QA Measurements Research Articles

Real-time delivered dose assessment in carbon ion therapy of moving targets

Abstract Purpose:
Real-time adaptive particle therapy is being investigated as a means to maximize the treatment delivery accuracy. To react to dosimetric errors, a system for fast and reliable verification of the agreement between planned and delivered doses is essential. This study presents a clinically feasible, real-time 4D-dose reconstruction system, synchronized with the treatment delivery and motion of the patient, which can provide the necessary feedback on the quality of the delivery.
Methods:
A GPU-based analytical dose engine capable of millisecond dose calculation for carbon ion therapy has been developed and interfaced with the next generation of the Dose Delivery System (DDS) in use at Centro Nazionale di Adroterapia Oncologica (CNAO). The system receives the spot parameters and the motion information of the patient during the treatment and performs the reconstruction of the planned and delivered 4Ddoses. After each iso-energy layer, the results are displayed on a graphical user interface by the end of the spill pause of the synchrotron, permitting verification against the reference dose.
The framework has been verified experimentally at CNAO for a lung cancer case based on a virtual phantom 4DCT. The patient’s motion was mimicked by a moving Ionization Chambers (ICs) 2D-array.
Results:
For the investigated static and 4D-optimized treatment delivery cases, real time dose reconstruction was-achieved with an average pencil beam dose calculation speed up to more than one order of magnitude smaller than the spot delivery. The reconstructed doses have been benchmarked against offline log-file based dose reconstruction with the TRiP98 treatment planning system, as well as QA measurements with the ICs 2D-array, where an average gamma-index passing rate (3%/3mm) of 99.8% and 98.3%, respectively, were achieved.
Conclusion:
This work provides the first real-time 4D-dose reconstruction engine for carbon ion therapy. The framework integration with the CNAO DDS paves the way for a swift transition to the clinics.

Comparative assessment and QA measurement array validation of Monte Carlo and Collapsed Cone dose algorithms for small fields and clinical treatment plans.

Many studies have demonstrated superior performance of Monte Carlo (MC) over type B algorithms in heterogeneous structures. However, even in homogeneous media, MC dose simulations should outperform type B algorithms in situations of electronic disequilibrium, such as small and highly modulated fields. Our study compares MC and Collapsed Cone (CC) dose algorithms in RayStation 12A. Under consideration are 6 MV and 6 MV flattening filter-free (FFF) photon beams, relevant for VMAT plans such as head-and-neck and stereotactic lung treatments with heterogeneities, as well as plans for multiple brain metastases in one isocenter, involving highly modulated small fields. We aim to investigate collimator angle dependence of small fields and performance differences between different combinations of ArcCHECK configuration and dose algorithm. Several verification tests were performed, ranging from simple rectangular fields to highly modulated clinical plans. To evaluate and compare the performance of the models, the agreements between calculation and measurement are compared between MC and CC. Measurements include water tank measurements for test fields, ArcCHECK measurements for test fields and VMAT plans, and film dosimetry for small fields. In very small or narrow fields, our measurements reveal a strong dependency of dose output to collimator angle for VersaHD with Agility MLC, reproduced by both dose algorithms. ArcCHECK results highlight a suboptimal agreement between measurements and MC calculations for simple rectangular fields when using inhomogeneous ArcCHECK images. Therefore, we advocate for the use of homogeneous phantom images, particularly for static fields, in ArcCHECK verification with MC. MC might offer performance benefits for more modulated treatment fields. In ArcCHECK results for clinical plans, MC performed comparable to CC for 6 MV. For 6 MV FFF and the preferred homogeneous phantom image, MC resulted in consistently better results (13%-64% lower mean gamma index) compared to CC.

Quality assurance of magnetic resonance imaging for a polymer gel dosimeter using a 3D-printed phantom

PurposeQuality assurance (QA) of magnetic resonance imaging (MRI) for gel dosimetry using an in-house phantom was established to ensure consistent dosimetric performance. MethodsA normoxic N-vinylpyrrolidone-based gel dosimeter sensitized with an inorganic salt was used. Each individual gel dosimeter, in addition to a specially designed image QA phantom, was imaged at three institutions with different MRI systems. ResultsBoth the gel dosimeter and QA phantom had linear dose–response curves at all institutions, with correlation coefficients = 1.00 and ≥ 0.99, respectively. Minor inter-institutional variations in the sensitivity (gel dosimeter, 0.096 ± 0.010 s−1Gy−1; QA phantom, 0.140 ± 0.003 s−1Gy−1) and offset value (gel dosimeter, 2.188 ± 0.222 s−1; QA phantom, 3.318 ± 0.081 s−1) were found. Dose uncertainties for the gel dosimeter ranged from 3.2% to 3.5% at 10 Gy and 1.7%–2.1% at 20 Gy, and these were comparable with the corresponding QA phantom uncertainties. Dose resolutions were 0.8–1.2 Gy for the gel dosimeter and 0.6–1.4 Gy for the QA phantom. Uniformity factors were ≥98% at all institutions. Spatial linearity had a geometric distortion ≤1.0%, Gamma passing rates of 95%–100% met recommended QA guidelines; strong correlations between the QA phantom and gel dosimeter were also obtained for each of these measures. ConclusionOur study demonstrates the viability of applying MRI acquisition parameters validated using a QA phantom to polymer gel dosimetry across diverse MRI systems. This approach enables robust QA measures, focusing on effective management of uniformity and dose uncertainty.

Implementation of an ionization chamber array for linear accelerator monthly dosimetry QA.

The IC Profiler (ICP) manufactured by Sun Nuclear Corporation (SNC) is an ionization chamber (IC) array used for linear accelerator dosimetry measurements. Previous work characterized response of the ICP under various conditions, but there is limited work of its implementation into monthly QA measurement procedures. This work quantifies ICP accuracy and variables that affect accuracy for beam output measurements, and demonstrates feasibility of using the ICP for all recommended monthly dosimetry measurements. A total of 1985 output measurements on six Varian TrueBeam and Edge linear accelerators were performed using three ICP with quad wedges (QWs) and were compared with conventional IC measurements. The accuracy of the ICP for beam output was characterized as the difference between the ICP and IC. Variables that affect ICP accuracy, including gain settings, calibrations, and template baselining as well as machine or energy-specific bias were investigated. Measurements of profile constancy, energy, dose rate constancy, wedge factors, and gating were performed. The initially observed mean output difference between the ICP and IC was 0.16% (0.61%). When gain settings were optimized, the output difference accuracy improved to -0.02% (0.38%). The output accuracy of the ICP was not dependent on array, dose, temperature and pressure calibrations, or template baselining. Statistically, ICP output accuracy was dependent on machine and beam energy, but clinically, all measurements fell within 0.5% of unity. ICP measurements of energy, dose rate constancy, and wedge factors matched passing results with conventional IC in water measurements. Gating and beam profile constancy measurements demonstrated good stability using the ICP. Finally, monthly dosimetry QA using ICP was completed in an average of 33 min compared to 66 min using the IC. This work demonstrated the feasibility and efficiency of using the ICP, with specific considerations, as a measurement device for dosimetric linear accelerator monthly QA.

#532 Association between blood flow measured within 30 days from AV access creation and the risk of thrombosis

Abstract Background and Aims The use of arteriovenous (AV) access for hemodialysis (HD) is hampered by the risk of dysfunction and thrombosis. Various factors affect the risk of AV access thrombosis, such as age, female sex, hematocrit, vessels’ diameter and anatomy, and patients’ comorbidities (e.g. diabetes mellitus, thrombophilia, chronic inflammatory disease). Previous studies have reported contradictory results on the prognostic value of intraoperative blood flow (Qa) measurements. Finally, only few studies have investigated the association between postoperative Qa measurements and thrombosis risk. Therefore, current guidelines report that there is not enough evidence to suggest routine assessment of AV access Qa for surveillance. This study aimed to analyze the association between Qa, measured at the first post-operative visit (within 30 days from creation), and thrombosis risk, in distal and proximal autologous AV fistulae (dAVF and pAVF), and AV grafts (AVG). Method This retrospective cohort study included all AV accesses created in San Paolo Hospital (Milan, IT) between January 1st 2013 and December 31st 2022, with a post-operative Qa measurement available within 30 days from the date of creation. AV accesses were classified as dAVF, pAVF and AVG. Qa was measured by ultrasound as the product of the cross-sectional area and the time-averaged mean velocity, in the feeding artery. The thrombosis-free survival of dAVF, pAVF and AVG was plotted using the Kaplan Meier method. The association between Qa and the risk of thrombosis was studied by Cox proportional hazard models, adjusted for patient's age and sex. Non-linearity was assessed using natural splines. Results A total of 218 AV accesses (92 dAVF, 76 pAVF, and 50 AVG) were created in 191 patients. At baseline, patients had a median age of 69 years, 70% were male, 42% were already on HD treatment, and overall, they had a high prevalence of hypertension, diabetes mellitus and cardiovascular comorbidities. At first post-operative visit, median Qa was 922 [698, 1278] ml/min, 1260 [917, 1825] ml/min, and 1665 [1195, 2508] ml/min, respectively for dAVF, pAVF, and AVG (p <0.001). During a median follow up time of 1.35 [0.55, 2.62] years, 66 AV access thrombosis occurred. The thrombosis-free survival rates for dAVF, pAVF and AVG were 84%, 97%, and 51% at 1 year, and 63%, 66%, and 19% at 5 years (Fig. 1). For dAVF we found a significant L-shaped association between Qa measured at the first post-operative visit and thrombosis risk, with the lowest risk found at Qa of 1289 ml/min and an exponential risk increase for dAVF with Qa <800 ml/min (Fig. 2). Particularly, for dAVF with Qa < 1289 ml/min, we found a 39% increased risk of thrombosis for every 100 ml/min decrease in Qa (aHR 1.39, 95% CI 1.10-1.75, p 0.006). In pAVF, Qa showed a trend of inverse linear association with the risk of thrombosis, although not reaching statistical significance (aHR 1.06, 95% CI 0.96-1.17, p 0.24, for every 100 ml/min decrease in Qa). In AVG, Qa was not associated with the risk of thrombosis (aHR 1.01, 95% CI 0.97-1.06, p 0.57, for every 100 ml/min decrease in Qa). Conclusion In our cohort we found that Qa, measured at the first post-operative visit, is associated with the risk of thrombosis in dAVF, but not in pAVF and AVG. In particular, in dAVF the association is non-linear with the lowest risk in our cohort found at Qa equal to 1289 ml/min, with an exponentially increasing risk for Qa <800 ml/min. Our findings suggest that post-operative flow assessment may be a useful tool to identify dAVF at higher risk of thrombosis and allow a closer monitoring or possible preventive interventions.

#2722 Hemodialysis vascular access flow assessment by ultrasound dilution method (NephroFlow™) and ultrafiltration method with NIKKISO DBB-EXA™

Abstract Background and Aims A functioning vascular access (VA) is needed to ensure optimal dialysis. Reduced or falling levels of VA flow volume (Qa) indicate VA dysfunction and predict thrombosis. Qa can be measured by a number of different techniques, most of all using dilution techniques such as ultrasound dilution to measure the amount of recirculation induced by reversal of blood lines. The NephroFlow™ hemodialysis monitor estimates Qa using ultrasound velocity measurements of flowing blood and their dilution with 0.9% NaCl. Nikisso DBB-EXATM dialysis machine has the capacity to measure the VA recirculation rate by photometry performing a controlled rise on ultrafiltration rate; Qa is subsequently estimated using the Krivitski formula. There is little literature on the comparison of the different methodologies, but it was described that the DBB-EXATM method underestimates Qa compared to others techniques [1]. Our study aims to compare the Qa between the NephroFlow™ ultrasound dilution technique and the Nikisso DBB-EXATM ultrafiltration method. Method We performed an observational study that included 45 hemodialysis patients followed at our unit. Measurements of VA flow using the NephroFlow® device and the Nikisso-EXATM Ultrafiltration method were performed in patients with a native arteriovenous fistula or a graft. They were hemodynamically stable without acute clinical conditions. Both studies were undertaken during the first 90 minutes of a mid-week dialysis session, under a constant dialysis blood pump flow with the ultrafiltration interrupted during the procedure after having reversed the dialysis lines. For each patient we calculated the average of two Qa evaluations conducted at Nikisso-EXATM, spaced 15 days apart, with an additional evaluation using NephroFlow® conducted between these assessments. Results Forty-five patients (male: 71.1%) with a mean age of 73.2 ± 8 years were selected. The main CKD causes were diabetic kidney disease (n = 13, 28.9%) and uncertain etiology (n = 13, 28.9%). The dialysis vintage of these patients was 142.6 months (minimum 19, maximum 168). An arteriovenous fistula was present in 38 patients (84.4%) and an arteriovenous graft in 7 patients (15.6%). In the entire group, the mean flow with NephroFlow™ was 986.9 ± 515 ml/min and with DBB-EXATM 859.7 ± 522 ml/min. The measured values of the two methods presented a significant positive correlation (by Pearson) (R = 0.822, P < 0.01) (Fig. 1). In the group of patients with arteriovenous fistula (n = 38, 84.4%), the mean flow with NephroFlow™ was 961.8 ± 395 ml/min and with DBB-EXATM 839.1 ± 577 ml/min. The measured values of the two methods presented a significant positive correlation (by Pearson) (R = 0.844, P < 0.01). In the group of patients with arteriovenous graft (n = 7, 15.6%), the mean flow with NephroFlow™ was 1017.3 ± 401 ml/min and with DBB-EXATM 909.2 ± 604 ml/min. The measured values of the two methods presented a significant positive correlation (by Pearson) (R = 0.906, P < 0.01). Conclusion We found a significantly positive correlation between the Qa measurements obtained using both methods. The measurement of Qa using the ultrafiltration method on Nikisso DBB-EXATM seems to be a valuable and easily implementable tool for VA surveillance in both arteriovenous fistula and grafts. However, a bigger number of studies are required in this field in order to clarify this correlation with an additional comparative analysis to assess the reproducibility of measurements.

Differentiation of Polygonatum Cyrtonema Hua from Different Geographical Origins by Near-Infrared Spectroscopy with Chemometrics.

The identification of the geographical origin of Polygonatum cyrtonema Hua is of particular importance because the quality and market value of Polygonatum cyrtonema Hua from different production areas are highly variable due to differences in the growing environment and climatic conditions. This study utilized near-infrared spectra (NIR) of Polygonatum cyrtonema Hua (n = 400) to develop qualitative models for effective differentiation of Polygonatum cyrtonema Hua from various regions. The models were produced under different conditions to distinguish the origins distinctly. Ten preprocessing methods have been used to preprocess the original spectra (OS) and to select the most optimal spectral preprocessing method. Principal component analysis (PCA), partial least-squares discriminant analysis (PLS-DA), and orthogonal partial least squares discriminant analysis (OPLS-DA) were used to determine appropriate models. For simplicity, the pretreated full spectrum was calculated by different wavelength selection methods, and the four most significant variables were selected as discriminant indicator variables. The results show that Polygonatum cyrtonema Hua from different regions can be effectively distinguished using spectra from a series of samples analyzed by OPLS-DA. The accuracy of the OPLS-DA model is also satisfactory, with a good differentiation rate. The study findings indicate the feasibility of using spectroscopy in combination with multivariate analysis to identify the geographical origins of Polygonatum cyrtonema Hua. The utilization of NIR spectroscopy combined with chemometrics exhibits high efficacy in discerning the provenance of herbal medicines and foods, thereby facilitating QA measures.

#5803 ARTERIOVENOUS FISTULA BLOOD FLOW ESTIMATION THROUGH AI MODEL EXPLOITING THE PATIENT'S ROUTINELY COLLECTED DATA

Abstract Background and Aims Clinical monitoring and surveillance are key pillars of Arteriovenous Fistula (AVF) management in hemodialysis patients. They are aimed at limiting the risk of suboptimal dialysis dose and Vascular Access (VA) failure. AVF blood flow (Qa) is commonly used to assess VA function. Several methods have been proposed for the measurement of Qa including doppler ultrasound, and Body Thermal Monitor, among others. All these diagnostic tests are, in general, time-consuming for clinical staff, operator dependent, and costly for healthcare providers. To overcome limitations in the uptake of current Qa measurement techniques, we used data automatically recorded by dialysis machine sensors and medical information captured in electronic charts records to estimate Qa using a machine learning technique. Method For this historical cohort study, we analyzed electronic health records (EHR) of adult patients from four different European countries (Czech Republic, Portugal, Slovakia, and Spain), receiving in-center hemodialysis therapy in Fresenius Medical Care dialysis clinics between January 1st, 2015 and June 30th, 2022 registered in the European Clinical Database (EuCliD®). All patients consented their pseudo-anonymized data be used for secondary data analysis. The input dataset included 49 variables representing the health status of the patients, functional parameters of AVF function and the HD treatment parameters. The input variables were collected in the 90 days before the Qa measure. We constructed metrics representing the 90-day average and trend of each functional and medical parameter. Qa was classified in three levels: < 525 ml/h (very low), 525 ml/h - 925 ml/h (low), > 925 ml/h (normal). We estimated Qa as ordinal classification problem. Therefore, we used 2 binary classifiers based on the XGBoost algorithm. The estimation of the first-class, P(Qa<525), is given by the first classifier and the estimation of the third class, P(Qa≥925), is given by the second classifier. The probability of the middle class is computed as P(Qa≥525)⋅(1-P(Qa≥925)). The Qa estimation accuracy was assessed computing mean absolute error (MAE), precision and recall, F1-score and confusion matrix. Results Our dataset included 46,292 Qa measurements referred to 5,940 different hemodialysis patients. We obtained an overall precision of 0.77, a recall of 0.76, a F1-score of 0.76, a MAE of 0.27. The same metrics for each class are shown in Table 1. The model was able to detect fistula with “very low flow” with a precision of 0.9 and 16% of the missed “very low flow” AVFs are predicted as “low flow” and 4.7% are predicted as normal flow. The Confusion Matrix for each class is shown in Figure 1. Conclusion In this study we showed that clinically relevant Qa classes can be accurately predicted by resorting to routinely collected clinical data extracted from a electronic health record without any additional effort from healthcare professionals, training or instrumentation. Qa assessment is an important parameter in the evaluation of AVF function. Our algorithm accurately discriminated patients with “very low flow” that may be referred to vascular surgeon evaluation. It might help the AVF surveillance process, without adding time-consuming procedures for clinical staff or costs for healthcare provider.

#3736 SACUBITRIL/VALSARTAN IMPROVES THE VASCULAR ACCESS FLOW OF HEMODIALYSIS PATIENTS WITH HEART FAILURE AND REDUCED EJECTION FRACTION

Abstract Background and Aims Sacubitril/valsartan improved left ventricular systolic and diastolic function in patients with heart failure with reduced ejection fraction<40% (HFrEF) and end-stage kidney disease. However, there was no report concerning the effect of sacubitril/valsartan on access flow (Qa) of vascular access in maintenance hemodialysis (HD) patients with HFrEF. We aimed to investigate the effect of sacubitril/valsartan on Qa of vascular access in HD patients with HFrEF. Method We retrospectively screened the HD patients who received echocardiogram and Qa measurement in Taipei Veterans General Hospital and Cheng Hsin General Hospital from Jan. 2019 to Dec. 2021. Patients with HFrEF received twice of both echocardiography and Qa at 1 year apart were enrolled and divided into two groups, including those with or without the treatment of sacubitril/valsartan. The difference of Qa at baseline and 1 year later was compared by Wilcoxon signed-rank test, with a statistical significance of P<0.05. Results A total of 33 HD patients with HFrEF were analyzed with a mean HD vintage of 4.7 years. Sixteen patients received sacubitril/valsartan and the other 17 patients received conventional treatment. The mean Qa increased significantly from 633.8 to 948.8 mL/min (P<0.001) and the Qa change correlated with LVEF change [Fig. 1. correlation coefficient (r) =0.70, P=0.006] after 1-year treatment of sacubitril/valsartan; however, there was neither significant change of Qa (from 637.7 to 621.8 mL/min, P=0.44) nor correlation with LVEF change (Fig. 2, r=0.41, P=0.13) for controls after one year. Conclusion Sacubitril/Valsartan improves Qa in HD patients with HFrEF possibly related to the increase of LVEF.

PO-1791 3D printed inhomogeneity phantom for radiotherapy commissioning and QA measurements

PO-1791 3D printed inhomogeneity phantom for radiotherapy commissioning and QA measurements

The effect of dose gradients on gamma comparison insensitivity in patient specific QA comparisons.

While many have speculated on the reasons for gamma comparison insensitivity for patient-specific quality assurance analysis, the true reasons for insensitivity have not yet been elucidated. Failing to understand the reasons for this technique's insensitivity limits our ability to either improve the gamma metric to increase sensitivity of the comparison or the capacity to develop new comparison techniques that circumvent the limitations of the gamma comparison. To understand the underlying cause(s) for gamma comparison insensitivity and determine if simple plan characteristics can quantitatively predict for gamma comparison sensitivity. Known MLC and MU errors of varying magnitudes were induced on simple test fields to preliminarily investigate where gamma failures first begin to appear as error magnitude is increased. Gamma value maps between error-induced plan calculations and error-free plan calculations were created for 20 IMRT and 20 VMAT cases, each on three different detector geometries-ArcCHECK, MapCHECK, and Delta4. Gamma value maps were qualitatively compared to dose-gradient maps, and quantitative comparisons were performed between various plan descriptors and the computed gamma sensitivity for five different classes of induced errors were utilized to determine if any plan descriptor could predict the gamma sensitivity on a case-by-case basis. All comparisons were performed in a calculation-only scenario to remove uncertainties introduced by comparisons made with real patient specific QA measurements. Gamma value maps with increasing induced error magnitude illustrated that gamma comparisons fail first in high-dose, low-gradient regions of the field. Conversely, in areas of high gradient, gamma values typically remain low, even in the presence of large errors, regardless of detector geometry and gamma normalization setting. Thus, the complex, and often overlapping, high dose gradients in plans appear to be a limiting factor in gamma comparison sensitivity as the number of points along these gradients may often outnumber the points available for failing the comparison in lower gradient regions of the field. None of the simple plan descriptors studied were able to quantitively predict gamma comparison sensitivity, suggesting that quantitatively predicting the sensitivity of gamma comparisons on a case-by-case basis may require a combination of multiple factors or metrics not studied here. Simple plan descriptors and the number of points in high-dose, low-gradient regions of the field did not quantitively predict for gamma comparison sensitivity. However, it is clear from gradient and gamma value maps that gamma comparisons fail first in high-dose, low-gradient regions of the field in the presence of known induced errors, which we have shown to be independent of detector geometry and gamma comparison normalization setting. Gamma comparison sensitivity is thus limited by the ever-increasing complexity of plans and is particularly important to consider as treatment volumes become smaller and the complexity of overlapping plan gradients increases. This suggests that new methods for patient-specific QA comparisons are required to circumvent this limitation.

Comparison of 116 Radiosurgery Treatment Plans for Multi-Leaf and Cone Collimator on a Varian Edge Linac: Are Cones Superior in the Daily Routine?

Delivering focused radiation doses via linear accelerators is a crucial component of stereotactic radiosurgery (SRS) for brain metastases. The Varian Edge linear accelerator provides highly conformal radiation therapy through a high-definition multi-leaf collimator (HD120 MLC) and conical collimator (CC). HD120 MLC adapts to the shape of the target volume using movable tungsten leaves, while CC has a block of conical shape (cones). CC in SRS treatments of small brain metastases is preferred due to its mechanical stability and steeper dose fall-off, potentially sparing organs at risk (OARs) and the brain better than HD120 MLC. This study aims to determine if CC offers significant advantages over HD120 MLC for SRS treatments. For 116 metastatic lesions, CC and HD120 MLC treatment plans were created in Varian Eclipse TPS and compared based on various dose parameters, robustness tests, and QA measurements. The results indicate that CC provides no significant advantages over HD120 MLC, except for slight, clinically insignificant benefits in brain sparing and dose fall-off for the smallest lesions. HD120 MLC outperforms CC in almost every aspect, making it a better choice for irradiating brain metastases with 0.1 cm3 or higher volumes.

The use of a mini-ridge filter with cyclotron-based pencil beam scanning proton therapy.

Pencil beam scanning (PBS) proton therapy allows for far superior dose conformality compared with passive scattering techniques. However, one drawback of PBS is that the beam delivery time can be long, particularly when treating superficial disease. Minimizing beam delivery time is important for patient comfort and precision of treatment delivery. Mini-ridge filters (MRF) have been shown to reduce beam delivery time for synchrotron-based PBS. Given that cyclotron systems are widely used in proton therapy it is necessary to investigate the potential clinical benefit of mini-ridge filters in such systems. To demonstrate the clinical benefit of using a MRF to reduce beam delivery time for patients with large target volumes and superficial disease in cyclotron-based PBS proton therapy. A MRF beam model was generated by simulating the effect of a MRF on our clinical beam data assuming a fixed snout position relative to the isocenter. The beam model was validated with a series of measurements. The model was used to optimize treatment plans in a water phantom and on six patient DICOM datasets to further study the effect of the MRF and for comparison with physician-approved clinical treatment plans. Beam delivery time was measured for six plans with and without the MRF to demonstrate the reduction achievable. Plans with and without MRF were reviewed to confirm clinical acceptability by a radiation oncologist. Patient-specific QA measurements were carried out with a two-dimensional ionization chamber array detector for one representative patient's plan optimized with the MRF beam model. Results show good agreement between the simulated beam model and measurements with mean and maximum deviations of 0.06mm (0.45%) and 0.61mm (4.9%). The increase in Bragg peak width (FWHM) ranged from 2.7mm at 226MeV to 6.1mm at 70 MeV. The mean and maximum reduction in beam delivery time observed per field was 29.1s (32.2%) and 79.7s (55.3%). MRFs can be used to reduce treatment time in cyclotron-based PBS proton therapy without sacrificing plan quality. This is particularly beneficial for patients with large targets and superficial disease such as in breast cancer where treatment times are generally long, as well as patients treated with deep inspiration breath hold (DIBH).

Clinical experience on patient-specific quality assurance for CBCT-based online adaptive treatment plan.

Ethos CBCT-based adaptive radiotherapy (ART) system can generate an online adaptive plan by re-optimizing the initial reference plan based on the patient anatomy at the treatment. The optimization process is fully automated without any room for human intervention. Due to the change in anatomy, the ART plan can be significantly different from the initial plan in terms of plan parameters such as the aperture shapes and number of monitor units (MUs). In this study, we investigated the feasibility of using calculation-based patient specific QA for ART plans in conjunction with measurement-based and calculation-based QA for initial plans to establish an action level for the online ART patient-specific QA. A cohort of 98 cases treated on CBCT-based ART system were collected for this study. We performed measurement-based QA using ArcCheck and calculation-based QA using Mobius for both the initial plan and the ART plan for analysis. For online the ART plan, Mobius calculation was conducted prior to the delivery, while ArcCheck measurement was delivered on the same day after the treatment. We first investigated the modulation factors (MFs) and MU numbers of the initial plans and ART plans, respectively. The γ passing rates of initial and ART plan QA were analyzed. Then action limits were derived for QA calculation and measurement for both initial and online ART plans, respectively, from 30 randomly selected patient cases, and were evaluated using the other 68 patient cases. The difference in MF between initial plan and ART-plan was 12.9% ± 12.7% which demonstrates their significant difference in plan parameters. Based on the patient QA results, pre-treatment calculation and measurement results are generally well aligned with ArcCheck measurement results for online ART plans, illustrating their feasibility as an indicator of failure in online ART QA measurements. Furthermore, using 30 randomly selected patient cases, the γ analysis action limit derived for initial plans and ART plans are 89.6% and 90.4% in ArcCheck QA (2%/2mm)andare 92.4% and 93.6% in Mobius QA(3%/2mm),respectively.According to the calculated action limits, the ArcCheck measurements for all the initial and ART plans passed QA successfully while the Mobius calculation action limits flagged seven and four failure cases respectively for initial plans and ART plans, respectively. An ART plan can be substantially different from the initial plan, and therefore a separate session of ART plan QA is needed to ensure treatment safety and quality. The pre-treatment QA calculation via Mobius can serve as a reliable indicator of failure in online ART plan QA. However, given that Ethos ART system is still relatively new, ArcCheck measurement of initial plan is still in practice. It may be skipped as we gain more experience and have better understanding of the system.

Optimal Flow Volume Measurements in Forearm versus Arm Arteriovenous Fistulas.

Successful haemodialysis is dependent on optimal arteriovenous (AV) access flow. Although 600 mL/min is frequently quoted as the critical level for functional flow volume (Qa) according to the National Kidney Foundation guideline, this may not be applicable for the different configurations of AV fistulas (AVF) or AV grafts (AVG). This study evaluates ultrasound derived Qa measurement in the inflow brachial artery to autologous AVF in the forearm radiocephalic and arm brachiocephalic/basilic configurations in relation to significant flow related AV dysfunction. Five hundred and eleven duplex ultrasound (DUS) scans were analysed in 193 patients. The end points were therapeutic intervention and/or thrombosis of AVF versus no complication within three months of the scan. Receiver operating characteristic (ROC) curves were used to determine the optimal threshold Qa of the brachial artery supplying the AVF. Of the 511 scans, 155 scans were assigned to the intervention group, ie, AVF requiring intervention or thrombosing within 3 months of the DUS. Using ROC curve analysis, the area under the curve (AUC) for all AVF is 0.90 (CI: 0.88-0.93) with an optimal threshold Qa of 686mL/min. In forearm AVF, the threshold Qa is 589mL/min while in arm AVF the threshold Qa is 877mL/min. Forearm Qa is statistically different from arm Qa. Forearm AVF Qa threshold at 589 mL/min is distinct from arm AVF Qa at 877 mL/min and these are predictive of the need for impending intervention or thrombosis due to flow-limiting stenosis. This article is protected by copyright. All rights reserved.

Evaluation of OrthoChromic OC-1 films for photon radiotherapy application.

A new film dosimetry system consists of the new OrthoChromic™ OC-1 film, and a novel calibration procedure was evaluated. Two films, C1 and C2, were exposed simultaneously using the 6FFF beam with a step-wedge pattern of five steps ranging from 590 to 3000cGy. C1 was used for calibration, and C2 was used for calibration curve validation. The second scan of C2 was done by rotating the film by 90-deg. To evaluate the effectiveness of the non-uniform scanner response correction with the new system, a film was exposed to a 20 × 20cm2 field. The beam profile measured with the film was compared to the IBA cc04 measurements in water. Films were irradiated to characterize the energy response, dynamic range and temporal growth effect. Open (MLC-defined) and clinical fields were radiated to evaluate the overall performance of the new system. The new calibration procedure was validated with an average dose difference of 1.6% and a gamma (2%,2mm) passing rate of 100%. With C2 scanned 90-deg rotated, the average dose difference was 1.3%. The average difference between cc04 and film was 0.4%. The St between films and diode/cc04 were within -0.3% difference for 1 × 1 to 14 × 14cm2 and -2.8% for 0.5 × 0.5cm2. For clinical fields, the average gamma (3%,2mm) was 98.8%. These results were consistent with EBT3 film and MapCheck measurements with a dose > 400cGy. The results have shown that the OC-1 film system can achieve accurate results for QA measurements, but more considerable uncertainty was observed within the low dose range.

A Method to Simplify Patient-Specific QA of Single Isocenter Multiple Met SRS Plans

<h3>Purpose/Objective(s)</h3> Pre-treatment quality assurance of single isocenter multi-met stereotactic radiosurgery (SRS) plans is standardly performed by acquiring a dose measurement using a detector array or ion chamber and film. The major efficiency issue with QA of these plans is how many, and which of the multiple metastases (mets) should be measured. The purpose of this study was to develop a workflow using an independent 3D dose calculation (DC) and trajectory log (TL) QA software to efficiently determine which of the brain mets should be further evaluated by measurement. <h3>Materials/Methods</h3> 30 single isocenter multiple metastasis patient SRS plans with a total of 82 mets were retrospectively studied. The clinical SRS plans were generated with an automated dynamic conformal arc SRS planning system (TPS). All plans were sent to the DC software for calculation. The gamma analysis (3%, 1 mm), percent difference between TPS and DC calculated mean and max dose to PTV, and the dose difference of the centroid point of each PTV was recorded. All plans were delivered to a high-resolution diode array designed for SRS and measurements were used to perform a gamma analysis (3%, 1 mm). In addition, the trajectory logs of the QA delivery were used to calculate 3D dose distributions and the same metrics acquired from the DC software were calculated and recorded. Pearson correlation between DC and TL calculated metrics was calculated. <h3>Results</h3> The results of the dosimetric metrics calculated by the DC and TL software are presented in Table 1. Overall agreement between DC and TPS was excellent, with all mean percent differences within 1.1% and mean gamma pass rates greater than 99%. Furthermore, dosimetric metrics calculated by DC and TL were highly correlated, with the correlation coefficients for all metrics at least 0.96, which suggests DC results can be used to predict TL QA results. To use DC to screen which PTVs should be further evaluated, an action level was set such that if any metric fell outside the 95% confidence interval (mean + 1.96 × SD), a QA measurement would be performed. For all metrics, a ±5% action level was calculated to cover the 95% confidence interval. DC results for 6 PTVs exceeded the action level. Of these plans, the measured patient specific QA passed for all plans. While all measured QA passed, DC and TL QA appeared to be more sensitive than the diode array measurement in identifying potential plan quality issues. <h3>Conclusion</h3> An efficient workflow for patient specific QA of single isocenter multiple met SRS plans was developed. An independent 3D dose calculation is used to calculate agreement of dosimetry metrics compared to the TPS. If the percent difference of a metric for a PTV exceeds a 5% action level, it is selected for further QA using a dose measurement.

FC097: Pitfalls in Assessing of Cardiopulmonary Recirculation in HD Patients

Abstract BACKGROUND AND AIMS Cardiopulmonary recirculation (CRP) is one of the most informative instrumental parameters, widely used to predict adverse cardiovascular events in patients on maintenance hemodialysis (HD). CRP calculation is based on two indirectly measured estimates: AVF volume blood flow (Qa) and cardiac output (CO). Large CRP variability may cause underestimation of cardiovascular risk in many HD patients. We aimed to study the inter- and inner-observer agreement of Qa and CO. As a second step, we studied the changes in Qa, CO and CPR before and after HD. METHOD The prospective study included 88 patients with native AVF. At the first step, we evaluated the inter- and inner-observer agreement of Qa measurement with color duplex ultrasound (Bland–Altman plots, Figure 1A–D). Two specialists with 5–7 years of experience measured Qa twice on brachial artery, twice on fistula vein. To estimate the maximum relative error in paired measurements we calculated the mean and the absolute mean difference for each pair. Then, the absolute mean difference was divided by mean; the greatest deviations from the average are given in %. After that, they measured CO twice before HD and once after HD. Ultrafiltration during HD was 1.6 ± 0.34 L. RESULTS We observed a good concordance between Qa measurements on brachial artery by one specialist (Figure 1A) and by two specialists (Figure. 1B): deviations were [–13.9%; 13.1%] and [–18.3%; 16.4%], respectively. There was a poor concordance between assessments on brachial artery and fistula vein (Figure 1C), and Qa measurement on fistula vein alone (Figure 1 D). In the latter case, the variance was very high even if measurements were performed by one specialist: deviations were [–37.9%; 39.6%] and [–59.3%; 93.6%], respectively. In all cases, the systematic bias was low, but the deviations were significantly different between measurement methods. The variance of bias increased along with increasing of Qa. We observed a good concordance between CO assessments made by one specialist and between specialists: deviations were [–12.4%; 12.3%] and [–10.5%; 9.1%], respectively. The main pitfall of CPR-based cardiovascular risk stratification is that CO changes significantly after HD (Fig. 2A), while Qa values remain relatively stable (Figure 2B): although Qa difference before and after HD is statistically significant, it seems to be modest. Median CO decrease after HD was 13.4% (26.6% max), while median of Qa decrease was 1.7% (6.1% max). This leads to a significant increase of CPR value after HD, which can reach 40%(! ) in some patients (absolute increase of 0.11) (Figure. 2C). CPR value more than 30% is considered to be an indication for Qa reduction. In our cohort, 17% of the patients had CPR &amp;gt;30% before HD and 42% after HD. CONCLUSION Qa assessment should be performed only on brachial artery. There is a significant decrease in CO after HD even if ultrafiltration was moderate, whereas Qa remains relatively stable. This leads to a significant increase of CPR value in some patients after HD. CPR assessment before HD may lead to underestimation of cardiovascular risk. The decision on Qa reduction to decrease CPR should be based on CPR assessment after HD session.

Dosimetric comparison of analytical anisotropic algorithm and the two dose reporting modes of Acuros XB dose calculation algorithmin volumetric modulated arc therapy of carcinoma lung and carcinoma prostate.

Volumetric Modulated Arc Therapy (VMAT) is an important modality for radical radiotherapy of all major treatment sites. This study aims to compare Analytical Anisotropic Algorithm (AAA) and the two dose-reporting modes of Acuros XB (AXB) algorithm -the dose to medium option (Dm) and the dose to water option (Dw) in Volumetric Modulated Arc Therapy (VMAT) of carcinoma lung and carcinoma prostate. We also compared the measured dose with Treatment Planning System calculated dose for AAA and the two dose reporting options of Acuros XB using Electronic Portal Imaging Device (EPID) and ArcCHECK phantom. Treatment plans of twenty patients each who have already undergone radiotherapy for cancer of lung and cancer of prostate were selected for the study. Three sets of VMAT plans were generated in Eclipse Treatment Planning System (TPS), one with AAA and two plans with Acuros-Dm and Acuros-Dw options. The Dose Volume Histograms (DVHs) were compared and analyzed for Planning Target Volume (PTV) and critical structures for all the plans. Verification plans were created for each plan and measured doses were compared with TPS calculated doses using EPID and ArcCHECK phantom for all the three algorithms. For lung plans, the mean dose to PTV in the AXB-Dw plans was higher by 1.7% and in the AXB-Dm plans by 0.66% when compared to AAA plans. For prostate plans, the mean dose to PTV in the AXB-Dw plans was higher by 3.0% and in the AXB-Dm plans by 1.6% when compared to AAA plans. There was no difference in the Conformity Index (CI) between AAA and AXB-Dm and between AAA and AXB-Dw plans for both sites. But the homogeneity worsened in AXB-Dw and AXB-Dm plans when compared to AAA plans for both sites. AXB-Dw calculated higher dose values for PTV and all the critical structures with significant differences with one or two exceptions. Point dose measurements in ArcCHECK phantom showed that AXB-Dm and AXB-Dw options showed very small deviations with measured dose distributions than AAA for both sites. Results of EPID QA also showed better pass rates for AXB-Dw and AXB-Dm than AAA for both sites when gamma analysis was done for 3%/3 mm and 2%/2 mm criteria. With reference to the results, it is always better to choose Acuros algorithm for dose calculations if it is available in the TPS. AXB-Dw plans showed very high dose values in the PTV when compared to AAA and AXB-Dm in both sites studied. Also, the volume of PTV receiving 107% dose was significantly high in AXB-Dw plans compared to AXB-Dm plans in sites involving high density bones. Considering the results of dosimetric comparison and QA measurements, it is always better to choose AXB-Dm algorithm for dose calculations for all treatment sites especially when high density bony structures and complex treatment techniques are involved. For patient specific QA purposes, choosing AXB-Dm or AXB-Dw does not make any significant difference between calculated and measured dose distributions.

Validation of the inverse method for the determination of the access flow with thermodilution

IntroductionThermodiluction is a widely used method for measuring vascular access flow (QA). Among the possibilities of TD, the reverse method (MI) can be beneficial in the execution time, without impact on the dialysis efficacy (Kt). However, it is not a sufficiently studied technique. MethodTransversal study of 117 arteriovenous fistulas (AVF). Two QA measurements were taken with the method described by the manufacturer (MR) and another with MI. MI is bases in the obtention of an inverted recirculation registry at the beginning of the session and a single subsequent recirculation measurement with the lines in normal position. In the concordance analysis, the Bland-Altman method and Cohen’s Kappa index were used. ResultsVery good concordance between MR and MI was evidenced for QA below 700 ml/min, but it worsens as flow increases. The median variability between the MR measurements (intra-method variability) was 3.4% (−17.13). This value did not differ from the median variability generated between MR and MI (inter-method variability), which was 2% (−14, 12) (P = 0.287). The degree of agreement between the two to identify AVFs susceptible to intervention was very good (K = 0.834). The time spent using the MI was significantly shorter (P = 0.000) without evidence of variations in the Kt of the measurement sessions (P = 0.201). ConclusionsThe thermodiluction MI is valid to determine the flow of the vascular access, especially in Qa lower than 700 ml/min, with great time savings, simplification of the procedure and without modifying the dialysis efficiency. The variability between the measurement by MR and MI is similar to that of MR. The concordance between methods in identifying potentially pathological AVFs is very good.