Amanda Deisher

Validation of proton stopping power ratio estimation based on dual energy CT using fresh tissue samples

Dual energy CT (DECT) has been shown, in theoretical and phantom studies, to improve the stopping power ratio (SPR) determination used for proton treatment planning compared to the use of single energy CT (SECT). However, it has not been shown that this also extends to organic tissues. The purpose of this study was therefore to investigate the accuracy of SPR estimation for fresh pork and beef tissue samples used as surrogates of human tissues. The reference SPRs for fourteen tissue samples, which included fat, muscle and femur bone, were measured using proton pencil beams. The tissue samples were subsequently CT scanned using four different scanners with different dual energy acquisition modes, giving in total six DECT-based SPR estimations for each sample. The SPR was estimated using a proprietary algorithm (syngo.via DE Rho/Z Maps, Siemens Healthcare, Forchheim, Germany) for extracting the electron density and the effective atomic number. SECT images were also acquired and SECT-based SPR estimations were performed using a clinical Hounsfield look-up table. The mean and standard deviation of the SPR over large volume-of-interests were calculated. For the six different DECT acquisition methods, the root-mean-square errors (RMSEs) for the SPR estimates over all tissue samples were between 0.9% and 1.5%. For the SECT-based SPR estimation the RMSE was 2.8%. For one DECT acquisition method, a positive bias was seen in the SPR estimates, having a mean error of 1.3%. The largest errors were found in the very dense cortical bone from a beef femur. This study confirms the advantages of DECT-based SPR estimation although good results were also obtained using SECT for most tissues.

Inter-centre variability of CT-based stopping-power prediction in particle therapy: Survey-based evaluation

Background and purpose Stopping-power ratios (SPRs) are used in particle therapy to calculate particle range in patients. The heuristic CT-to-SPR conversion (Hounsfield Look-Up-Table, HLUT), needed for treatment planning, depends on CT-scan and reconstruction parameters as well as the specific HLUT definition. To assess inter-centre differences in these parameters, we performed a survey-based qualitative evaluation, as a first step towards better standardisation of CT-based SPR derivation. Materials and methods A questionnaire was sent to twelve particle therapy centres (ten from Europe and two from USA). It asked for details on CT scanners, image acquisition and reconstruction, definition of the HLUT, body-region specific HLUT selection, investigations of beam-hardening and experimental validations of the HLUT. Technological improvements were rated regarding their potential to improve SPR accuracy. Results Scan parameters and HLUT definition varied widely. Either the stoichiometric method (eight centres) or a tissue-substitute-only HLUT definition (three centres) was used. One centre combined both methods. The number of HLUT line segments varied widely between two and eleven. Nine centres had investigated influence of beam-hardening, often including patient-size dependence. Ten centres had validated their HLUT experimentally, with very different validation schemes. Most centres deemed dual-energy CT promising for improving SPR accuracy. Conclusions Large inter-centre variability was found in implementation of CT scans, image reconstruction and especially in specification of the CT-to-SPR conversion. A future standardisation would reduce time-intensive institution-specific efforts and variations in treatment quality. Due to the interdependency of multiple parameters, no conclusion can be drawn on the derived SPR accuracy and its inter-centre variability.

External Arrhythmia Ablation Using Photon Beams

Background— This study sought to investigate external photon beam radiation for catheter-free ablation of the atrioventricular junction in intact pigs. Methods and Results— Ten pigs were randomized to either sham irradiation or irradiation of the atrioventricular junction (55, 50, 40, and 25 Gy). Animals underwent baseline electrophysiological evaluation, cardiac gated multi-row computed tomographic imaging for beam delivery planning, and intensity-modulated radiation therapy. Doses to the coronary arteries were optimized. Invasive follow-up was conducted ⩽4 months after the irradiation. A mean volume of 2.5±0.5 mL was irradiated with target dose. The mean follow-up length after irradiation was 124.8±30.8 days. Out of 7 irradiated animals, complete atrioventricular block was achieved in 6 animals of all 4 dose groups (86%). Using the same targeting margins, ablation lesion size notably increased with the delivered dose because of volumetric effects of isodose lines around the target volume. The mean macroscopically calculated atrial lesion volume for all 4 dose groups was 3.8±1.1 mL, lesions extended anteriorly into the interventricular septum. No short-term side effects were observed. No damage was observed in the tissues of the esophagus, phrenic nerves, or trachea. However, histology revealed in-field beam effects outside of the target volume. Conclusions— Single-fraction doses as low as 25 Gy caused a lesion with interruption of cardiac impulse propagation using this respective target volume. With doses of ⩽55 Gy, maximal point-doses to coronary arteries could be kept <7Gy, but target conformity of lesions was not fully achieved using this approach.

A comparison of relative proton stopping power measurements across patient size using dual- and single-energy CT

Abstract Purpose: To evaluate the accuracy and precision across phantom size of a dual-energy computed tomography (DECT) technique used to calculate relative proton stopping power (SPR) in tissue-simulating materials and a silicone implant relative to conventional single-energy CT (SECT). Material and methods: A 32u2009cm lateral diameter (CIRS model 062M, Norfolk, Virginia) electron density phantom containing inserts which simulated the chemical composition of eight tissues in a solid-water background was scanned using SECT and DECT. A liquid water insert was included to confirm CT number accuracy. All materials were also placed in four water tanks, ranging from 15 to 45u2009cm in lateral width and scanned using DECT and SECT. A silicone breast implant was scanned in the same water phantoms. SPR values were calculated based on commercial software (syngo CT Dual Energy, Siemens Healthcare GmbH) and compared to reference values derived from proton beam measurements. Accuracy and precision were quantified across phantom size using percent error and standard deviation. Graphical and regression analysis were used to determine whether SECT or DECT was superior in estimating SPR across phantom size. Results: Both DECT and SECT SPR data resulted in good agreement with the reference values. Percent error was ±3% for both DECT and SECT in all materials except lung and dense bone. The coefficient of variation (CV) across materials and phantom sizes was 1.12% for SECT and 0.96% for DECT. Material-specific regression and graphical analysis did not reveal size dependence for either technique but did show reduced systematic bias with DECT for dense bone and liver. Mean percent error in SPR for the implant was reduced from 11.46% for SECT to 0.49% for DECT. Conclusions: We demonstrate the superior ability of DECT to mitigate systematic bias in bones and liver and estimate SPR in a silicone breast implant.

Theoretical and experimental analysis of photon counting detector CT for proton stopping power prediction

PURPOSEnPhoton counting detectors (PCDs) are being introduced in advanced x-ray computed tomography (CT) scanners. From a single PCD-CT acquisition, multiple images can be reconstructed, each based on only a part of the original x-ray spectrum. In this study, we investigated whether PCD-CT can be used to estimate stopping power ratios (SPRs) for proton therapy treatment planning, both by comparing to other SPR methods proposed for single energy CT (SECT) and dual energy CT (DECT) as well as to experimental measurements.nnnMETHODSnA previously developed DECT-based SPR estimation method was adapted to PCD-CT data, by adjusting the estimation equations to allow for more energy spectra. The method was calibrated directly on noisy data to increase the robustness toward image noise. The new PCD SPR estimation method was tested in theoretical calculations as well as in an experimental setup, using both four and two energy bin PCD-CT images, and through comparison to two other SPR methods proposed for SECT and DECT. These two methods were also evaluated on PCD-CT images, full spectrum (one-bin) or two-bin images, respectively. In a theoretical framework, we evaluated the effect of patient-specific tissue variations (density and elemental composition) and image noise on the SPR accuracy; the latter effect was assessed by applying three different noise levels (low, medium, and high noise). SPR estimates derived using real PCD-CT images were compared to experimentally measured SPRs in nine organic tissue samples, including fat, muscle, and bone tissues.nnnRESULTSnFor the theoretical calculations, the root-mean-square error (RMSE) of the SPR estimation was 0.1% for the new PCD method using both two and four energy bins, compared to 0.2% and 0.7% for the DECT- and SECT-based method, respectively. The PCD method was found to be very robust toward CT image noise, with a RMSE of 2.7% when high noise was added to the CT numbers. Introducing tissue variations, the RMSE only increased to 0.5%; even when adding high image noise to the changed tissues, the RMSE stayed within 3.1%. In the experimental measurements, the RMSE over the nine tissue samples was 0.8% when using two energy bins, and 1.0% for the four-bin images.nnnCONCLUSIONSnIn all tested cases, the new PCD method produced similar or better results than the SECT- and DECT-based methods, showing an overall improvement of the SPR accuracy. This study thus demonstrated that PCD-CT scans will be a qualified candidate for SPR estimations.

Deformable registration of radiation isodose lines to delayed contrast-enhanced magnetic resonance images for assessment of myocardial lesion formation following proton beam therapy

Ventricular tachycardia is increasingly treated with ablation therapy, a technique in which catheters are guided into the ventricle and radiofrequency energy is delivered into the myocardial tissue to interrupt arrhythmic electrical pathways. Recent efforts have investigated the use of noninvasive external beam therapy for treatment of ventricular tachycardia where target regions are identified in the myocardium and treated using external beams. The relationship between the planned dose map and myocardial tissue change, however, has not yet been quantified. In this work, we use a deformable registration algorithm to align dose maps planned from baseline computed-tomography scans to delayed contrast-enhanced magnetic resonance imaging scans taken at 4 week intervals following proton beam therapy. From this data, the relationship between the planned dose and image enhancement, which serves as a surrogate for tissue change, can be quantified.

Quantitative assessment of cardiac motion using multiphase computed tomography imaging with application to cardiac ablation therapy

Cardiac arrhythmias, a condition in which the heart beats irregularly, are typically treated with drug or cardiac ablation therapy. More recently, external beam ablation therapy has been proposed as a potential approach for treating cardiac arrhythmias. Currently, a significant challenge regarding external beam ablation therapy in the heart is compensation for cardiac motion to ensure precise targeting. Porcine animal models are often used for evaluating image-guided intervention systems for cardiac applications; however, to date there have been relatively few studies evaluating motion in the swine heart. In this study, we model and quantify cardiac motion in the left atrium and left ventricle of three beating porcine hearts by tracking anatomic landmarks across twenty phases of the cardiac cycle from multi-phase computed tomography images. 10 landmarks are tracked for each porcine heart, 5 in the left atrium and 5 in the left ventricle. The mean (std) displacement for the 5 left atrial landmarks is 5.5(3.5) mm in x, 5.0(2.9) mm in y, and 5.6(3.3) mm in z. The mean (std) displacement for the 5 left ventricular landmarks is 7.1(3.8) mm in x, 9.9(5.2) mm in y, and 7.7(3.1) mm in z.

SU-F-T-290: Modeling MLC Penumbra Within the Eclipse TPS - A Major Mode of IMRT QA Failure

PURPOSEnTo illustrate how inaccurate modeling of the MLC penumbra within the Eclipse TPS can lead to dosimetric differences and decreased IMRT QA passing rates.nnnMETHODSnGafchromic film was used to compare 6 MV measured profiles from 2×2 cm2 MLC-defined fields to profiles calculated by the Eclipse TPS (v13.6) under two different scenarios. The first scenario involved calculations using the original MLC opening (original Eclipse). In the second, we selectively broadened the penumbra by summing two appropriately weighted fields: One with 95% of MUs delivered using the original MLC opening, and a second with the remaining MUs delivered with all MLCs opened an additional 5 mm (modified Eclipse). MATLAB code was written to generate MLC patterns and MU weighting in this manner and used to calculate modified Eclipse doses for 25 different clinical IMRT and VMAT plans. Patient specific QA measurements were made with film and compared to both original and modified Eclipse calculations.nnnRESULTSnFor the 2×2 cm2 fields, agreement was observed between measured and original Eclipse profiles everywhere except in the penumbral region 5-10 mm from the end of the MLC where doses 10-30% higher than expected were measured. Near perfect agreement throughout the entire profile was observed when comparing measurement to the modified Eclipse calculation with the broadened penumbra. For the 25 clinical treatment plans, the average 2%/0 mm gamma passing rate using the original Eclipse calculations was 82.3%, while for the modified Eclipse calculations it improved to 90.1%.nnnCONCLUSIONnThe approximation used by the Eclipse TPS to model the rounded end of the MLC is a failure mode that could lead to poor quality and/or failing QA for IMRT and VMAT plans. Improved modeling of the penumbra in the region 5-10 mm from the end of the MLC is required to completely correct this discrepancy.

WE-EF-BRA-03: Catheter- Free Ablation with External Photon Radiation: Treatment Planning, Delivery Considerations, and Correlation of Effects with Delivered Dose

Purpose: To plan, target, and calculate delivered dose in atrioventricular node (AVN) ablation with volume-modulated arc therapy (VMAT) in an intact porcine model. Methods: Seven pigs underwent AVN irradiation, with prescription doses ranging between 25 and 55Gy in a single fraction. Cardiac CT scans were acquired at expiration. Two physicians contoured AVN targets on 10 phases, providing estimates of target motion and inter-physician variability. Treatment planning was conducted on a static phase-averaged CT. The volume designated to receive prescription dose covered the full extent of AVN cardiac motion, expanded by 4mm for setup uncertainty. Optimization limited doses to risk structures according to single-fraction tumor treatment protocols. Orthogonal kV images were used to align bony anatomy at time of treatment. Localization was further refined with respiratory-gated cone-beam CT, and range of cardiac motion was verified under fluoroscopy. Beam delivery was respiratory-gated for expiration with a mean efficiency of 60%. Deformable registration of the 10 cardiac CT phases was used to calculate actual delivered dose for comparison to electro-anatomical and visually evident lesions. Results: The mean [minimum,maximum] amplitude of AVN cardiac motion was LR 2.9 [1.7,3.9]mm, AP 6.6 [4.4,10.4]mm, and SI 5.6 [2.0,9.9]mm. Incorporating cardiac motion into the dose calculation showed the volume receiving full dose was 40–80% of the volume indicated on the static planning image, although the contoured AVN target received full dose in all animals. Initial results suggest the dimensions of the electro-anatomical lesion are correlated with the 40Gy isodose volume. Conclusion: Image-guidance techniques allow for accurate and precise delivery of VMAT for catheter-free arrhythmia ablation. An arsenal of advanced radiation planning, dose optimization, and image-guided delivery techniques was employed to assess and mitigate effects of cardiac and respiratory motion. Feasibility of delivery to the pulmonary veins and left ventricular myocardium will be investigated in future studies. D. Packer Disclosures: Abiomed, Biosense Webster, Inc., Boston Scientific Corp., CardioFocus, Inc., Johnson and Johnson, Excerpta Medica, Ortho-McNeil-Jannsen, Sanofi Aventis, CardioInsight Technologies, InfoBionic, SIEMENS, Medtronic, Inc., CardioDx, Inc., CardioInsight Technologies, FoxP2 Medica, Mediasphere Medical, Wiley-Blackwell, St. Jude Medical, Endosense, Thermedical, EP Advocate LLC, Hansen Medical, American Heart Association, EpiEP, NIH