Single Gantry Research Articles

TH‐C‐BRB‐10: Clinical Implication of the New Dosimetry Formalism in IMRT Quality Assurance

Purpose: To apply the new dosimetry formalism [Med. Phys. 35, 5179 (2008)] to clinical IMRT quality assurance (QA). Methods: 20 different linear accelerator (Varian Clinac 21 EX)‐based clinical IMRT fields were transferred to the CT images of a 30×30×17 cm3 Solid Water phantom to create IMRT QA fields. The phantom position was adjusted for each QA field to place the detector or chamber at the lowest dose gradient region in a virtual PTV. The reference doses in the IMRT QA and 10×10 cm2 fields were measured using a PTW micro liquid ion chamber (microLion). Based on the new dosimetry formalism, the clinical correction factor of each IMRT QA field was measured for a calibrated Exradin A12 Farmer‐type chamber in a fully‐rotated delivery and a delivery at a single gantry angle, a collapsed delivery. For each QA field, the measured dose with the correction factor was compared with a calculated dose using Analytical Anisotropic Algorithm (AAA) or Monte Carlo (MC) methods. Results: The clinical correction factor deviated from unity by up to 2.4% and 3.7% in the fully‐rotated and collapsed deliveries, respectively, depending on the dose homogeneity at the Exradin A12 collecting volume. In the fully‐rotated delivery, the measured dose with the correction factor is different from the calculated dose to within 5% and 3% for the AAA and MC, respectively. In the collapsed delivery, the discrepancy between the measured and AAA‐calculated doses was to within 8%, while it was improved to within 3.5% compared with the MC‐calculated dose. When applying the clinical correction factor, the decrease of the measured and calculated dose discrepancy is more significant for an IMRT QA field having higher dose heterogeneity. Conclusions: This work proves that the suggested dosimetry technique is effective to improve the dosimetric consistency of clinical IMRT QA.

TH‐C‐BRB‐11: Comparability of Four Different Planar IMRT QA Methods

Purpose: To investigate comparability of four different planar IMRT QA Methods: Patient Gantry Angle Composite (PGAC), Single Gantry Angle Composite (SGAC), Field by Field (FBF), and Patient Gantry Angle Composite using film (FPGAC). Methods: IMRT verification plans were generated by Varian Eclipse TPS for twelve heavily modulated head and neck (H&N) and twelve moderately modulated Prostate treatment plans. Sun Nuclear MapCHECK2 (2D diode array with detector spacing of 7.07 mm for SGAC, PGAC, and FBF) and GAFCHROMIC EBT2 film (0.34 mm/pixel for FPGAC) respectively sandwiched with 10 cm thick water equivalent phantom were used for the IMRT plan verification. All measurements were analyzed with Sun Nuclear ‘Patient’ software using the gamma criteria (MapCHECK2/Film) of 1%‐1mm/2%‐1mm (G1), 2%‐2mm/3%‐2mm (G2), and 3%‐3mm/4%‐3mm (G3) considering known 1% inherent dosimetric uncertainty of MapCHECK2. The Student's t‐test was used to compare passing rates with ‘Absolute Dose’ comparison and 10% threshold. Results: The average passing rates (H&N/Prostate) were 98.9/99.7% (PGAC), 99.6/100.0% (SGAC), 99.5/100.0% (FBF), and 92.0/95.8% (FPGAC) with the G3 criteria. Three MapCHECK2 QAs had statistically comparable passing rates for all comparisons (p‐value>0.05), while they did not show any significant difference based on the plan modulation. FPGAC showed systematically lower passing rates than the MapCHECK2 methods and statistically distinguished the degrees of modulation with all gamma criteria (p‐values of H&N against Prostate: 0.037 (G1), 0.012 (G2), and 0.016 (G3)). Conclusions: Three IMRT QA methods using MapCHECK2 are found to be comparably used for routine IMRT QA for both H&N and Prostate plans. This is against the belief that PGAC is more effective IMRT QA than FBF or SGAC. FPGAC showed significantly different passing rate based on the degree of plan modulation. These findings imply that the gamma passing rate heavily depends on the spatial resolution of detectors and there are intrinsic limitations of the gamma analysis.

In-gantry or remote patient positioning? Monte Carlo simulations for proton therapy centers of different sizes

PurposeWe estimated the potential advantage of remote positioning (RP) vs. in-room positioning (IP) for a proton therapy facility in terms of patient throughput. Materials and methodsMonte Carlo simulations of facilities with one, two or three gantries were performed. A sensitivity analysis was applied by varying the imaging and setup correction system (ICS), the speed of transporters (for RP) and beam switching time. Possible advantages of using three couches (for RP) or of switching the beam between fields was also investigated. ResultsFor a single gantry facility, an average of 20% more patients could be treated using RP: ranging from +45%, if a fast transporter and slow ICS were simulated, to −14% if a slow transporter and fast ICS was simulated. For two gantries, about 10% more patients could be treated with RP, ranging from +32% (fast transporter, slow ICS) to −12% (slow transporter, fast ICS). The ability to switch beam between fields did not substantially influence the throughput. In addition, the use of three transporters showed increased delays and therefore a slight reduction of the fractions executables. For three gantries, RP and IP showed similar results. ConclusionsThe advantage of RP vs. IP strongly depends on ICS and the speed of the transporters. For RP to be advantageous, reduced transport times are required. The advantage of RP decreases with increasing number of gantries.

Time-Resolved Interventional Cardiac C-arm Cone-Beam CT: An Application of the PICCS Algorithm

Time-resolved cardiac imaging is particularly interesting in the interventional setting since it would provide both image guidance for accurate procedural planning and cardiac functional evaluations directly in the operating room. Imaging the heart in vivo using a slowly rotating C-arm system is extremely challenging due to the limitations of the data acquisition system and the high temporal resolution required to avoid motion artifacts. In this paper, a data acquisition scheme and an image reconstruction method are proposed to achieve time-resolved cardiac cone-beam computed tomography imaging with isotropic spatial resolution and high temporal resolution using a slowly rotating C-arm system. The data are acquired within 14 s using a single gantry rotation with a short scan angular range. The enabling image reconstruction method is the prior image constrained compressed sensing (PICCS) algorithm. The prior image is reconstructed from data acquired over all cardiac phases. Each cardiac phase is then reconstructed from the retrospectively gated cardiac data using the PICCS algorithm. To validate the method, several studies were performed. Both numerical simulations using a hybrid motion phantom with static background anatomy as well as physical phantom studies have been used to demonstrate that the proposed method enables accurate reconstruction of image objects with a high isotropic spatial resolution. A canine animal model scanned in vivo was used to further validate the method.

CT Angiographic Measurement of Vascular Blood Flow Velocity by Using Projection Data

To determine whether flow velocity can be measured by using projection data from computed tomographic (CT) scans obtained during contrast material injection in a phantom model. The authors constructed a 12.7-mm-diameter single-channel flow phantom with constant water flow velocity settings of 25.3, 43.9, and 70.5 cm/sec. For each flow velocity, serial axial scans were obtained with 16-section multidetector CT while a 10-mL bolus of contrast material was injected upstream of the imaging plane. For each bolus injection, the CT projection data from the scan with the sharpest increase in magnitude of detected contrast material was used for flow velocity measurements. Flow velocity was calculated as the ratio of distance between CT detector rows and the corresponding time lag in the contrast enhancement curves and was correlated with the reference velocities. Five separate contrast material injections and CT measurements were made for each flow velocity setting. The correlation coefficient between the CT measurements of flow velocity and the reference measurements was 0.98 (P < .05). The mean CT measurements of flow velocity were 34.2, 53.9, and 80.8 cm/sec for slow, moderate, and fast velocity settings, respectively, overestimating the corresponding actual flow velocities by 26%, 18%, and 13% and showing precision values (coefficients of variation) of 5.2%, 3.7%, and 6.6%. Flow velocity can be measured from row-to-row multidetector CT projectional data obtained during a single gantry revolution as a bolus of contrast material flows through a vascular phantom. With further development, this novel technique could potentially provide physiologic information to complement the anatomic CT angiographic findings of vascular disease.

VMAT Reduces Moderate to High Integral Dose When Compared to Conventional IMRT for Treatment of Prostate Cancer

VMAT Reduces Moderate to High Integral Dose When Compared to Conventional IMRT for Treatment of Prostate Cancer

SU-E-T-187: A Dedicated Phantom for Electron Density Calibration and Image QA of Linac Cone Beam CT Options

Purpose: In order to use linac Cone‐Beam‐CT (CBCT) data for dosimetric planning an electron density calibration is compulsory. Studies on available phantoms for CT performance checks/QA have shown that these are not sufficient for an adequate HU calibration. Therefore we designed a new dedicated phantom and carried out a planning study to determine the usability of such calibrated CBCT data in comparison with diagnostic CT data. Methods: Available linacCBCT options acquire in a single gantry rotation a volumetric CT data set. In order to 3D‐calibrate the system with regard to HU a special phantom has been designed to include the whole imager area. Removable inserts allow flexible positioning of the probes in x‐z as well as z‐direction. Furthermore a performance and image QA part according AAPM recommendations No. 1 has been integrated and evaluated. We compared 20 plan setups (10 in the pelvic and 10 in the thoracic region). Results: Within the field of view the HU variation is up to 5%. Towards the imager edges hounsfield units show small deviations relative to the imager center (max. 8%). Relative dose distributions in CBCT‐based plans show minor differences to plans calculated using a diagnostic CTimage dataset (p=0.002) and absolute dosage deviations are within 1% (p=0.001). Image quality comparisons using the performance and image QA part showed that CBCTimages are of lower quality than diagnostic images, i.e. spatial and low contrast resolution are 2–3 levels below. Conclusions: The newly designed phantom allows real 3D‐calibration of linacCBCT. It is made of water equivalent material and consists of probes with tissue equivalent materials. The phantom has additional scatter material to simulate a real patients' situation. CBCT data can be used for treatment planning. Furthermore the image QA part allows to carry out recommended constancy checks in straightforward approach. We received financial support from Varian Medical Systems to manufacture the prototypes.

TU‐E‐BRC‐01: Volumetric 4D Computed Tomography with a 320‐Detector Row Scanner for Radiotherapy Simulation

Purpose: We propose to overcome the limitations of conventional helical 4D computed tomography (4DCT) using a volumetric 4DCT approach with a 320‐slice scanner. This scanner is able to acquire image volumes with 16 cm of superior‐inferior coverage with a single gantry rotation and no couch translation, preventing artifacts caused by retrospective image slice sorting. Image volume datasets over multiple breathing cycles can be acquired using this technique. Methods: A respiration motion phantom containing a cube and two sphere objects was scanned on a 320‐slice scanner using a volumetric approach and on a 16‐slice CT scanner using a helical scan technique. An irregular breathing pattern taken from a previous patient scan was programmed into the phantom. Volumes and centroid positions of these objects were segmented for volume and displacement measurements. A model was developed which predicted the number of samples of peak‐to‐peak displacement of a target that could be obtained with a conventional or volumetric scan technique. Results: Using a volumetric 4DCT scan technique to scan an irregular breathing pattern reduced the percent error in the measured volumes relative to the ground truth volumes of the cube and spheres by as much as 81%. No sorting artifacts or missing slices in these object volumes were present using the volumetric technique, unlike the conventional approach. The model predicted that the helical scan technique was limited to measuring 1–2 displacement samples. This limitation was not present with the volumetric scan technique, with which multiple samples of peak‐to‐peak displacement can be acquired. Conclusions: Volumetric 4DCT is a superior scan technique to the conventional helical approach, and its ability to acquire target volumes and displacements more accurately for irregular breathing patterns than the helical approach could help to improve internal target volume definitions for radiotherapy treatment planning.

Comparison of 128-Section Single-Shot Technique with Conventional Spiral Multisection CT for Imaging of the Temporal Bone

Computed tomography is an essential modality for imaging of the temporal bone. Newest generation scanners allow the coverage of large examination volumes with a single gantry rotation. The objective of this study was to compare a 128-section SST (1 single rotation of the x-ray tube) with conventional spiral MSCT (ultra-high-resolution mode) for imaging of the temporal bone. Fifty-four temporal bones in 27 patients were scanned with both a conventional MSCT and 128-section SST. After blinding and randomization of both examinations, 2 observers assessed the visualization of 38 anatomic structures (eg, various segments of the facial nerve canal, mallear ligaments) by using multiplanar reconstructions in the axial and coronal planes. The differences in evaluation scores obtained for the 2 techniques were analyzed by using a Wilcoxon signed rank test, with a P value of < .05 considered significant. For both methods, imaging time and radiation exposure were noted. Overall visualization of anatomic structures did not differ significantly between the 2 techniques (P > .05). When we compared the anatomic structures separately, there was better visualization of the lateral mallear ligament with MSCT, whereas the cochlear septa were ranked higher with SST (P < .05). Imaging time and average DLP for MSCT were 12.3 seconds and 306 mGy cm, respectively; for SST, values they were 1 second and 64 mGy cm, respectively (ie, a dose reduction of 79%). For imaging of the temporal bone with adequate diagnostic quality, 128-section SST can be used. The main advantages over MSCT are the dramatic reductions of imaging time and radiation exposure, which are particularly important when scanning uncooperative patients or children.

Volumetric modulated arc therapy planning for distal oesophageal malignancies

Volumetric modulated arc therapy (VMAT) is a novel form of intensity-modulated radiation therapy that allows the radiation dose to be delivered in a single gantry rotation using conformal or modulated fields. The capability of VMAT to reduce heart and cord dose, while maintaining lung receiving 20 Gy <20%, was evaluated for chemoradiation for oesophageal cancer. An optimised forward-planned four-field arrangement was compared with inverse-planned coplanar VMAT arcs with 35 control points for 10 patients with lower gastro-oesophageal tumours prescribed 54 Gy in 30 fractions. Conformal (cARC) and intensity-modulated (VMATi) arcs were considered. Plans were assessed and compared using the planning target volume (PTV) irradiated to 95% of the prescription dose (V95), volumes of lung irradiated to 20 Gy (V20), heart irradiated to 30 Gy (V30), spinal cord maximum dose and van't Riet conformation number (CN). The monitor units per fraction and delivery time were recorded for a single representative plan. VMATi provided a significant reduction in the heart V30 (31% vs 55%; p=0.02) with better CN (0.72 vs 0.65; p=0.01) than the conformal plan. The treatment delivery was 1 min 28 s for VMAT compared with 3 min 15 s. For similar PTV coverage, VMATi delivers a lower dose to organs at risk than conformal plans in a shorter time, and this has warranted clinical implementation.

Volumetric Modulated Arc Therapy for Prostate Radiotherapy

Intensity modulated radiotherapy (IMRT) is an effective treatment modality for prostate carcinoma. A new technology, Volumetric Modulated Arc Therapy (VMAT), allows the radiation dose to be delivered continuously in a single gantry rotation. This technology promises to shorten delivery time as well as reduce the total MUs. This retrospective treatment planning study compares dosimetric quality and delivery efficiency of VMAT plans to clinically delivered IMRT plans.

Diagnostic Accuracy of 320-Row Multidetector Computed Tomography Coronary Angiography to Noninvasively Assess In-Stent Restenosis

Percutaneous coronary intervention with stent implantation is routinely performed to treat patients with obstructive coronary artery disease. However, thus far, noninvasive assessment of in-stent restenosis has been challenging. Recently, 320-row multidetector computed tomography coronary angiography (CTA) was introduced, allowing volumetric image acquisition of the heart in a single heart beat or gantry rotation. The aim of this study was to evaluate the diagnostic performance of 320-row CTA in the evaluation of significant in-stent restenosis. Invasive coronary angiography (ICA) served as the standard of reference, using a quantitative approach. The population consisted of patients with previous coronary stent implantation who were clinically referred for cardiac evaluation because of recurrent chest pain and who underwent both CTA and ICA. CTA studies were performed using a 320-row CTA scanner with 320 detector-rows, each 0.5 mm wide, and a gantry rotation time of 350 milliseconds. Tube voltage and current were adapted to body mass index and thoracic anatomy. The entire heart was imaged in a single heart beat, with a maximum of 16-cm craniocaudal coverage. During the scan, the ECG was registered simultaneously for prospective triggering of the data. First, CTA stent image quality was assessed using a 3-point grading scale: (1) good image quality, (2) moderate image quality, and (3) poor image quality. Subsequently, the presence of in-stent restenosis was determined on a stent and patient basis by a blinded observer. Significant in-stent restenosis was defined as >or=50% luminal narrowing in the stent lumen or the presence of significant stent edge stenosis. Overlapping stents were considered to represent a single stent. Results were compared with ICA using quantitative coronary angiography. In addition, CTA stent image quality and diagnostic accuracy were related to stent characteristics and heart rate during CTA image acquisition. The population consisted of 53 patients (37 men, mean age: 65 +/- 13 years) with a total of 89 stents available for evaluation. ICA identified 12 stents (13%) with significant in-stent restenosis. A total of 7 stents (8%) were of nondiagnostic CTA stent image quality, and were considered positive. Sensitivity, specificity, positive, and negative predictive values were 92%, 83%, 46%, and 98%, respectively on a stent basis. Five CTA studies (9%) were of nondiagnostic quality for the evaluation of in-stent restenosis and were considered positive. Sensitivity, specificity, positive, and negative predictive values were 100%, 81%, 58%, and 100%, respectively on a patient level. Stent diameter <3 mm as well as stent strut thickness >or=140 mum were associated with decreased CTA stent image quality and diagnostic accuracy. Heart rate during CTA acquisition and stent overlap were not associated with image degradation. The present results show that 320-row CTA allows accurate noninvasive assessment of significant in-stent restenosis. However, stents with a large diameter and thin struts allowed better in-stent visualization than stents with a small diameter or thick struts. Consequently, noninvasive assessment of in-stent restenosis using CTA may be an attractive and feasible alternative particularly in carefully selected patients.

Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields

To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields. A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [Alfonso et al., Med. Phys. 35, 5179-5186 (2008)], a candidate plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10 x 10 cm2 field was measured using three radiation detectors: Gafchromic EBT films, a diamond detector, and a guarded liquid-filled ionization chamber developed in-house (GLIC-03). Pcsr correction factors [formula in text] were determined for five different types of air-filled ionization chambers (Exradin A12, NE-2571, Exradin A1SL, Exradin A14, and PinPoint 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery). The combined standard uncertainty in measuring the correction factor [formula in text] using the three dosimetry techniques was 0.3%. For all the air-filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than +/- 0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955-0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922-1.0048. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber-type dependence. The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factors [formula in text] for air-filled ionization chambers at the 0.3% 1sigma uncertainty level. These techniques can be used to determine criteria for the selection of plan-class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.

Clinical Application of CT Coronary Angiography: State of the Art

In recent years, multi-slice computed tomography (MSCT) technology has developed rapidly, allowing high-resolution non-invasive imaging of the coronary arteries and surrounding structures. Since the introduction of MSCT, acquisition time, detector number, spatial and temporal resolution have continuously improved with each new scanner generation, resulting in excellent image quality and diagnostic accuracy in the detection of coronary artery disease (CAD). At the same time, developments in MSCT technology have focused on reduction of the radiation dose. In particular, the availability of dose modulation and prospective ECG gating have drastically reduced patient radiation dose. Moreover, with the introduction of 320-slice MSCT, volumetric scanning of the entire heart has become possible in a single heart beat or gantry rotation, thereby eliminating oversampling and stair-step artifact. The present article provides an overview of state of the art clinical applications of cardiac MSCT, including the diagnosis of CAD, evaluation of plaque morphology and composition, prognostification, and the evaluation of left ventricular function and aortic and mitral valve anatomy.

A Survey on the Quality Assurance Procedures Used in Intensity Modulated Radiation Therapy (IMRT) at Indian Hospitals

A national survey to obtain information about the Quality Assurance (QA) procedures and methods being followed at Indian radiotherapy centers for intensity modulated radiation therapy (IMRT) was conducted. A questionnaire containing parameters relevant to IMRT QA was evolved to collect the information pertaining to the QA of IMRT delivery system, QA of IMRT treatment planning system, and patient specific IMRT QA. The questionnaire was circulated to 40 hospitals in the country and responses of 31 centers were received. Survey results showed that 71% centers are having adequate machine specifi c IMRT QA programme, 19% centers have inadequate machine specific IMRT QA programme and 9% centers have irrelevant machine specific IMRT QA programme. No specific answer for question of QA tests of TPS specific to IMRT were received from the user. Almost all the centers have programme of setup verification of the patient by means of EPID/DRR/OBI. However, 91% of centers could not provide any information about the QA methodology of the devices used for setup verification. For patient specific dosimetric QA, almost all the hospitals have the program of pre-treatment dose verification using calibrated ionization chambers of sensitive volumes in the range of 0.01 to 0.65 cc. Dosimetric verification is performed by combining dose from all gantry angles to a single gantry angle. Two dimensional (2D) dosimetry systems such as radiographic and radiochromic films, 2D array of ionization chambers/ semiconductor diodes and EPID are also used in patient specific dosimetry verifications. Majority of the centers (about 48%) accept the plan with 3% dose difference and 3 mm dose to distance agreement criteria with gamma index less than unity. However, a number of other acceptance criteria specific to institution and tumor site are being also followed. This survey reveals that a variety of IMRT QA program is being followed at the Indian hospitals. This study has brought into focus the need to evolve a national protocol for IMRT QA so that treatment outcomes of all the IMRT centers of country can be compared.

Variable dose rate single-arc IMAT delivered with a constant dose rate and variable angular spacing

Single-arc intensity-modulated arc therapy (IMAT) has gained worldwide interest in both research and clinical implementation due to its superior plan quality and delivery efficiency. Single-arc IMAT techniques such as the Varian RapidArc™ deliver conformal dose distributions to the target in one single gantry rotation, resulting in a delivery time in the order of 2 min. The segments in these techniques are evenly distributed within an arc and are allowed to have different monitor unit (MU) weightings. Therefore, a variable dose-rate (VDR) is required for delivery. Because the VDR requirement complicates the control hardware and software of the linear accelerators (linacs) and prevents most existing linacs from delivering IMAT, we propose an alternative planning approach for IMAT using constant dose-rate (CDR) delivery with variable angular spacing. We prove the equivalence by converting VDR-optimized RapidArc plans to CDR plans, where the evenly spaced beams in the VDR plan are redistributed to uneven spacing such that the segments with larger MU weighting occupy a greater angular interval. To minimize perturbation in the optimized dose distribution, the angular deviation of the segments was restricted to ⩽± 5°. This restriction requires the treatment arc to be broken into multiple sectors such that the local MU fluctuation within each sector is reduced, thereby lowering the angular deviation of the segments during redistribution. The converted CDR plans were delivered with a single gantry sweep as in the VDR plans but each sector was delivered with a different value of CDR. For four patient cases, including two head-and-neck, one brain and one prostate, all CDR plans developed with the variable spacing scheme produced similar dose distributions to the original VDR plans. For plans with complex angular MU distributions, the number of sectors increased up to four in the CDR plans in order to maintain the original plan quality. Since each sector was delivered with a different dose rate, extra mode-up time (xMOT) was needed between the transitions of the successive sectors during delivery. On average, the delivery times of the CDR plans were approximately less than 1 min longer than the treatment times of the VDR plans, with an average of about 0.33 min of xMOT per sector transition. The results have shown that VDR may not be necessary for single-arc IMAT. Using variable angular spacing, VDR RapidArc plans can be implemented into the clinics that are not equipped with the new VDR-enabled machines without compromising the plan quality or treatment efficiency. With a prospective optimization approach using variable angular spacing, CDR delivery times can be further minimized while maintaining the high delivery efficiency of single-arc IMAT treatment.

Optimization of a Linac-based 4D Cone-beam CT Acquisition Protocol for Image-guided Radiation Therapy

Materials/Methods: We have developed a 4D CBCT acquisition technique to support IGRTfor free-breathing thoracic andabdominal cancer patients. The Varian OBI imager was displaced (half-fan acquisition geometry) extending the FOV to 450 mm. The projections were acquired in 360 gantry rotation arcs. The 8 to 12 minute scanning time was set to cover 120 breathing cycles (BC), with a sampling rate of 20 projections per BC. We used the Varian Real-time positioning and management system as an external surrogate to correlate the patient respiration with the projection acquisition. View aliasing artifacts for single and multiple gantry arc acquisitions were evaluated on patient datasets. Using a synthetic 4D CBCT dataset derived from a 10 minute scan, we evaluated the distribution of gantry angle increments as a function of number of gantry rotations assuming 1 minute rotation time and 10 frames per second. Actual patient respiratory traces were used to sort the projections into phase bins. IQ was assessed as function x-ray exposure (mAs) per projection.

VMAT Plans Solution with Single and Double Gantry Rotation Compared with IMRT Step & Shoot for Different Treatment Sites

Volumetric Modulated Arc Therapy (VMAT) can ensure dose conformality and sparing of OARs reducing MUs and treatment times with respect to standard IMRT techniques. We compared IMRT step& shoot with VMAT plans for treatments of prostate and hypo-fractionated treatments of spinal and abdominal tumors, analyzing advantages and drawbacks of a single arc solution (VMAT1) with respect to a 2 rotations approach (VMAT2).

Volumetric Modulated Arc Therapy Improves Dosimetry and Reduces Treatment Time Compared to Conventional Intensity-Modulated Radiotherapy for Locoregional Radiotherapy of Left-Sided Breast Cancer and Internal Mammary Nodes

Volumetric modulated arc therapy (VMAT) is a novel extension of conventional intensity-modulated radiotherapy (cIMRT), in which an optimized three-dimensional dose distribution may be delivered in a single gantry rotation. VMAT is the predecessor to RapidArc (Varian Medical System). This study compared VMAT with cIMRT and with conventional modified wide-tangent (MWT) techniques for locoregional radiotherapy for left-sided breast cancer, including internal mammary nodes. Therapy for 5 patients previously treated with 50 Gy/25 fractions using nine-field cIMRT was replanned with VMAT and MWT. Comparative endpoints were planning target volume (PTV) dose homogeneity, doses to surrounding structures, number of monitor units, and treatment delivery time. For VMAT, two 190 degrees arcs with 2-cm overlapping jaws were required to optimize over the large treatment volumes. Treatment plans generated using VMAT optimization resulted in PTV homogeneity similar to that of cIMRT and MWT. The average heart volumes receiving >30 Gy for VMAT, cIMRT, and MWT were 2.6% +/- 0.7%, 3.5% +/- 0.8%, and 16.4% +/- 4.3%, respectively, and the average ipsilateral lung volumes receiving >20 Gy were 16.9% +/- 1.1%, 17.3% +/- 0.9%, and 37.3% +/- 7.2%, respectively. The average mean dose to the contralateral medial breast was 3.2 +/- 0.6 Gy for VMAT, 4.3 +/- 0.4 Gy for cIMRT, and 4.4 +/- 4.7 Gy for MWT. The healthy tissue volume percentages receiving 5 Gy were significantly larger with VMAT (33.1% +/- 2.1%) and IMRT (45.3% +/- 3.1%) than with MWT (19.4% +/- 3.7%). VMAT reduced the number of monitor units by 30% and the treatment time by 55% compared with cIMRT. VMAT achieved similar PTV coverage and sparing of organs at risk, with fewer monitor units and shorter delivery time than cIMRT.

Adaptive two‐pass cone‐beam artifact correction using a FOV‐preserving two‐source geometry: A simulation study

The evolution to ever wider detector arrays that are able to cover whole organs with a single circular gantry sweep has revitalized the research efforts toward finding improved axial scanning algorithms and protocols. The authors propose a computed tomography scan and reconstruction concept using two sources, a single detector and a two-pass cone-beam correction method, as an integral part of the reconstruction. Compared with standard circular acquisition and reconstruction methods, the new concept excels with improved coverage and very low cone-beam artifact level also for short scan acquisitions, which makes it especially attractive for cardiac applications.