Ramani Ramaseshan

Dosimetric evaluation of Plastic Water Diagnostic Therapy

High‐precision radiotherapy planning and quality assurance require accurate dosimetric and geometric phantom measurements. Phantom design requires materials with mechanical strength and resilience, and dosimetric properties close to those of water over diagnostic and therapeutic ranges. Plastic Water Diagnostic Therapy (PWDT: CIRS, Norfolk, VA) is a phantom material designed for water equivalence in photon beams from 0.04 MeV to 100 MeV; the material has also good mechanical properties. The present article reports the results of computed tomography (CT) imaging and dosimetric studies of PWDT to evaluate the suitability of the material in CT and therapy energy ranges. We characterized the water equivalence of PWDT in a series of experiments in which the basic dosimetric properties of the material were determined for photon energies of 80 kVp, 100 kVp, 250 kVp, 4 MV, 6 MV, 10 MV, and 18 MV. Measured properties included the buildup and percentage depth dose curves for several field sizes, and relative dose factors as a function of field size. In addition, the PWDT phantom underwent CT imaging at beam qualities ranging from 80 kVp to 140 kVp to determine the water equivalence of the phantom in the diagnostic energy range. The dosimetric quantities measured with PWDT agreed within 1.5% of those determined in water and Solid Water (Gammex rmi, Middleton, WI). Computed tomography imaging of the phantom was found to generate Hounsfield numbers within 0.8% of those generated using water. The results suggest that PWDT material is suitable both for regular radiotherapy quality assurance measurements and for intensity‐modulated radiation therapy (IMRT) verification work. Sample IMRT verification results are presented. PACS number: 87.53Dq

Template-based breast IMRT planning for increased workload efficiency

BackgroundTo be less resource intensive, we developed a template-based breast IMRT technique (TB-IMRT). This study aims to compare resources and dose distribution between TB-IMRT and conventional breast radiation (CBR).MethodsTwenty patients with early stage breast cancer were planned using CBR and TB-IMRT. Time to plan, coverage of volumes, dose to critical structures and treatment times were evaluated for CBR and TB-IMRT. Two sided-paired t tests were used.ResultsTB- IMRT planning time was less than CBR (14.0 vs 39.0 min, p < 0.001). Fifteen patients with CBR needed 18 MV, and 11 of these were planned successfully with TB-IMRT using 6 MV. TB-IMRT provided better homogeneity index (0.096 vs 0.124, p < 0.001) and conformity index (0.68 vs 0.59, p = 0.003). Dose to critical structures were comparable between TB-IMRT and CBR, and treatment times were also similar (6.0 vs 7.8 min, p = 0.13).ConclusionsTB- IMRT provides reduction of planning time and minimizes the use of high energy beams, while providing similar treatment times and equal plans compared to CBR. This technique permits efficient use of resources with a low learning curve, and can be done with existing equipment and personnel.

Applications for Low Frequency Impedance Analysis Systems

We present methodology and instrumentation used to carry out and log automated multiple electrical impedance measurements using multiplexed control. We also address the issue of measurement error introduced by the instrumentation, and demonstrate how we reduce these effects in our experiments. Finally, we present two potential applications for our automated electrical impedance analysis systems: tissue scanning and mapping via impedance measurements between arrays of electrodes, and materials testing of novel conductive polymer materials.

Embedded process for flexible conductive electrodes for applications in tissue electrical impedance scanning (EIS)

We present the fabrication and testing of micro-patternable flexible conductive nanoparticle composite polymer (C-NCP) electrode arrays for electrical impedance scanning (EIS). We attempt to minimize EIS issues of mechanical skin contact and resolution through the use of highly compliant micropatternable elastomeric C-NCPs. We anticipate an increase in spatial resolution as the electrodes can be patterned into high density arrays using a new multi-level process presented here for the first time. We characterize the conductivity of the electrodes (average resistivity of 2.98×10−4 ohm-m +/− 8.3% at 60 wt-% of silver nanoparticles), compare the baseline impedance map with a new circuit phantom, and demonstrate anomaly detection in a gelatin tissue phantom using highly flexible Ag/AgCl C-NCP electrodes.

Initial experiments with flexible conductive electrodes for potential applications in cancer tissue screening

We present initial results on the fabrication and testing of micropatternable conductive nanocomposite polymer (C-NCP) electrodes for tissue impedance measurements. We present these proof-of-concept results as a first step toward the realization of our goal: an improved Electrical Impedance Scanning (EIS) system, whereby tissue can be scanned for cancerous tissue and other anomalies using large arrays of highly flexible microfabricated electrodes. Previous limitations of existing EIS system are addressed by applying polymer based microelectromechanical system (MEMS) technology. In particular, we attempt to minimize mechanical skin contact issues through the use of highly compliant elastomeric polymers, and increase the spatial resolution of measurements through the development of microelectrodes that can be micropatterned into large, highly dense arrays. We accomplish these improvements through the development of C-NCP electrodes that employ silver nanoparticle fillers in an elastomer polymer base that can be easily patterned using conventional soft lithography techniques. These new electrodes are tested on conventional tissue phantoms that mimic the electrical characteristics of human tissue. We characterize the conductivity of the electrodes (average resistivity of 7x10-5 ohm-m +/- 14.3% at 60 wt-% of silver nanoparticles), and further employ the electrodes for impedance characterization via Cole-Cole plots to show that measurements employing C-NCP electrodes are comparable to those obtained with normal macroscopic metal electrodes. We also demonstrate anomaly detection using our highly flexible Ag/AgCl C-NCP electrodes on a tissue phantom.

SU‐FF‐I‐14: Investigation of Metallic Filters On the Detectability of Soft Tissues in Cone Beam Computed Tomography Using Flat Panel Detector of Acuity Simulator

Purpose: The purpose of this work is to investigate the image quality with metallic filters partially or completely covering the CBCTdetector.Materials and methods:Aluminum metallic filters of various thicknesses were placed on top of the detector covering them either completely or partially. The influence of these filters on the detectability of soft tissue was evaluated quantitatively. The parameters investigated included signal to noise ratio (SNR),contrast to noise ratio (CNR),image uniformity and CT number accuracy. The studies were carried out using the Catphan phantom imaged in the half fan configuration mode. Assessment was carried using a 36mm2 region of interest (ROI). CNR was determined with ROIs within the insert and adjacent to the insert of the low contrast object. SNR was assessed in the uniform water region. CT numbers were measured for air, PMP, LDPE, polystyrene, acrylic, delrin, Teflon and water. All the measurements were carried out at 125kVp and 80mA with slice thickness of 2.5mm. Results: The SNR for half fan mode improved by 31% for the 0.6mm filter covering the detector uniformly while the 1.5mm filter partially covering the detector (on the non bowtie side) improved SNR by 34%. For the similar setup the CNR improved by 33% and 10% respectively. These improvements could be due to metallic filter selectively absorbing low energy scattered photons. No noticeable improvement in image uniformity was observed. The CT number accuracy was not compromised by the use of filter. Conclusion: The results of our current investigation suggest use of metallic aluminum filter on flat panel detector can improve the ability to detect low contrast objects. A series of experiments are currently underway to evaluate the effect of different filters under different geometries for both full and half fan image acquisitions.

An approach for measuring the spatial orientations of a computed-tomography simulation system

The quality assurance tests for measuring the spatial orientations between tabletop, external patient positioning lasers, couch longitudinal moving direction, and imaging plane in a CT simulation system are a complicated and time‐consuming process. We proposed a simple and efficient approach to acquire the angular deviations of spatial orientations between these components. An in‐house cross‐jig was used in this study. We found a relationship between the orientations of the jigs arms shown on the CT images and the orientations of the components in a CT simulator. We verified this relationship with 16 misalignment orientations of known errors, to simulate all possible deviation situations. Generally, the tabletop and external lasers system are mounted separately in a CT simulation system; the former is on the couch trail, the later is on the wall and ceiling. They are independent to each other and will cause different effects on CT images. We only need two scans to acquire the angular deviations of our system: i) when aligning the cross‐jig with tabletop, we can check the orientations between the tabletop, couch longitudinal moving direction, and imaging plane; ii) while aligning the cross‐jig with the external axial lasers, we will know the angular deviation between the lasers, couch longitudinal moving direction, and imaging plane. The CT simulator had been carefully examined by performing the QA procedures recommended by the AAPM Task Group 66. The measurements of the spatial orientations using the proposed method agree well with TG 66 recommendations. However, the time taken to perform the QA using our method is considerably shorter than the method described in TG 66 — 5 minutes versus 30 minutes. The deliberate misalignment orientations tests with known errors were detected successfully by our in‐house analysis program. The maximum difference between the known errors and the measured angles is only 0.07°. We determined that the relationship between the orientations of the jigs arms and the orientations of the CT components. By means of quantifying the deviations in degree we can correct the errors accurately. This approach can also be used to inspect the spatial orientations of other imaging systems, such as PET‐CT and MRI. PACS number: 87.57.Q‐

Poster — Thur Eve — 47: Evaluation of the ArcCHECK device for commissioning and patient‐specific QA

The most promising method of accurately verifying VMAT treatments is by direct dose measurement over the three dimensions of irradiated volume. ArcCHECK device (Sun Nuclear, Melbourne, FL) have the potential to detect delivery errors on the treatment machine due to mechanical problems resulting from gantry and MLC motion. The estimation of the dosimetric leaf gap (DLG) parameter for Varian MLC (Varian Medical Systems, Palo Alto, CA) was attempted using ArcCHECK. Finding the optimal DLG value for use in TPS requires a measuring device like ArcCHECK to be employed especially in highly intensity modulated fields. In addition, ArcCHECK was used to assess the effect of positional error of MLC leaf in a given VMAT plan. Patient-specific QA tests were performed using the ArcCHECK device. QA results of patient plans that failed considering portal dosimetry technique were reassessed with ArcCHECK measurements for IMRT plans. The preliminary test results and performance of the ArcCHECK device were very encouraging. VMAT plans for head and neck cases were generated and their delivery was evaluated using ArcCHECK. Results have shown a success rate greater than 90% in the quality assurance of individual plans. Optimal DLG value was detected using ArcCHECK. Also, the device showed enough sensitivity to identify failed QA plans. Moreover, MLC central leaf pair position offset in a VMAT plan of the order of 1mm was fairly distinguished by ArcCHECK measurements.

Poster — Thur Eve — 57: Craniospinal irradiation with jagged‐junction IMRT approach without beam edge matching for field junctions

PURPOSE Craniospinal irradiation were traditionally treated the central nervous system using two or three adjacent field sets. A intensity-modulated radiotherapy (IMRT) plan (Jagged-Junction IMRT) which overcomes problems associated with field junctions and beam edge matching, improves planning and treatment setup efficiencies with homogenous target dose distribution was developed. METHODS AND MATERIALS Jagged-Junction IMRT was retrospectively planned on three patients with prescription of 36 Gy in 20 fractions and compared to conventional treatment plans. Planning target volume (PTV) included the whole brain and spinal canal to the S3 vertebral level. The plan employed three field sets, each with a unique isocentre. One field set with seven fields treated the cranium. Two field sets treated the spine, each set using three fields. Fields from adjacent sets were overlapped and the optimization process smoothly integrated the dose inside the overlapped junction. RESULTS For the Jagged-Junction IMRT plans vs conventional technique, average homogeneity index equaled 0.08±0.01 vs 0.12±0.02, and conformity number equaled 0.79±0.01 vs 0.47±0.12. The 95% isodose surface covered (99.5±0.3)% of the PTV vs (98.1±2.0)%. Both Jagged-Junction IMRT plans and the conventional plans had good sparing of the organs at risk. CONCLUSIONS Jagged-Junction IMRT planning provided good dose homogeneity and conformity to the target while maintaining a low dose to the organs at risk. Jagged-Junction IMRT optimization smoothly distributed dose in the junction between field sets. Since there was no beam matching, this treatment technique is less likely to produce hot or cold spots at the junction in contrast to conventional techniques.

SU‐E‐I‐95: Using Angular Magnitude Evaluate the Spatial Orienations in Computed‐Tomography Simulation System

Purpose: To quantify the spatial orientations of the tabletop and external patient positioning lasers with respect to the couch moving direction and imaging plane in a CT simulation system by indicating as angular deviation. Method and Materials:A home‐made cross‐jig is used in this study. The longitudinal arm of the jig is orthogonal with the transverse arm and some BB markers inlaid in the jig to indicate the arms on CTimages. We found there are some relationships between the orientations of the jig arms which shown on the images and the orientations among the tabletop, external lasers, couch moving direction, and imaging plane. First, the orientations of CT simulation system had been carefully confirmed by using the approaches mentioned on TG 66. To simulate all possible deviating situations, we designed 8 orientation arrangements for the tabletop and external lasers, respectively, due to these two components cause different effects on CTimages. When attaching the cross‐jig on tabletop we can check the orientations between the tabletop, couch moving direction, and imaging plane. While aligning the cross‐jig with the external lasers we will know the angular deviation between the external lasers, couch moving direction, and imaging plane. Results: Based on the preliminary analysis, the maximum error is 0.12 degree. After considering the inherent angular deviations in our CT simulator, image analysis errors due to the size of BB and imagespatial resolution, and the jig setup errors we can say all of the results conformed to our expectation. Conclusions: Only two scans, then we will know the spatial orientations in CT simulation. By means of accurately quantifying the deviations we can easily correct the error if it has to be. The method proposed in this study could also be used for detecting an angular deviation in any other orthogonal imagingsystem.