method resulted in an extremely low beam flux. Conclusion: A conventional x‐ray irradiator can be converted to a micro‐beam array but with limitations. The primary restriction is caused by machines large irradiation angular coverage. Advanced method should be developed to evaluate micro‐beam dose from film.

SU‐FF‐T‐125: Commissioning of Monaco Monte Carlo IMRT Treatment Planning System

Purpose: To commission CMS Monaco IMRTtreatment planning system for clinical applications using a standard test suite. Methods and Materials: A system test including IMRT planning and plan delivery was performed for CMS Monaco IMRTtreatment planning system, version 1.0.2. Four cases from TG‐119 were studied: one head and neck case, one prostate case, and two C shape cases. IMRT plans were generated with the dose goals set by TG‐119 and were compared with the plans generated using CMS XiO treatment planning system. Analysis metrics were for dose coverage, number of segments, total MU, MU efficiency, number of segments per beam, and delivery time. The IMRT plans were delivered on an Elekta Synergy linear accelerator with step‐and‐shoot technique. This linac has 4‐mm MLC.IMRT QA was performed with field by field review using a diode array device Mapcheck. Results: Monaco plans showed similar target dose coverage as XiO plans and improved organ sparing in some cases. The ratios (Monaco/XiO) of number of total segments, total MU, number of segments per beam, and MU efficiency (Dose/MU) were 0.21–0.76, 0.56–0.77, 0.21–0.50, and 1.31–1.86, respectively. The delivery time of a Monaco plan was shorter than the XiO plan having the same number of beams by approximately 33%. IMRT QA pass rates of the Monaco plans were 93.8%–97.6% for the criteria of 3‐mm, 3%, and 10% threshold, which in three cases were higher than the XiO plans (93.1%–96.6%). Conclusion: The IMRT plans from the Monaco treatment planning system showed advantages over XiO plans. These advantages include smaller number of segments, smaller MU, smaller number of segments per beam, and higher MU efficiency. Additionally, the delivery time was shorter. The QA pass rates were similar to or higher than the XiO plans.

SU‐FF‐J‐27: Calibration of the Dual‐Transducer Focused Acoustic Radiation Field for a Multimodal Sono‐Contrast NIR Spectroscopy System

Purpose: To calibrate the dual‐transducer focused acoustic radiation field, register the focus center in the imagingultrasound system, and establish a monthly calibration protocol for a noninvasive multi‐modal breast cancer scanning system. Method and Materials: The scanhead incorporated two 1MHz focused transducers aligned orthogonally to deliver acoustic radiation force (ARF) in a 2.4cm region. A commercial ultrasound probe, located in the same vertical plane as the focused transducers, was used in the system for image co‐registration. Dielectric fluid was filled into the scanhead as the ultrasound transmission media. The scanhead was merged into a water tank filled with degassed distilled water. A function generator connected with an amplifier was used to drive the focused transducers. The imagingultrasound probe was connected to a commercial ultrasound system. The focused transducers were excited by 1MHz continuous sine wave with amplitude of 200mv from a function generator, amplified by the amplifier. A hydrophone was aligned vertically below the bottom plate and moved by a 3D motion platform (resolution 0.05mm) in the 3D Cartesian space at a step of 0.1mm. An oscilloscope was connected with the hydrophone for data display and acquisition of the voltage signals for calculating the acoustic intensity. Results: The focal spot that has the maximum peak‐positive voltage signal on the oscilloscope was found at 2.34 cm below the bottom plate, which is the designed intersection of the axes of the two focused transducers. The Spatial Peak Temporal Average Intensity was determined to be 0.4 W⋅cm−2, below the FDA therapeutic ultrasound limits (0.72 W⋅cm−2). The focal spot was then registered in the imagingultrasoundimages.Conclusions: This study established the baseline of the dual‐transducer focus acoustic radiation field. A monthly calibration protocol has been developed following the same procedure for the focused acoustic radiation field. Acknowledgement: supported by NCI Grant CA107860.

SU‐FF‐T‐36: Prostate Brachytherapy Seed Immobilization: Three Novel Techniques as Potential Candidates for Robotic Implementation

Purpose: Local movement and distant migration of radioactive seeds from prostate implants are known to occur frequently. We have developed three novel techniques amendable to robotic delivery for reducing seed migration/movement. This study compares the efficacy of these techniques and investigates the effect of these techniques on I‐125 radioactive seeds. Method and Materials: Three techniques developed for preventing seed movement/migration were: surgical glue (BioGlue®) to seal needle track, Diathermy Coagulation, and Laser Coagulation. The selection of mode and power for laser and diathermy were based on the effectiveness of tissue welding with least tissue lesion, charring and smoke. Dummy seeds were implanted in fresh bovine liver phantoms with and without applying immobilization techniques; then the phantoms were given 10hr continuous rocking motion. Seed movements were measured from initial and final x‐ray films. Sustainability of radioactive seeds in these techniques was evaluated by exposing live I‐125 radioactive seeds (0.63mCi/seed) in surgical glue, laser and diathermy energy. Activities of the seeds were individually measured and recorded before and after exposing to glue, laser and diathermy. Wipe test was performed to ensure no leakage. Results: Without applying the immobilization techniques, seed movements were (max/mean): 34.4mm/18.7mm. Seed movements after using immobilization techniques were (max/mean): in Surgical glue 1.1mm/0.9mm; in Diathermy coagulation 2.1mm/1mm; in Laser coagulation 2.9mm/1.1mm. Evaluation of dosimetric distribution indicated that non‐immobilized seed movement would alter planned dose distribution considerately, while by using immobilization techniques the deviations in dose distribution were greatly reduced. No significant effects of immobilization techniques were observed on radioactivity of the seeds. Wipe test ensured no leakage. Conclusions: Results from the experiments revealed that these three novel techniques are quite promising for improving dose distribution by reducing seed movement and migration. Moreover, these methods did not alter normal radioactivity of I‐125 seeds and preserved encapsulation. Acknowledgement: supported by DOD‐W81XWH‐06‐1‐0227.

SU‐GG‐T‐520: Inhomogeneity Corrected/Non‐Corrected Plan Evaluation for Simple and Conformal Plans For Radiation Therapy of Lung Cancer

Purpose: To investigate the effects on dose to critical structures for density corrected planning on simple (APPA and off‐cord) and conformal (4–5 field) lung plans compared to historical experience assuming homogeneous density. Method and Materials: Ten patients were planned using both techniques with XIO CMS software. The conformal plans were normalized to percentage coverage of target volumes and the simple plans were calculated to midplane. All plans were calculated using homogeneous density as the standard. After planning, CT density corrections were implemented and monitor units were matched to the homogeneous plan; the plans were not re‐optimized. Dose volume histograms were reviewed for critical structures of cord, “hot spot” D5(PTV), and V20(lung). Plan uniformity was assessed by the slope of the PTV dose curve using the equation [(D5–D95)/Dmean]. Results: The V20 increased with corrections: 23.7±1.7 to 24.6±7.2 for complex plans; 25.2±10.8 to 26.1±11.3 for simple plans. The conformal plans degraded in uniformity by a factor twice that of the simple plans and D5 increased accordingly. Cord dose increased slightly in conformal plans (33.65Gy±12.0Gy to 34.38±12.2Gy) and remains similar in simple plans (37.83±16.5Gy to 38.11±16.8Gy). The range of cord dose variations in corrected conformal plans was from −0.22–3.24Gy; one increase raised the cord V1% from 45.95Gy to 48.84Gy. Additionally, since the simple plans historically were assumed to approximate zero cord dose for the off‐cord obliques, the calculated/delivered cord dose is much higher. The effect of density corrections increased notably in the conformal plans where more beams transverse the lung.Conclusion: Although the effect of homogeneity corrections are well documented, the outcome on plan uniformity and critical structure tolerances should be further examined when transitioning from simple to multi‐field beam arrangements including IMRT. This study provides clinical dosimetric reference for adoption of heterogeneity corrected planning.