Significant Dose Errors Research Articles

A physically constrained Monte Carlo-Neural Network coupling algorithm for BNCT dose calculation.

In boron neutron capture therapy (BNCT)-a form of binary radiotherapy-the primary challenge in treatment planning systems for dose calculations arises from the time-consuming nature of the Monte Carlo (MC) method. Recent progress, including the use of neural networks (NN), has been made to accelerate BNCT dose calculations. However, this approach may result in significant dose errors in both the tumor and the skin, with the latter being a critical organ in BNCT. Furthermore, owing to the lack of physical processes in purely NN-based approaches, their reliability for clinical dose calculations in BNCT is questionable. In this study, a physically constrained MC-NN (PCMC-NN) coupling algorithm is proposed to achieve fast and accurate computation of the BNCT three-dimensional (3D) therapeutic dose distribution. This approach synergizes the high precision of the MC method with the speed of the NN and utilizes physical conservation laws to constrain the coupling process. It addresses the time-consuming issue of the traditional MC method while reducing dose errors. Clinical data were collected from 113 glioblastoma patients. For each patient, the 3D dose distributions for both the coarse and detailed dose grids were calculated using the MC code PHITS. Among these patients, the data from 14 patients were allocated to the test set, 9 to the validation set, and the remaining to the training set. A neural network, 3D-Unet, was built based on the coarse grid dose and patient CT information to enable fast and accurate computation of the 3D detailed grid dose distribution of BNCT. Statistical evaluations, including relative deviation, dose deviation, mean absolute error (MAE), and mean absolute percentage error (MAPE) were conducted. Our findings suggested that the PCMC-NN algorithm substantially outperformed the traditional NN and interpolation methods. Furthermore, the proposed algorithm significantly reduced errors, particularly in the skin and GTV, and improved computational accuracy (hereinafter referred to simply as 'accuracy') with a MAPE range of 1.6%-4.0% and a maximum MAE of 0.3Gy (IsoE) for different organs. The dose-volume histograms generated by the PCMC-NN aligned well with those obtained from the MC method, further validating its accuracy. The PCMC-NN algorithm enhanced the speed and accuracy of BNCT dose calculations by combining the MC method with the NN algorithm. This indicates the significant potential of the proposed algorithm for clinical applications in optimizing treatment planning.

Couch and multileaf collimator tracking: A clinical feasibility study for pancreas and liver treatment.

Real-time tumor tracking through active correction by the multileaf collimator or treatment couch offers a promising strategy to mitigate delivery uncertainty due to intrafractional tumor motion. This study evaluated the performance of MLC and couch tracking using the prototype iTools Tracking system in TrueBeam Developer Mode and the application for abdominal cancer treatments. Experiments were carried out using a phantom with embedded Calypso transponders and a motion simulation platform. Geometric evaluations were performed using a circular conformal field with sinusoidal traces and pancreatic tumor motion traces. Geometric tracking accuracy was retrospectively calculated by comparing the compensational MLC or couch motion extracted from machine log files to the target motion reconstructed from real-time MV and kV images. Dosimetric tracking accuracy was measured with radiochromic films using clinical abdominal VMAT plans and pancreatic tumor traces. Geometrically, the root-mean-square errors for MLC tracking were 0.5 and 1.8mm parallel and perpendicular to leaf travel direction, respectively. Couch tracking, in contrast, showed an average of 0.8mm or less geometric error in all directions. Dosimetrically, both MLC and couch tracking reduced motion-induced local dose errors compared to no tracking. Evaluated with five pancreatic tumor motion traces, the average 2%/2mm global gamma pass rate of eight clinical abdominal VMAT plans was 67.4% (range: 26.4%-92.7%) without tracking, which was improved to 86.0% (range: 67.9%-95.6%) with MLC tracking, and 98.1% (range: 94.9%-100.0%) with couch tracking. In 16 out of 40 deliveries with different plans and motion traces, MLC tracking did not achieve clinically acceptable dosimetric accuracy with 3%/3mm gamma pass rate below 95%. This study demonstrated the capability of MLC and couch tracking to reduce motion-induced dose errors in abdominal cases using a prototype tracking system. Clinically significant dose errors were observed with MLC tracking for certain plans which could be attributed to the inferior MLC tracking accuracy in the direction perpendicular to leaf travel, as well as the interplay between motion tracking and plan delivery for highly modulated plans. Couch tracking outperformed MLC tracking with consistently high dosimetric accuracy in all plans evaluated, indicating its clinical potential in the treatment of abdominal cancers.

Medication Dosing Safety for Pediatric Patients: Recognizing Gaps, Safety Threats, and Best Practices in the Emergency Medical Services Setting. A Position Statement and Resource Document from NAEMSP

Background Millions of patients receive medications in the Emergency Medical Services (EMS) setting annually, and dosing safety is critically important. The need for weight-based dosing in pediatric patients and variability in medication concentrations available in the EMS setting may require EMS providers to perform complex calculations to derive the appropriate dose to deliver. These factors can significantly increase the risk for harm when dose calculations are inaccurate or incorrect. Methods We conducted a scoping review of the EMS, interfacility transport and emergency medicine literature regarding pediatric medication dosing safety. A priori, the authors identified four research topics: (1) what are the greatest safety threats that result in significant dosing errors that potentially result in harm to patients, (2) what practices or technologies are known to enhance dosing safety, (3) can data from other settings be extrapolated to the EMS environment to inform dosing safety, and (4) what impact could standardization of medication formularies have on enhancing dosing safety. To address these topics, 17 PICO (Patient, Intervention, Comparison, Outcome) questions were developed and a literature search was performed. Results After applying exclusion criteria, 70 articles were reviewed. The methods for the investigation, findings from these articles and how they inform EMS medication dosing safety are summarized here. This review yielded 11 recommendations to improve safety of medication delivery in the EMS setting. Conclusion These recommendations are summarized in the National Association of EMS Physicians® position statement: Medication Dosing Safety for Pediatric Patients in Emergency Medical Services.

Continuous aperture dose calculation and optimization for volumetric modulated arc therapy

Although VMAT delivery features continuous gantry rotation and leaf motion, dose calculation is often performed under the dual assumption of discrete apertures changing instantaneously from one discrete angle to the next. In this work, the validity of these two approximations is determined, as well as their impact on the quality of optimized plans. Further, an accurate method of fluence calculation is derived which does not use the discrete aperture approximation, but instead calculates the fluence as the multi-leaf collimator leaves sweep from one position to another. This continuous aperture fluence calculation is integrated in the VMAT optimization process using the open-source treatment planning system matRad. The three-step approach of VMAT optimization is used: fluence map optimization followed by leaf sequencing and direct aperture optimization, with variable leaf speed, gantry rotation speed, and MU rate. The benefit of the continuous aperture VMAT method over the discrete aperture method is determined by comparing the plan quality of discrete aperture and continuous aperture optimized plans, when the former is recalculated using the continuous aperture fluence calculation. Discrete aperture VMAT plans calculated at 4° spacing result in significant dose errors (10%–35%, depending on the anatomical site) as compared to the reference dose (continuous aperture fluence calculation at 0.5° spacing). These errors are greatly reduced (to 0.8%–2%) when the continuous aperture fluence calculation method was used at the same 4° spacing, implying that the dose error is primarily due to the discrete aperture approximation. Whereas all dose objectives were met by the discrete aperture VMAT optimized plan, many of them failed when the dose was recalculated with the continuous aperture fluence calculation. All objectives were met once again when the plan was optimized with the new continuous aperture VMAT optimization. Further, using only half of the beam angles, the continuous aperture VMAT optimization can achieve the same degree of accuracy with only 40% of the computing time as compared with the standard discrete aperture VMAT.

Hypertensive Crisis During Norepinephrine Syringe Exchange

A 67-year critically ill patient suffered from a hypertensive crisis (200 mm Hg) because of a norepinephrine overdose. The overdose occurred when the clinician exchanged an almost-empty syringe and the syringe pump repeatedly reported an error. We hypothesized that an object between the plunger and the syringe driver may have caused the exertion of too much force on the syringe. Testing this hypothesis in vitro showed significant peak dosing errors (up to +572%) but moderate overdose (0.07 mL, +225%) if a clamp was used on the intravenous infusion line and a large overdose (0.8 mL, +2700%) if no clamp was used. Clamping and awareness are advised.

Abstract TP293: Errors in Estimation of Body Weight in Thrombolysis for Acute Ischemic Stroke

Introduction: Intravenous tissue plasminogen activator (IVtPA) is a proven treatment in acute ischemic stroke but relies on weight-based dosing in a time sensitive manner. Errors in body weight estimation are common and may result in over or under-dosing of IVtPA. Methods: With IRB approval, we prospectively screened 99 consecutive patients who received IVtPA for suspected acute ischemic stroke from November 1, 2015 to July 1, 2016 as part of a prospective IVtPA study. Patients were included in this analysis who had complete data. Data was collected on patient demographics, National Institutes of Health Stroke Scale (NIHSS), body weight, and times of last known normal (LKN) and IVtPA administration. A significant error in IVtPA dosing was defined as a >10% difference in actual versus calculated IVtPA dose. Results: Among 42 patients prospectively enrolled, the mean age was 65.8 years (range, 32 to 89) and 30 (79%) of patients were transfers from outside Emergency Rooms. Mean time from LKN to IVtPA administration was 2.4 hours (range. 0.7 to 4.8). Initial pre-IVtPA mean NIHSS was 14 (range, 3 to 26). The mean % error in IVtPA dose was +1.9% (range, -29% to + 21%). Ten patients (24%) had a significant error in IVtPA dosing of > 10% of the dose, and 8 (80%) were over-doses. Patients with significant IVtPA dosing error did not differ in terms of age (65.8 vs 65.8, p 0.99), sex (female 55.6% vs 38.9%, p 0.45), race (white 70% vs 47%, p 0.28), height (164.8 vs 171.0 cm, p 0.1), or time from LKN to IVtPA (2.8 vs 2.3, p 0.15). Patients with significant IVtPA dosing errors had significantly lower baseline weight (70.3 vs 90.0 kilograms, p 0.029) and higher NIHSS (18.7 vs 12.6, p 0.018). Conclusion: Dosing errors of > 10% of the calculated IVtPA dose occur in nearly one-quarter of acute stroke patients treated in the community, and patients with lower weight and higher NIHSS were at higher risk of dosing errors.

Dosing errors in preterm neonates due to flow rate variability in multi-infusion syringe pump setups: An in vitro spectrophotometry study

BackgroundDrug administration on the neonatal intensive care unit is often associated with adverse events. This may be due to dosing errors caused by multi-infusion setups. We aim to investigate these dosing errors. Material and methodsN=3 experiment using a medication schedule, multi-infusion setup (three pumps) and disposables as applied on the NICU. In-line and real-time absorption spectrophotometry was used with dyes as substitutes for pharmaceuticals. Three flow rate changes lasting 1h were initiated. Subsequently, the possible dosing errors were estimated in the parallel pumps. In addition, startup durations, the times the flow rates required to reach steady state after significant dosing errors, as well as the total dosing error were measured. ResultsContribution of the start-up delays to the cumulative dosing errors was the largest. However, initiated flow rate changes resulted in significant dosing errors in the parallel pumps as well. The total dosing error was not significant. The significant peak errors were between 48.2% and −32.5% at flow rate increase and decrease, respectively. Startup delays of up to 42.6min were measured. ConclusionsApplying multi-infusion while following a neonatal medication schedule may temporarily result in dosing errors, which can be relevant for fast-acting medications. Awareness may mitigate the risks.

SU‐F‐T‐311: Comparison of Measurements From Two Diode Array QA Devices to Deep Point Dose Measurements with Two Treatment Planning Model Settings for Brain VMAT SRT Patient‐Specific QA

Purpose:To investigate the sensitivity of traditional gamma‐index‐based analysis using diode arrays to detect significant dosimetric errors in patientspecific measurements for VMAT brain SRT plans.Methods:Quality assurance measurements were performed using an ion chamber and two different diode array devices for four brain SRT treatment plans. All cases were planned with Eclipse AAA RapidArc using 6MV VMAT for a TrueBeam STx. Ion chambers measurements were performed with a PinPoint 3D micro‐ion chamber in both the Standard Imaging Stereotactic Dose Verification (SI) Phantom and in the ArcCheck (AC) cavity plug. Point dose measurements were taken at isocenter within the target. Array measurements were acquired with the ArcCheck and gantry‐mounted MapCheck (MC) and assessed using the SNC patient software with a 3%/3mm absolute dose gamma‐index analysis with global normalization and a 10% dose threshold. Calculations and their corresponding measurements were performed and compared before and after a dynamic leaf gap (DLG) adjustment.Results:The measured doses at isocenter dropped from 6.3%–8.7% in the SI phantom and 4.0%–9.3% for AC down to −0.1%–1.1% in the SI phantom and 0.8%–2.9% in the AC after adjusting the DLG. However, the cumulative dose differences to MC's central diode ranged −2.0%–1.9% before the DLG adjustment and were −7.7%–0.2% afterwards. Similarly, the gamma‐index analysis results for MC were found to be 94.1%–100% before the DLG adjustment and 94.4%–100% afterwards. The gamma‐index results from AC were 72.9%–98.7% before and 95.9%–99.7% after.Conclusion:Our data suggest that traditional passing criteria using gantry‐mounted MapCheck may not have the sensitivity to catch significant dose errors introduced by DLG settings. ArcCheck measurements did flag some dose differences, however there appears to be a possibility that dose discrepancies may not always be observed for VMAT SRT treatments, and point dose measurements are recommended for verification.

SU‐E‐J‐173: Dosimetric Effect of Patient Positioning Uncertainties for IMRT and SBRT

Purpose:To quantify the dosimetric effects of patient positioning uncertainties on PTV coverage and normal tissue sparing and find the tolerance for IMRT and SBRT positioning errors.Method and Materials: Patient positioning uncertainties were represented by isocenter shifts. Three head and neck IMRT, three prostate IMRT, and two lung SBRT plans were included in this study. The potential positioning uncertainties were introduced by shifting the isocenters by 0.1 cm, 0.3 cm, 0.5 cm, .0 cm, and 1.5 cm in Eclipse. Actual isocenter shifts (clinical shift) based on OBI/CBCT were also applied to the plans. The plans with isocenter shifts were recalculated in Eclipse and the results were compared with the original plans.Results:The dose errors increased with isocenter shifts. For head and neck patients, the PTV D95 dropped by 3.3% and the cord dose increased by 3.0% for 3‐mm shift, but D95 dropped 6.8 % with 5 mm shift, and spinal cord dose increased by 6.9%. For prostate, 3‐mm shift reduced 3.4% in D95 and increased 1.2% rectum dose and 2.7% bladder dose, while 5 mm shift caused 9.8% of reduction in D95 and 14.2% increase in bladder dose, but 7.1% decrease in rectum mean dose. For lung SBRT, 1‐mm shift caused 2.2% decrease in D95 and 0.7% decrease in spinal cord, but 3 mm shifts led to 14.9% of decrease in D95, while spinal cord dose also decreased by 1.5%.ConclusionThe patient positioning uncertainties reduced the PTV coverage and changed the dose to the normal tissues. However, the significance of dosimetric effect of patient positioning uncertainties varied with different tumor sites. This study suggested that the positioning uncertainty should be within 3 mm for regular IMRT treatment, and 1 mm for SBRT to avoid significant dose errors.

PO-0710 Dosing Errors And Clinical Impact In Preterm Infants Due To Flow Rate Variability In Mult-infusion Therapy

<h3>Background and aims</h3> Almost all preterm infants on the NICU receive continuous intra-venous infusion therapy. Commonly, multiple pharmaceuticals are administered through one catheter (multi-infusion). Due to the mutual influence of infusion pumps, dosing errors can occur that may lead to adverse events. We designed an <i><i>in vitro</i></i> experiment to measure flow rate variability and dosing errors. Subsequently, possible clinical impact was investigated. <h3>Methods</h3> We conducted an n = 3 experiment with 3 syringe pumps and disposables as used in our NICU. A clinically relevant medication schedule was simulated using laser dyes as substitutes for pharmaceuticals. Real-time, inline, absorption spectrometry was used to measure dye concentrations and, subsequently, analyse flow rate variability. After changing the flow rate we registered temporary dosing errors in the parallel pumps, in addition, we registered start-up delays. A one-compartment pharmacological model was used to investigate the clinical impact of these errors. <h3>Results</h3> The significant temporary dosing errors were between 48.1% ± 12.9% and -32.5% ± 22.5% over- and under-dose respectively. Start-up delays were up to 0.71 ± 0.11 h. Our pharmacological model indicates that these dosing errors could lead to haemodynamic instability for commonly used inotropes. <h3>Conclusions</h3> Potential clinical impact includes hypertension, hypotension and intraventricular haemorrhages. We conclude that applying multi-infusion with currently used NICU infusion setups results in dosing errors with potential clinical impact. It is advised not to combine high-alert medication on a mutual lumen or line. In addition, it is advised to raise awareness about these phenomena. This study was co-funded by the EMRP.

SU-E-T-192: FMEA Severity Scores - Do We Really Know?

Purpose: Failure modes and effects analysis (FMEA) is a subjective risk mitigation technique that has not been applied to physics-specific quality management practices. There is a need for quantitative FMEA data as called for in the literature. This work focuses specifically on quantifying FMEA severity scores for physics components of IMRT delivery and comparing to subjective scores. Methods: Eleven physical failure modes (FMs) for head and neck IMRT dose calculation and delivery are examined near commonly accepted tolerance criteria levels. Phantom treatment planning studies and dosimetry measurements (requiring decommissioning in several cases) are performed to determine the magnitude of dose delivery errors for the FMs (i.e., severity of the FM). Resultant quantitative severity scores are compared to FMEA scores obtained through an international survey and focus group studies. Results: Physical measurements for six FMs have resulted in significant PTV dose errors up to 4.3% as well as close to 1 mm significant distance-to-agreement error between PTV and OAR. Of the 129 survey responses, the vast majority of the responders used Varian machines with Pinnacle and Eclipse planning systems. The average years of experience was 17, yet familiarity with FMEA less than expected. Survey reports perception of dose delivery error magnitudemore » varies widely, in some cases 50% difference in dose delivery error expected amongst respondents. Substantial variance is also seen for all FMs in occurrence, detectability, and severity scores assigned with average variance values of 5.5, 4.6, and 2.2, respectively. Survey shows for MLC positional FM(2mm) average of 7.6% dose error expected (range 0–50%) compared to 2% error seen in measurement. Analysis of ranking in survey, treatment planning studies, and quantitative value comparison will be presented. Conclusion: Resultant quantitative severity scores will expand the utility of FMEA for radiotherapy and verify accuracy of FMEA results compared to highly variable subjective scores.« less

SU‐E‐T‐15: A Home Grown Process Improvement Tracking Tool

Purpose:To evaluate the use of a web based error tracking system.Methods:A web based error tracking system called the PIT (Process Improvement Tracking Tool) was used by all staff members to enter errors found at all parts of the treatment process in four satellite facilities. An error is defined as an action that did not fully comply with policy and procedure. Entries were made in real time by all staff members and require that the user enters the date of the incident, the patient medical record number, the physical location of the site, the process during which the error was found and categorize the incident type.Results:Number of errors is approximately proportional to the number of patient treatments. Errors: VBMC 45%, UROC 25%, FROC 13%, PHC 17%. Treatments: VBMC 38%, UROC 32%, FROC 16%, PHC 14%. Staff members making the most mistakes are therapists (36%), dosimetrists (23%) and physicists (15%). Processes where errors were found: physics weekly 25%, therapist weekly 18%, physics second check 15%, physics peer review 6%, therapists check 5%, treatment 5% and various other steps 26%.Conclusion:Proportionality of the number of patients treated to the number of errors implies homogeneity across four satellites. Differences between staff member error percentages need to be investigated more thoroughly, but possible explanations are workload and staffing levels. Step in the process where errors are found implies the need for improved processes upstream. Specifically, 5% errors discovered during treatment is considered very high and a specific attempt to reduce it will be undertaken. The vast majority of the errors discovered during treatment were found before the treatment was delivered. Only four were discovered after delivery and none involved significant dose errors. Further work is required to identify the exact procedures within the department that will achieve this goal.

Importance of Blood Glucose Meter and Carbohydrate Estimation Accuracy

The clinical significance of blood glucose meter (BGM) error in the presence of increasing carbohydrate errors in diabetes patients who use both the BGM result and the carbohydrate estimation to dose insulin is unknown. This Monte Carlo simulation modeled diabetes patients who calculate insulin dosages based on BGM results and carbohydrate estimations. It evaluated the likelihood of on-target insulin dosing and clinically significant insulin dose errors based on data from five BGMs with different levels of performance (expressed as bias and imprecision [coefficient of variation (%CV)]) and increasing levels of carbohydrate estimation errors. The study was performed across three separate preprandial glucose (PPG) ranges (90-150, 150-270, and 270-450 mg/dl). When carbohydrate estimation is accurate (%CV = 0%), the likelihood for on-target insulin doses ranged 50.1-98.5%. The likelihood depended on BGM performance and PPG range. In the presence of carbohydrate estimation errors (%CV = 5-20%), the likelihood of on-target insulin dosages markedly decreased (range, 27.2-80.1%) for all BGMs, the likelihood of insulin underdosing (range, 0-12.8%) and overdosing (range, 0-32.3%) increased, and the influence of BGM error on insulin dosing accuracy was blunted. Even in the presence of carbohydrate error, the BGM with the best performance (bias 1.35% and %CV = 4.84) had the highest probability for on-target insulin dosages. Both BGM and carbohydrate estimation error contribute to insulin dosing inaccuracies. The BGM with the best performance was associated with the most on-target insulin dosages.

Dofetilide Dose Calculation Errors in Elderly Associated with Use of the Modification of Diet in Renal Disease Equation

To report 2 cases of drug dosage calculation errors that occurred when the Modification of Diet in Renal Disease (MDRD) equation was used for initiating drug therapy with dofetilide in elderly patients with chronic kidney disease. An 83-year-old woman and a 92-year-old man were admitted for dofetilide treatment initiation and cardioversion for atrial fibrillation. The estimated glomerular filtration rate (eGFR) determined with use of the MDRD equation was significantly higher than the estimated creatinine clearance (eCrCl) determined with use of the Cockcroft-Gault equation for both cases (85 vs 43 mL/min for the man and 40 vs 24 mL/min for the woman). Initial dofetilide dosages calculated by the MDRD equation were 2-fold higher than those calculated by eCrCl in both cases. Initiation of dose based on the MDRD in the first patient led to a 32% increase in the QTc interval from baseline. Dofetilide therapy was adjusted for QTc interval prolongation based on eCrCl and reinitiated at a lower dose, and the patient did not develop further significant increases in the QTc interval. In the second patient, the lower dose based on eCrCl was initiated and the QTc interval remained within an acceptable range. The initial dosing of dofetilide is based on eCrCl as specified by the drug manufacturer. Recent widespread use and automated reporting of the eGFR by clinical laboratories has tempted some clinicians to consider using eGFR for calculating drug doses. However, recent data suggest that the eGFR, calculated by the MDRD equation, consistently overestimates eCrCl, leading to dose discrepancies, particularly in the elderly. The cases reported here illustrate the drug dose calculation errors that may occur when using the MDRD equation for initiating doses of dofetilide. Use of the eGFR or MDRD equation for calculation of doses in renal dysfunction has not been validated, and significant drug dose errors have been reported. The use of eGFR to calculate doses of dofetilide should be avoided.

Continuous quality improvement using intelligent infusion pump data analysis

The use of continuous quality-improvement (CQI) processes in the implementation of intelligent infusion pumps in a community teaching hospital is described. After the decision was made to implement intelligent i.v. infusion pumps in a 413-bed, community teaching hospital, drug libraries for use in the safety software had to be created. Before drug libraries could be created, it was necessary to determine the epidemiology of medication use in various clinical care areas. Standardization of medication administration was performed through the CQI process, using practical knowledge of clinicians at the bedside and evidence-based drug safety parameters in the scientific literature. Post-implementation, CQI allowed refinement of clinically important safety limits while minimizing inappropriate, meaningless soft limit alerts on a few select agents. Assigning individual clinical care areas (CCAs) to individual patient care units facilitated customization of drug libraries and identification of specific CCA compliance concerns. Between June 2007 and June 2008, there were seven library updates. These involved drug additions and deletions, customization of individual CCAs, and alterations of limits. Overall compliance with safety software use rose over time, from 33% in November 2006 to over 98% in December 2009. Many potentially clinically significant dosing errors were intercepted by the safety software, prompting edits by end users. Only 4-6% of soft limit alerts resulted in edits. Compliance rates for use of infusion pump safety software varied among CCAs over time. Education, auditing, and refinement of drug libraries led to improved compliance in most CCAs.

Recombinant human erythropoietin dosing errors due to concentrated EPO

Recombinant human erythropoietin (rHuEPO) is widely used to treat anemia. A dialysis provider enacted a policy utilizing 20,000 U/ml multi-dose vials for rHuEPO dosing. The purpose of this study was to determine the accuracy and precision in administering small rHuEPO doses from this vial. Ten registered nurses (RNs) were selected at random, supplied with a rHuEPO vial refilled with water, and instructed to withdraw the following amounts (1,200 U, 2,400 U, 3,600 U) using standard procedures and assuming the standard rHuEPO concentration of 20,000 U/ml. Samples were drawn up in duplicate and placed into 1.5 ml micro-centrifuge tubes. The volumes were measured using P-100 or P-200 microliter pipettes. The equivalent amount of rHuEPO was calculated by multiplying the volume by 20 U/microl. The rHuEPO dosing errors were large and on occasion greater than 100% at the 1,200 U dose. The variability for each RN, while large, was less than the inter-RN variability (within-RN % error 9.6% vs. 29.8% between-RN % error at the 1,200 unit dose). Errors occurred in both directions, both under- and overdosing. Utilizing concentrated rHuEPO resulted in significant dosing errors at low rHuEPO doses. The implications include inaccurate medication administration and disparity between administered and billed dosages. Policy decisions that effect medication administration need to be carefully evaluated to determine their impact on patient well-being and safety.

WE-E-AUD A-07: Should a Gaussian Probability Density Function Be Used to Approximate Respiration Induced Dosimetric Effects for Proton Radiotherapy?

Purpose: To compare the dose distributions generated by convolving a static dose distribution using a patient specific respiratory probability density function (R-PDF) with those generated using a generic Gaussian PDF (G-PDF) for proton therapy of lung cancer. Method and Materials: The R-PDFs were obtained by identifying the centroid motion of the targets from the 4D-CT scans of a phantom (CIRS Model 008 Dynamic Thorax) and a representative lung cancer patient. The CMS XiO® Treatment Planning System commissioned with 208 MeV nominal proton beam data from a passive scattering beam line at a proton therapy center was used for the static dose calculation. The dose convolution results from four different G-PDFs with standard deviations (SD) of 0.2, 0.3, 0.4, and 0.5 multiplying by the peak-to-peak motion amplitude (letter “A”, 1.60cm in the phantom and 1.75cm in the patient) were compared to the R-PDF convolved dose distributions using a commercial dosimetry analysis package (OmniPro I'mRT). Results: Respiration-induced dose error was 29% and 16% of the prescribed dose (PD) compared to the static doses in the phantom and patient, respectively. The G-PDF with SD of 0.4A most closely approximates the R-PDF whilst the maximum dose disagreements (MDDs) between the convolved doses using the two methods were 6% and 4% of the PD in phantom and patient, respectively. When G-PDFs with SD of 0.2A and 0.5A were used to approximate the R-PDF, the resulting MDDs were 19% and 12% in the phantom respectively, and 12% and 10% in the patient, respectively. When the G-PDF with SD of 0.3A was used to approximate the R-PDF, the resulting MDDs were 10% in the phantom and 8% in the patient. Conclusion: A Gaussian function should not be used to approximate a patient specific respiratory PDF since it can lead to clinically significant dose errors.

Hybrid computational phantoms of the 15-year male and female adolescent: applications to CT organ dosimetry for patients of variable morphometry.

Currently, two classes of the computational phantoms have been developed for dosimetry calculation: (1) stylized (or mathematical) and (2) voxel (or tomographic) phantoms describing human anatomy through mathematical surface equations and three-dimensional labeled voxel matrices, respectively. Mathematical surface equations in stylized phantoms provide flexibility in phantom design and alteration, but the resulting anatomical description is, in many cases, not very realistic. Voxel phantoms display far better anatomical realism, but they are limited in terms of their ability to alter organ shape, position, and depth, as well as body posture. A new class of computational phantoms--called hybrid phantoms-takes advantage of the best features of stylized and voxel phantoms-flexibility and anatomical realism, respectively. In the current study, hybrid computational phantoms representing reference 15-year male and female body anatomy and anthropometry are presented. For the male phantom, organ contours were extracted from the University of Florida (UF) 14-year series B male voxel phantom, while for the female phantom, original computed tomography (CT) data from two 14-year female patients were used. Polygon mesh models for the major organs and tissues were reconstructed for nonuniform rational B-spline (NURBS) surface modeling. The resulting NURBS/polygon mesh models representing body contour and internal anatomy were matched to anthropometric data and reference organ mass data provided by the Centers for Disease Control and Prevention (CDC) and the International Commission on Radiation Protection (ICRP), respectively. Finally, two hybrid 15-year male and female phantoms were completed where a total of eight anthropometric data categories were matched to standard values within 4% and organ masses matched to ICRP data within 1% with the exception of total skin. To highlight the flexibility of the hybrid phantoms, 10th and 90th weight percentile 15-year male and female phantoms were further developed from the 50th percentile phantoms through adjustments in the body contour to match the total body masses given in CDC pediatric growth curves. The resulting six NURBS phantoms, male and female phantoms representing their 10th, 50th, and 90th weight percentiles, were used to investigate the influence of body fat distributions on internal organ doses following CT imaging. The phantoms were exposed to multislice chest and abdomen helical CT scans, and in-field organ absorbed doses were calculated. The results demonstrated that the use of traditional stylized phantoms yielded organ dose estimates that deviate from those given by the UF reference hybrid phantoms by up to a factor of 2. The study also showed that use of reference, or 50th percentile, phantoms to assess organ doses in underweight 15-year-old children would not lead to significant organ dose errors (typically less than 10%). However, more significant errors were noted (up to approximately 30%) when reference phantoms are used to represent overweight children in CT imaging dosimetry. These errors are expected to only further increase as one considers CT organ doses in overweight and obese individuals of the adult patient population, thus emphasizing the advantages of patient-sculptable phantom technology.

SU-FF-T-210: Evaluation of CT Extended Field of View Imaging Impact On Radiation Therapy Treatment Planning

Purpose: CT imaging of patients for radiotherapy treatment planning frequently includes anatomy that extends beyond the 50cm nominal field-of-view (nFOV): 28.35% of the 575 CT scans we acquired from July–December 2006. The purpose of this study was to evaluate 1) the degradation of Hounsfield units (HU) in the extended field of view (eFOV), and 2) the dosimetric impact of ignoring or correcting this degradation within the treatment planning system (TPS). Method and Materials: CT images were acquired at maximum FOV (82cm) on a diamond-shaped 30×30×13cm solid-water phantom, challenging the reconstruction algorithm's ability to model the truncated projection data adequately. A unique dataset was acquired for 19 phantom locations (1cm intervals outside of the nFOV) and each imported into a TPS. Each phantom was contoured using a threshold of −200HU. A single treatment beam with isocenter placed at the center of the phantom was planned to deliver 100cGy to isocenter for each CT data set; dose map comparisons with and without homogenous correction for density (HU>−700 =1gm/cc) were performed relative to the control phantom within the nFOV. Results: Significant variation of HU was observed as a function of phantom displacement outside the nFOV (range +513 to −873HU); most dramatically ∼5cm beyond the nFOV border. If uncorrected, these changes in HU produced significant dose errors. Plans were compared to control plans and dose difference maps were generated; uncorrected images 3cm outside the nFOV demonstrated >5% difference, where overriding HU values above −700HU maintained <5% error for phantom positions ∼10cm beyond nFOV. Conclusion: HU values differ significantly for anatomy 2cm outside the nFOV, can be visualized and should be corrected for dose calculations. These results show <5% dose error can be accomplished for anatomy extending ∼10cm beyond the nFOV (which accounts for 98% of eFOV patients here).

Quantification of the impact of MLC modeling and tissue heterogeneities on dynamic IMRT dose calculations

This study quantifies the dose prediction errors (DPEs) in dynamic IMRT dose calculations resulting from (a) use of an intensity matrix to estimate the multi-leaf collimator (MLC) modulated photon fluence (DPE(IGfluence) instead of an explicit MLC particle transport, and (b) handling of tissue heterogeneities (DPE(hetero)) by superposition/convolution (SC) and pencil beam (PB) dose calculation algorithms. Monte Carlo (MC) computed doses are used as reference standards. Eighteen head-and-neck dynamic MLC IMRT treatment plans are investigated. DPEs are evaluated via comparing the dose received by 98% of the GTV (GTV D 98%), the CTV D 95%, the nodal D 90%, the cord and the brainstem D 02%, the parotid D 50%, the parotid mean dose (D (Mean)), and generalized equivalent uniform doses (gEUDs) for the above structures. For the MC-generated intensity grids, DPE(IGfluence) is within +/- 2.1% for all targets and critical structures. The SC algorithm DPE(hetero) is within +/- 3% for 98.3% of the indices tallied, and within +/- 3.4% for all of the tallied indices. The PB algorithm DPE(hetero) is within +/- 3% for 92% of the tallied indices. Statistical equivalence tests indicate that PB DPE(hetero) requires a +/- 3.6% interval to state equivalence with the MC standard, while the intervals are < 1.5% for SC DPE(hetero) and DPE(IGfluence). Overall, these results indicate that SC and MC IMRT dose calculations which use MC-derived intensity matrices for fluence prediction do not introduce significant dose errors compared with full Monte Carlo dose computations; however, PB algorithms may result in clinically significant dose deviations.