L. Hong

Application of AAPM TG 119 to volumetric arc therapy (VMAT)

The purpose of this study was to create AAPM TG 119 benchmark plans for volumetric arc therapy (VMAT) and to compare VMAT plans with IMRT plan data. AAPM TG 119 proposes a set of test clinical cases for testing the accuracy of IMRT planning and delivery system. For these test cases, we generated two treatment plans, the first plan using 7–9 static dMLC IMRT fields and a second plan utilizing one‐ or two‐arc VMAT technique. Dose optimization and calculations performed using 6 MV photons and Eclipse treatment planning system. Dose prescription and planning objectives were set according to the TG 119 goals. Plans were scored based on TG 119 planning objectives. Treatment plans were compared using conformity index (CI) for reference dose and homogeneity index (HI) (for D5‐D95). F or test cases prostate, head‐and‐neck, C‐shape and multitarget prescription dose are 75.6 Gy, 50.4 Gy, 50 Gy and 50 Gy, respectively. VMAT dose distributions were comparable to dMLC IMRT plans. Our planning results matched TG 119 planning results. For treatment plans studied, conformity indices ranged from 1.05–1.23 (IMRT) and 1.04–1.23 (VMAT). Homogeneity indices ranged from 4.6%–11.0% (IMRT) and 4.6%–10.5% (VMAT). The ratio of total monitor units necessary for dMLC IMRT to that of VMAT was in the range of 1.1–2.0. AAPM TG 119 test cases are useful to generate VMAT benchmark plans. At preclinical implementation stage, plan comparison of VMAT and IMRT plans of AAPM TG 119 test case allowed us to understand basic capabilities of VMAT technique. PACS number: 87.55.Qr

A method incorporating 4DCT data for evaluating the dosimetric effects of respiratory motion in single-arc IMAT.

This study introduces a method incorporating 4DCT data to determine the impact of respiratory motion in single-arc intensity-modulated arc therapy (IMAT). Simulation was done by re-warping the static dose distribution of all phases of a 4DCT image set with a 3D deformation map to reference CT images at end-inspiration and end-expiration. To calculate the dose received during respiration under IMAT, the control points were interpolated and re-distributed into separate IMAT plans corresponding to each respiratory phase. This study also investigated the role that plan complexity may play in the dosimetric impact of the respiratory motion in the delivery of IMAT. The dosimetric impact of organ motion was evaluated by analyzing the degradation of D(95,) D(50) and D(05) of the CTV and PTV. From the results shown for the patients in this study who had maximum organ motion displacement approximately 15 mm, the dosimetric impact is rather small. Therefore, our preliminary results suggest that respiratory motion of less than 1.5 cm may be ignored for both moderately and highly modulated IMAT, irrespective of the number of fractions. Specifically, highly modulated plans only increased the degradation of D(95) of the DVH curves for a single fraction by 2% in the CTV and 9% in the PTV compared to the expected value of the multi-fraction plan.

Biological impact of geometric uncertainties: what margin is needed for intra-hepatic tumors?

BackgroundTo evaluate and compare the biological impact on different proposed margin recipes for the same geometric uncertainties for intra-hepatic tumors with different tumor cell types or clinical stages.MethodThree different margin recipes based on tumor motion were applied to sixteen IMRT plans with a total of twenty two intra-hepatic tumors. One recipe used the full amplitude of motion measured from patients to generate margins. A second used 70% of the full amplitude of motion, while the third had no margin for motion. The biological effects of geometric uncertainty in these three situations were evaluated with Equivalent Uniform Doses (EUD) for various survival fractions at 2 Gy (SF2).ResultsThere was no significant difference in the biological impact between the full motion margin and the 70% motion margin. Also, there was no significant difference between different tumor cell types. When the margin for motion was eliminated, the difference of the biological impact was significant among different cell types due to geometric uncertainties. Elimination of the motion margin requires dose escalation to compensate for the biological dose reduction due to the geometric misses during treatment.ConclusionsBoth patient-based margins of full motion and of 70% motion are sufficient to prevent serious dosimetric error. Clinical implementation of margin reduction should consider the tumor sensitivity to radiation.

SU‐FF‐T‐288: Initial Experience with a Commercial Monte Carlo Electron Treatment Planning System

Introduction:Monte Carlo modeling of clinical electron beams has the potential to substantially improve accuracy and quality of treatment planning, as excessive compute time and lack of commercial availability has hindered its application. Here we present a preliminary evaluation of a commercial electron Monte Carlo algorithm.Methods and Materials: Percent depth dose and profiles of 6‐20 MeV electrons and 6×6 – 25×25 cm cones were measured in a water tank at 100 cm SSD using a Farmer chamber for electrons. Absolute output was measured at 110 and 100 cm SSDs. Outputs and distributions of two extreme test cases were measured: a 2.1 cm × 3 cm insert in a 6×6 cone and a 2.8 cm × 15.7 cm long slit on a 25×25 cone. The algorithms ability to accurately model relative and absolute dose of an obliquely (30°) oriented beam was evaluated ionization chamber measurements. Clinical cases with were checked using Mosfet dosimeters in vivo. Monte Carlo calculations were performed with a 2 mm grid, and smoothing filters provided with the algorithm were applied to minimize noise in the data. Results: Agreement of 2% of measured and modeled doses was found over the evaluated range of energies, cones, obliquities and SSDs. Compute times of 1–5 minutes were a function of increasing field size. Visual comparison of the shapes of the profiles was in agreement with measurement. Profiles of the eccentric geometry test cases appeared to be to be physically unrealistic (e.g. an inverted V) in the smaller dimension suggesting that the beam model was not valid. Conclusions: The Monte Carlo electron algorithm provides accurate distributions for most clinical cases. For extreme cases, measurements should be made to test the accuracy of the system, and further development of the algorithm is recommended.

SU‐E‐J‐135: Measurements of Non‐Linearity Features of Breathing Patterns Using Recurrence Quantification Analysis (RQA) and Dynamic Complexity (DC)

Purpose: To investigate if there exists a difference in breathing patterns between patients treated with SBRT and IMRT using RQA and DC (using K2: correlation entropy and D2: correlation dimension) measures. Methods: 9 patients treated with SBRT and 8 treated with IMRT were scanned with 4D CT and the breathing patterns were acquired. One of the SBRT patients was scanned with and without meditation. Each breathing signal consisted of a scalar time series and recurrence quantification analysis (RQA) was utilized to determine the following measures: Periodicity of the system as percentage of recurrence points (%RR), determinism (DET), maximum diagonal line length (L_max) whose inverse the divergence (DIV) is measure for how fast trajectories diverge from each other, the average diagonal line length (L) that can be interpreted as the mean prediction time of the signal, and the entropy (ENTR) a measure of information complexity. In addition the invariant measures of K2 and D2 were also estimated. A locally nonlinear forecast was applied to predict future breathing signals of N time step ahead for the patient with and without meditation. Results: Our results showed %RR has significant correlation with L_max and has inverse correlation with DIV. DET has significant correlation with Lmax, L, ENTR and DIV. Independent t test suggests there is no difference between the SBRT and IMRT groups in terms of the RQA measures and K2. Patient that had undergone meditation showed improvement in %RR, L_max, DIV, K2 and had an estimated correlation dimension of 1.7. Prediction showed similar results for one and three time step ahead but meditation one had better prediction horizon when time step was higher. Conclusion: RQA is a powerful tool that allows one to analyze the dynamic nature of breathing pattern. No significant difference was found in the dynamical complexity of SBRT and IMRT patients.

SU‐E‐T‐172: Portal Dosimetry of Gated VMAT; with and Without Gating

PURPOSEnTo study duty cycle and respiratory period influence on VMAT delivery using portal dosimetry.nnnMETHODSnFor this study, we selected 12 ARC fields from four different gated VMAT plans with Varian, TrueBeam 6 MV photon beam and HD120 MLC. For Planning and portal dose prediction we used Varian, Eclipse (Version 10.0) AAA and PDIP algorithms respectively. We acquired integrated portal images with and without gated arc fields delivered to EPID at SID 140cm. We simulated respiratory periods with Quasar, Motion Phantom. For gated delivery, we used planned duty cycle for patient respiratory trace, and 6 sec per breath (SPB) and duty cycles 40%, 30%, 20% and 10% with 3SPB. Eclipse, Portal dosimetry software was used for Gamma analyses of measured and predicted portal dose images. In addition, we compared with and without gated portal dose images of same arc. MLC log file analyses of delivered arcs were done with Mobius, Dose lab software. Gamma passing criteria is to have gamma<1 for greater than 90% data points.nnnRESULTSnFor all arcs, predicted vs. measured dose distributions passed with criteria ΔD = 3% & DTA = 3mm, excluding few 10% duty cycle measurements. For these, pass criteria is ΔD = 5% & DTA = 4mm. Similarly, with and without gated portal dose image comparison passed the criteria of ΔD = 1% & DTA = 1mm excluding few 10% duty cycle measurements. For these pass criteria is ΔD = 2% & DTA = 2mm. For all delivered arcs, Leaf error 95 percentile is < 0. 1mm and error RMS is below 0.05mmConclusion: Except 10 % duty cycle, the gating influence on VMAT delivery is minimal. Therefore, gated VMAT QA can be done without gating. Duty cycles larger than 20% are recommended to minimize delivery errors.

SU‐E‐T‐604: 16‐MV Photon Beams Do Not Improve Plan Quality Compared to 6‐MV Photon Beams in Prostate Cancer IMRT

PURPOSEnPhoton energies 10 MV or higher are generally considered optimal for treatment of deep-seated pelvic targets. We performed dosimetric quality assessment of Prostate IMRT plans in patients treated with 6-MV and 16-MV Photons, to discern if 16-MV plan quality was superior to 6-MV treatment plans.nnnMETHODSnFrom our institutional database, treatment plans of 84 patients previously treated for early stage prostate cancers were included in this retrospective study. Forty-two patients were treated with 6-MV and forty-two with 16-MV. Beam energy choice was based on linac capability, physician preference and not on patient separation. All patients were planned with a coplanar 7-F IMRT technique. The prescription dose (75.6-Gy), optimization technique and planning objectives were similar in all patients. Dose distributions were evaluated using various indices-Conformity-Index (CI), Healthy-Tissue Conformity Index (HTCI), Homogeneity-Index (HI), Gradient-Index (GI), Conformity-Number (CN), Normal-Tissue Integral Dose (NTID), Body-mass-index (BMI) and quality of coverage (QC). Rectal and Bladder dose-volume indices were evaluated per RTOG guidelines. Non-parametric Mann-Whitney test was applied in the statistical analysis and for a p-value <0.05, the null hypothesis is rejected.nnnRESULTSnMean PTV was 197.9cc (±13.1) for the 6-MV group and 191.8cc (±10.3) for the 16-MV group. MUs per fraction were 905 (±32) for 6-MV and 862 (±41) for 16-MV plans. The CI, HTCI and GI were statistically similar between 6-MV and 16-MV plans (p=0.22). Indices HI, QC and CN all showed statistically significant improvement for 6-MV plans compared to 16-MV plans (p<0.03). NTID was slightly lower for 16-MV plans, but not statistically significant, compared to 6-MV plans. NTID correlated with BMI for 16-MV group (r=0.70) and weakly for 6-MV group (r=0.28). Rectal V65, V40 and Bladder V65 were similar between 6-MV and 16-MV plans.nnnCONCLUSIONnWe conclude that 16-MV photon beams do not provide additional dosimetric advantage compared to 6-MV photon beams in Prostate IMRT.

Multi-scale Regularization Approaches of Non-parametric Deformable Registrations

Most deformation algorithms use a single-value smoother during optimization. We investigate multi-scale regularizations (smoothers) during the multi-resolution iteration of two non-parametric deformable registrations (demons and diffeomorphic algorithms) and compare them to a conventional single-value smoother. Our results show that as smoothers increase, their convergence rate decreases; however, smaller smoothers also have a large negative value of the Jacobian determinant suggesting that the one-to-one mapping has been lost; i.e., image morphology is not preserved. A better one-to-one mapping of the multi-scale scheme has also been established by the residual vector field measures. In the demons method, the multi-scale smoother calculates faster than the large single-value smoother (Gaussian kernel width larger than 0.5) and is equivalent to the smallest single-value smoother (Gaussian kernel width equals to 0.5 in this study). For the diffeomorphic algorithm, since our multi-scale smoothers were implemented at the deformation field and the update field, calculation times are longer. For the deformed images in this study, the similarity measured by mean square error, normal correlation, and visual comparisons show that the multi-scale implementation has better results than large single-value smoothers, and better or equivalent for smallest single-value smoother. Between the two deformable registrations, diffeormophic method constructs better coherence space of the deformation field while the deformation is large between images.

SU‐FF‐T‐647: RapidArc Versus DMLC IMRT: Planning Comparison Utilizing AAPM TG119 Guidelines

Purpose: To compare treatment plans produced by RapidArc® and dMLC IMRT techniques for test cases proposed in AAPM TG119 report. Materials and Methods: AAPM‐TG119 proposes a set of mock clinical cases for testing the accuracy of IMRT planning and delivery system. For these mock cases, we generated two treatment plans, the first plan using 7–9 static IMRT fields and a second plan utilizing 1 or 2 arc RapidArc® technique. Dose optimization and calculations were performed using 6 MV photons and Eclipse® treatment planning system (Varian Medical Systems). Dose prescription and planning objectives were set according to the TG119 goals. Plans were scored based on TG119 planning objectives. Treatment plans were compared using‐ Conformity Index (CI) for reference isodose, Homogeneity Index (D5‐D95), dose gradient (mean radius difference between V50 and V100 of total volume), Normal Tissue Integral‐dose (NTID) and total MU. Results: RapidArc® dose distributions were comparable to dMLC IMRT plans. Our planning results matched published TG119 planning results. For treatment plans studied, conformity indices were ranged from 1.05 – 1.10 (IMRT) and 1.04 – 1.09 (RapidArc®) respectively. Homogeneity indices ranged from 4.6 – 11.0% (IMRT) and 4.6 – 10.5% (RapidArc®) respectively. For IMRT plans, the dose gradient measure and NTID ranges were 1.5 – 2.5 cm and 8.6 – 17.7 respectively. In case of RapidArc® plans, the gradient measure and NTID ranges were 1.4–2.5cm and 8.5–16.8 respectively. The ratio of total Monitor Units necessary for dMLC IMRT to that of RapidArc® was in the range 1.1–2.0 Conclusion: RapidArc® treatment plans were similar to dose distributions achieved by 7‐ or 9‐ field dynamic‐IMRT plans. The advantage of RapidArc® technique lies in the decreased number of total monitor units necessary which can further lead to reduction in patient “beam‐on” time and out of field scatter doses.