S Wang

A carbon nanotube field emission multipixel x-ray array source for microradiotherapy application

The authors report a carbon nanotube (CNT) field emission multipixel x-ray array source for microradiotherapy for cancer research. The developed multipixel x-ray array source has 50 individually controllable pixels and it has several distinct advantages over other irradiation source including high-temporal resolution (millisecond level), the ability to electronically shape the form, and intensity distribution of the radiation fields. The x-ray array was generated by a CNT cathode array (5×10) chip with electron field emission. A dose rate on the order of >1.2 Gy∕min per x-ray pixel beam is achieved at the center of the irradiated volume. The measured dose rate is in good agreement with the Monte Carlo simulation result.

Stationary micro-CT scanner using a distributed multi-beam field emission x-ray source: a feasibility study

Current micro-CT scanners use either one or two x-ray tubes that are mechanically rotated around an object to collect the projection images for CT reconstruction. The rotating gantry design hinders the performance of the micro-CT scanner including the scanning speed. Based on the newly emerged carbon nanotube based distributed multi-beam x-ray array technology, we have proposed to build a stationary gantry-free multi-beam micro-CT (MBμCT) scanner. To investigate the feasibility of this concept, a prototype system using a source array with 20 individually controlled x-ray beams has been designed and tested. The prototype CT scanner can generate a scanning x-ray beam to image an object from different viewing angles (coverage of 36°) without any rotation. The electronics and software for system control and data have been implemented. The projected performance of the prototype MBμCT imaging system was discussed and some preliminary imaging results were presented.

WE‐E‐BRB‐02: Current IMRT QA Metrics Are Not Correlated with Clinically Relevant Dosimetric Errors in Prostate and Head Neck Treatments

Purpose: To investigate if the degree of IMRT QA error characterized by the conventional metrics is correlated with clinically relevant dosimetric errors for prostate and head‐neck treatments. Methods: We examined 37 head‐neck and 41 prostate treatments, each had at least one field failed IMRT QA on MapCHECK judged by one of the standards m 95% passing rate at 3%/3mm, 5%/4mm, or 7%/5mm. The resulting deviation on target and critical normal‐structure doses was computed using in‐house TPS PlanUNC. PLanUNC reproduced the QA‐failed treatment plan using MapCheck‐measured beam characteristics. The modified plan represents the actual treatment that patient would receive if treatment (that failed QA) had been delivered. The degree of IMRT QA error was correlated with its dosimetric impact on minimum dose to 95% of the target volume [D95], mean and maximum doses to critical structures using Pearson correlation. A commercial software (3DVH) for 3D dosimetry study using MapCHECK QA data is also evaluated. Results: There is a lack of general correlation between QA passing rate and resulting error in PTV D95 and critical structures ((bladder, rectum, femur heads for prostate and brainstem, cord, parotids for head and neck) mean and max. dose (Pearson r value −0.146, range −0.519  0.130). More stringent QA criterion does not have better correlation with the 3D dosimetry error than less stringent criterion (r = −.154+/−0.15 for 3%/3mm, r= −1.93+/−0.192 for 5%/4mm, and r= −1.85+/− 0.14 for 7%/5mm). The IMRT QA errors for all cases studied, if left uncorrected, would lead to errors of 0.7% for the D95 (range −4.0–2.8%), 2.15% for the cord dose (−3.3–21.9%), and 0.6% (−9.5–4.9%) for all other critical structures. Conclusions:We observed no strong and consistent correlation between the degree of IMRT QA error and the degree of clinically relevant patient dose error.

Design, optimization and testing of a multi-beam micro-CT scanner based on multi-beam field emission x-ray technology

As a widely adopted imaging modality for pre-clinical research, micro-CT is constantly facing the need of providing better temporal as well as spatial resolution for a variety of imaging applications. Faster CT scanning speed is also preferred for higher imaging throughput. We recently proposed a gantry-free multi-beam micro-CT (MBμCT) design which has the potential to overcome some of the intrinsic limitations of current rotating-gantry CT technology. To demonstrate its feasibility, we have constructed a testing system with a multi-beam field emission x-ray (MBFEX) source array with a linear array of 20 individually controllable x-ray emitting pixels. Based on simulations of the electron optics and preliminary experimental measurements the design of the MBFEX source has been further optimized. The newly designed imaging system has been characterized and commissioned following our standard imaging protocol. It has clearly shown improved system stability and enhanced imaging capability. As a result of reduced mechanical rotation during imaging acquisition, we are expecting to achieve higher CT scanning speed without significantly sacrificing imaging quality. This prototype MBμCT system, although still in its early development phase, has been proved to be an ideal testing platform for the proposed gantry-free micro-CT scanner.

3D Analysis of Intensity-Modulated Radiation Therapy Quality Assurance Measurement using a 2D Diode Array

Intensity-modulated radiation therapy (IMRT) quality assurance (QA) is often performed using a 2D device and compares measured and computed fluence maps to determine if a field passes or fails certain dose and position criteria. The effects of a measured deviation to the 3D patient spatial dosimetry and dose volume histogram (DVH) are largely unknown because they cannot be analyzed using commercial 2D array IMRT QA systems. We report an in-house treatment planning system (TPS) PLanUNC based 3D IMRT QA analysis approach that has been used in our institution for the past ten years when 2D fluence map IMRT QA failed. In this approach the measured 2D fluence maps are imported back to PLanUNC and used to re-compute 3D patient dosimetry including DVHs. The 2D fluence map IMRT QA criteria is that the measured dose for 95% of the detectors is within 5% of the planned dose, and that the distance-to-agreement be within 4mm (5%/4mm). 22 IMRT plans that had at least one field fail initial QA using MapCHECK 2 are examined using our 3D QA approach. The DVH analysis shows that 19/22 plans that failed initial QA were within 2% of the planned target and critical structure DVHs. 3/22 IMRT plans were found to have DVH difference greater than 2%. The 3D analysis of 2D IMRT QA result shows that when a fluence map QA fails for a single field, often it is clinically insignificant in terms of patient 3D dosimetry

Fabrication and Characterization of Individually Controlled Multi- Pixel Carbon Nanotube Cathode Array Chip for Micro-RT Application for Cancer Research

We report here the development of a new carbon nanotube (CNT) field emission multi-pixel cathode array chip, a vital component for the multi-pixel beam x-ray micro-radiotherapy (micro-RT) system under development in our group for cancer research. The CNT field emission cathode array chip has up to 25 (5 × 5) individually addressable cathode pixels, each 1 mm in diameter and with center-to-center distance of 2 mm. The fabrication is a two-step process: first a Cr/Cu electrical contact was fabricated on Si substrates with a 5 μm SiO(2) dielectric layer using photolithography; and second the CNTs were selectively deposited on 1 mm-diameter predefined Cr/Cu contact dots by using a combined photolithography/electrophoresis deposition technique. The electron pixel beams produced from the multi-pixel array chips are uniform and individually controllable. Each pixel beam is expected to generate a dose rate in the order of 100 cGy/min based on our Monte Carlo simulations.

SU‐E‐T‐166: Clinical Impact of Minor Errors Discovered in Conventional IMRT QA On Patient Treatment

PURPOSE To demonstrate that conventional IMRT QA metrics and passing criteria should be replaced by DVH-based criteria that can efficiently interpret the clinical impact of minor errors found for head and neck and prostate cancer patient treatments. METHODS 41 prostate cancer and 31 head & neck (H&N) cancer clinical IMRT QAs using MapCheck were retrospectively analyzed using three different tightness of the pass/fail metrics (3%/3mm, 5%/4mm, and 7%/5mm). An in-house TPS, PLanUNC, was used to reproduce the accelerator-delivered plan from the beams measured by MapCheck in IMRT QA. The impact of the deviation/error discovered in the per-field IMRT QA on PTV D95, mean dose and max. dose of major OARs are computed and correlated with the conventional IMRT QA pass rate and tightness of IMRT metrics, and compared with the impact of a 2mm random setup error on patient dosimetry. RESULTS Statistically there is no correlation between the quality of DVH-based dosimetry for PTV and OARs and the conventional IMRT QA metrics and the criteria tightness. The standard deviation of errors in PTV D95 for H&N and prostate treatments due to a 2mm random setup error are 0.7% and 2.2%, respectively. The same standard deviation due to errors discovered in IMRT QA is 1.1% and 1.0% for H&N and prostate, respectively. Our data shows that the clinical impact of a given error found in conventional IMRT QA is treatment site dependent. Based on this study, reducing IMRT errors found in H&N patients may have more clinical impact than for prostate patients. CONCLUSION While conventional per-field IMRT QA metrics are effective in catching gross error for patient safety it fails to interpret their clinical impact. A DVH-based IMRT QA evaluation should be used instead to timely and effectively identify and reduce the minor IMRT QA errors that have clinical significance.

TU‐C‐BRB‐08: Preliminary Study of DNA Damage and Repair of Tumor Cells under Microbeam Radiation Therapy Using an Integrated Carbon Nanotube‐Based Field Emission and Micropallet Array System

Purpose: This study aims at integration of carbon nanotube‐based field emission and micropallet array technologies to investigate the effects of microbeam radiation therapy(MRT) irradiation on the DNA damage and repair mechanisms of tumor cells in cancerradiation therapy.Methods: A carbon nanotube‐based field emissionelectron beamMRT device was used to deliver a 0.2 – 200 Gy electron MRTbeam to a monolayer of A549 lung cancer cells grown on glass coverslips. Cells were irradiated (100 Gy) and then fixed at various times for analysis. Fixed cells were stained for the stress‐induced protein phospho‐p38 using fluorescently tagged antibodies and fluorescence microscopy. Fluorescence data were analyzed using ImageJ analysis software. Results: The electron MRTbeam has a width of ∼40 microns and a dose rate of ∼100 Gy/s. Following irradiation, levels of phospho‐p38 increased rapidly peaking at six hours and then returning to basal levels by 24 hours. We observed that stress signaling such as phospho‐p38 is upregulated in a remarkably spatially discrete manner in monolayer cells irradiated with the microbeam irradiator. DNA damage as measured by gamma‐H2AX in A549 cells displayed kinetics similar to those for phospho‐p38 following irradiation. Conclusions: We have shown the feasibility of integrating our carbon nanotube‐based field emission and micropallet array system for MRT mechanistic study. Our initial study indicates that the DNA damage and repair from MRT may be different than from conventional therapeutic radiation but the details are still under investigation and clarification. Acknowledgements: This work is partially supported by the North Carolina University Cancer Research Fund. This work is partially supported by the University of North Carolina Cancer Research Fund

SU‐E‐T‐347: Evaluation of DQA Results Using a Super‐Sampling Dose Calculation in Helical Tomotherapy

Purpose: The aim of this work is to evaluate the impact of a new supersampling dose calculation method on delivery quality assurance (DQA) results for helical tomotherapy patient plans. Methods: Accurays Tomotherapy treatment planning system performs its dose calculation by approximating the continuous beam of a full gantry rotation into 51 discrete beam projections, with one dose calculation per projection (TomoHD version 1.0). In a recent software release, TomoHD version 1.1, Accuray enhanced this technique by employing three dose calculation samples per projection. This ‘super‐sampling’ methodology is meant to improve agreement between measured and calculated dose. For this study, we compare the results of the 24 patient DQA plans calculated in our clinic with the newer version of dose calculation with the previous 24 patient plans which were calculated with the older method. The plans were delivered to a SunNuclear ArcCHECK cylindrical detector array, and data were compared using a γ evaluation, with criteria of 3%/3mm. To quantify the results, the percentage of points with γ 95%. Results: 21 of 24 DQA plans (87%) calculated with the older TomoHD 1.0 algorithm passed our (Pγ 95% criteria, while all 24 DQA Plans (100%) generated with the TomoHD 1.1 super‐sampling dose calculation passed. The average values for (Pγ<l) was 97.9% and 98.9% for the original and super‐sampling calculation, respectively. The standard deviation for the older software was 2.1, versus 1.5 for the newer super‐sampling method. Conclusions: The increased number of samples per projection angle employed in the new TomoHD Version 1.1 software leads to a reduction in the dose discrepancies seen in patient DQA plan results. This can improve the agreement between the calculated dose and delivered dose to patients.

MO‐G‐BRB‐06: Integrating Carbon Nanotube and Micropallet Technologies for Microbeam Radiotherapy Biology Research

Purpose: Microbeam radiotherapy(MRT) is a preclinical therapy that has been shown in animal experiments to have a selective ability to eradicate tumor cells while sparing normal tissue. However, potential MRT clinical application is hindered by the lack of understanding of the biological mechanisms involved. To study DNA damage and repair mechanisms and cell survival under MRTirradiation, our objective is to integrate a carbon nanotubefield emissiontechnology‐enabled high dose rate (100s Gy/s) electron microbeam with a micropallet array technology‐enabled cell retrieval and analysis system. The focus of this presentation is a feasibility demonstration of the integrated system for MRT cellular irradiation. Methods: We have upgraded our carbon nanotube field emission‐based prototype cell irradiator system to produce a single or multiple ∼40 micron wide electron MRT beams at a dose rate range of several hundred Gy per second. Dosimetry is measured using GAFCHROMIC HD‐810 film. SW480 colon cancer cells are plated on individual square micropallets of 60 or 100 micron in size, placed in a customized culture dish. Cells can be collected from each individual micropallet while remaining adherent to their growth surface for further analysis. Results: We have demonstrated the MRT cellular irradiation using gamma‐H2AX, a nuclear histone protein that becomes phosphorylated in proximity to IR‐induced DNA double strand breaks. The MRT radiation beam profile shown by staining fixed cells post‐ IR for phospho‐H2AX was consistent with calculation. Conclusions: We have developed an integrated MRT cellular irradiation and analysis system using carbon nanotubefield emissiontechnology and micropallet array technology. We have characterized the dosimetry and demonstrated the feasibility of MRT beam irradiation in cellular irradiation for future mechanistic research on microbeam therapy. Acknowledgements: This work is partially supported by North Carolina University Cancer Research Fund.