G Tang

Dose calculation for hypofractionated volumetric-modulated arc therapy: approximating continuous arc delivery and tongue-and-groove modeling*

Hypofractionated treatments generally increase the complexity of a treatment plan due to the more stringent constraints of normal tissues and target coverage. As a result, treatment plans contain more modulated MLC motions that may require extra efforts for accurate dose calculation. This study explores methods to minimize the differences between in‐house dose calculation and actual delivery of hypofractionated volumetric‐modulated arc therapy (VMAT), by focusing on arc approximation and tongue‐and‐groove (TG) modeling. For dose calculation, the continuous delivery arc is typically approximated by a series of static beams with an angular spacing of 2°. This causes significant error when there is large MLC movement from one beam to the next. While increasing the number of beams will minimize the dose error, calculation time will increase significantly. We propose a solution by inserting two additional apertures at each of the beam angle for dose calculation. These additional apertures were interpolated at two‐thirds’ degree before and after each beam. Effectively, there were a total of three MLC apertures at each beam angle, and the weighted average fluence from the three apertures was used for calculation. Because the number of beams was kept the same, calculation time was only increased by about 6%‐8%. For a lung plan, areas of high local dose differences (>4%) between film measurement and calculation with one aperture were significantly reduced in calculation with three apertures. Ion chamber measurement also showed similar results, where improvements were seen with calculations using additional apertures. Dose calculation accuracy was further improved for TG modeling by developing a sampling method for beam fluence matrix. Single element point sampling for fluence transmitted through MLC was used for our fluence matrix with 1 mm resolution. For Varian HDMLC, grid alignment can cause fluence sampling error. To correct this, transmission volume averaging was applied. For three paraspinal HDMLC cases, the average dose difference was greatly reduced in film and calculation comparisons with our new approach. The gamma (3%, 3 mm) pass rates have improved significantly from 74.1%, 90.0%, and 90.4% to 99.2%, 97.9%, and 97.3% for three cases, for calculation without volume averaging and calculation with volume averaging, respectively. Our results indicate that more accurate MLC leaf position and transmission sampling can improve accuracy and agreement between calculation and measurement, and are particularly important for hypofractionated VMAT that consists of large MLC movement. PACS number(s): 87.55.kd

Low-dose 2.5 MV cone-beam computed tomography with thick CsI flat-panel imager

Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high‐energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low‐dose megavoltage cone‐beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high‐efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone‐beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone‐beam and 6X megavoltage cone‐beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone‐beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone‐beam images is 2.4−3.4 MU compared to 26 MU in 6 MV cone‐beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone‐beam, the 2.5 MV imaging system does not suffer from high‐Z image artifacts. This can be very useful for treatment planning in cases where high‐Z prostheses are present. PACS number(s): 87.57.Q‐Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high-energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low-dose megavoltage cone-beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high-efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone-beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone-beam and 6X megavoltage cone-beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone-beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone-beam images is 2.4-3.4 MU compared to 26 MU in 6 MV cone-beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone-beam, the 2.5 MV imaging system does not suffer from high-Z image artifacts. This can be very useful for treatment planning in cases where high-Z prostheses are present. PACS number(s): 87.57.Q.

SU-F-T-173: One-Scan Protocol: Verifying the Delivery of Spot-Scanning Proton Beam

PURPOSE Radiochromic film for spot-scanning QA provides high spatial resolution and efficiency gains from one-shot irradiation for multiple depths. However, calibration can be a tedious procedure which may limit widespread use. Moreover, since there may be an energy dependence, which manifests as a depth dependence, this may require additional measurements for each patient. We present a one-scan protocol to simplify the procedure. METHODS We performed the calibration using an EBT3 film at depths of 18, 20, 24cm of Plastic Water exposed by a 6-level step-wedge plan on a Proteus Plus proton system (IBA, Belgium). The calibration doses ranged 65-250 cGy(RBE) for proton energies of 170-200MeV. A clinical prostate+nodes plan was used for validation. The planar doses at selected depths were measured with EBT3 films and analyzed using one-scan protocol (one-scan digitization of QA film and at least one film exposed to known dose). The Gamma passing rates, dose-difference maps, and profiles of 2D planar doses measured with EBT3 film, IBA MatriXX PT, versus TPS calculations were analyzed and compared. RESULTS The EBT3 film measurement results matched well with the TPS calculation data with an average passing rate of ∼95% for 2%/2mm and slightly lower passing rates were obtained from an ion chamber array detector. We were able to demonstrate that the use of a proton step-wedge provided clinically acceptable results and minimized variations between film-scanner orientation, inter-scan, and scanning conditions. Furthermore, it could be derived from no more than two films exposed to known doses (one could be zero) for rescaling the master calibration curve at each depth. CONCLUSION The use of a proton step-wedge for calibration of EBT3 film increases efficiency. The sensitivity of the calibration to depth variations has been explored. One-scan protocol results appear to be comparable to that of the ion chamber array detector. One author has a research grant from Ashland Inc., the manufacturer of the GafChromic film.

SU-F-T-226: QA Management for a Large Institution with Multiple Campuses for FMEA

PURPOSE To redesign our radiation therapy QA program with the goal to improve quality, efficiency, and consistency among a growing number of campuses at a large institution. METHODS A QA committee was established with at least one physicist representing each of our six campuses (22 linacs). Weekly meetings were scheduled to advise on and update current procedures, to review end-to-end and other test results, and to prepare composite reports for internal and external audits. QA procedures for treatment and imaging equipment were derived from TG Reports 142 and 66, practice guidelines, and feedback from ACR evaluations. The committee focused on reaching a consensus on a single QA program among all campuses using the same type of equipment and reference data. Since the recommendations for tolerances referenced to baseline data were subject to interpretation in some instances, the committee reviewed the characteristics of all machines and quantified any variations before choosing between treatment planning system (i.e. treatment planning system commissioning data that is representative for all machines) or machine-specific values (i.e. commissioning data of the individual machines) as baseline data. RESULTS The configured QA program will be followed strictly by all campuses. Inventory of available equipment has been compiled, and additional equipment acquisitions for the QA program are made as needed. Dosimetric characteristics are evaluated for all machines using the same methods to ensure consistency of beam data where possible. In most cases, baseline data refer to treatment planning system commissioning data but machine-specific values are used as reference where it is deemed appropriate. CONCLUSION With a uniform QA scheme, variations in QA procedures are kept to a minimum. With a centralized database, data collection and analysis are simplified. This program will facilitate uniformity in patient treatments and analysis of large amounts of QA data campus-wide, which will ultimately facilitate FMEA.

SU-G-TeP2-01: Can EPID Based Measurement Replace Traditional Daily Output QA On Megavoltage Linac?

PURPOSE To investigate the long term stability and viability of using EPID-based daily output QA via in-house and vendor driven protocol, to replace conventional QA tools and improve QA efficiency. METHODS Two Varian TrueBeam machines (TB1&TB2) equipped with electronic portal imaging devices (EPID) were employed in this study. Both machines were calibrated per TG-51 and used clinically since Oct 2014. Daily output measurement for 6/15 MV beams were obtained using SunNuclear DailyQA3 device as part of morning QA. In addition, in-house protocol was implemented for EPID output measurement (10×10 cm fields, 100 MU, 100cm SID, output defined over an ROI of 2×2 cm around central axis). Moreover, the Varian Machine Performance Check (MPC) was used on both machines to measure machine output. The EPID and DailyQA3 based measurements of the relative machine output were compared and cross-correlated with monthly machine output as measured by an A12 Exradin 0.65cc Ion Chamber (IC) serving as ground truth. The results were correlated using Pearson test. RESULTS The correlations among DailyQA3, in-house EPID and Varian MPC output measurements, with the IC for 6/15 MV were similar for TB1 (0.83-0.95) and TB2 (0.55-0.67). The machine output for the 6/15MV beams on both machines showed a similar trend, namely an increase over time as indicated by all measurements, requiring a machine recalibration after 6 months. This drift is due to a known issue with pressurized monitor chamber which tends to leak over time. MPC failed occasionally but passed when repeated. CONCLUSION The results indicate that the use of EPID for daily output measurements has the potential to become a viable and efficient tool for daily routine LINAC QA, thus eliminating weather (T,P) and human setup variability and increasing efficiency of the QA process.

SU‐E‐T‐328: Dosimetric Impact of Cobalt‐Chrome Stabilization Hardware in Paraspinal Radiation Therapy

Purpose: Due to saturation, high density materials Result in an apparent density of 3.2 g/cm3 in CT images. The true density of traditional titanium stabilization rods (∼4.4 g/cm3) is typically ignored in treatment planning. This may not be acceptable for new cobalt-chrome rods with a density of 8.5 g/cm3. This study reports the dosimetric impact of cobalt-chrome rods in paraspinal radiotherapy. Methods: For titanium and cobalt-chrome rods, two planning studies were done for both IMRT and VMAT in Varian Eclipse using AAA. 1) The effect of planning without assigning the true rod density was assessed by comparing plans generated with the apparent density and recalculated with the true density for titanium and cobalt-chrome. 2) To test if TPS can compensate for high density rods during optimization. Furthermore, TPS calculation accuracy was verified using MapCheck for a single 20 x 10 cm2 field. The MapCheck was incrementally shifted to achieve measurement resolution of 1 mm. Results: PTV coverage was ∼0.3% and ∼4.7% lower in plans that were recalculated with the true rod density of titanium and cobalt-chrome, respectively. PTV coverage can be maintained if the correct density is used in optimization. Measurements showed that TPS overestimated the dose locally by up to 11% for cobalt-chrome rods and up to 4% for titanium rods if the density is incorrect. With density corrected, maximum local differences of 6% and 3% were seen for cobalt-chrome and titanium rods, respectively. At 2 cm beneath a rod, electrons scattered from the side of the rod increased the lateral dose and diminished as depth increases. TPS was not able to account for this effect properly even with the true rod density assigned. Conclusion: Neglecting the true density of cobalt-chrome rods can cause under coverage to the PTV. Assigning the correct density during treatment planning can minimize unexpected decrease in PTV dose.

SU-E-T-440: Elliptical Source Model for IMRT and VMAT with Complex Modulation

PURPOSE To improve dose calculation accuracy for highly modulated fields in IMRT and VMAT using an elliptical source model. METHODS For 2 TrueBeam linacs equipped with Millennium and HD MLCs, the source model of 6 and 15 MV beams was optimized with adjustments of the focal spot dimensions in the Varian Eclipse TPS. Starting from the measured focal spot size of 1.75 × 1.75 mm, the y-dimension of the source was varied while the x-dimension was kept constant. In addition, all other modeling parameters such as the extrafocal source and dosimetric leaf gap remained unchanged. The different models were compared with measured penumbra for jaws and MLC using ion chamber and diode. For 2 IMRT and 2 VMAT cases, the calculated doses were compared with radiochromic film and EPID measurements. RESULTS A source size of 1.75 × 0.75 mm was found to be optimal for beam modeling. The calculated penumbra (80-20) of a MLCdefined open field was within 0.8 mm compare to measurement for both the inline and crossline profile for various field sizes and depths. The dose gradients in the direction perpendicular to the MLC travel direction was better modeled compared to the source model with a circular focal spot of 1.75 mm for the fields with rigorous MLC modulation. Local differences up to 8% were found in those regions. CONCLUSION The conventional source model using a symmetric focal spot tends to overestimate the penumbra perpendicular to the MLC travel direction, resulting in an inaccurate dose calculation especially for fields with complex MLC modulation. This can be improved with an elliptical source model and it can be considered as a standard as IMRT and VMAT treatments are becoming a norm for the clinic.

SU-E-T-52: Beam Data Comparison for 20 Linear Accelerators in One Network

PURPOSE To compare photon beam data for the 20 Varian linear accelerators (TrueBeam, iX, and EX models) in use at five centers in the same network with the intent to model with one set of beam data in Eclipsec. METHODS Varian linear accelerators, TrueBeam (3), 21 EX, iX, and Trilogy (14), and 6 EX (3), installed between 1999 and 2014 have their 6 MV and 15 MV x-ray beams reevaluated. Full commissioning, including output factors (St), percent depth doses (PDD), and off-axis profiles, was recently performed for a TrueBeam with a cc04 ion chamber in an IBA Blue phantom. Similarly, a subset of beam data for each of the other accelerators was measured recently as follows: for 3×3, 10×10, and 30×30 cm2 field sizes, flatness and penumbra (80-20%) were measured at dmax and 10 cm depths, PDD were measured at 10 and 20 cm depths, and St were measured at 5 cm depth. Measurement results for all machines were compared. RESULTS For 15 high-energy (6 and 15 MV) and 3 low-energy machines (6MV only): 1) PDD agreed within 1.4% at 10 and 20 cm depths; 2) penumbra agreed within 1.0 mm at dmax and 10 cm depths; 3) flatness was within 1.3% at dmax and 10 cm depths; and 4) with exception of the three low energy machines, output factors were within 1.1% and 0.5% for 3×3 and 30×30 cm2 , respectively. Measurement uncertainty, not quantified here, accounts for some of these differences. CONCLUSION Measured beam data from 15 high-energy Varian linacs are consistent enough that they can be classified using one beam data set in Eclipse. Two additional high-energy machines are removed from this group until their data are further confirmed. Three low-energy machines will be in a separate class based upon differences in output factors (St).

SU‐E‐T‐33: A Comparative Study of Commissioning Data of Two TrueBeam LINACs with TrueBeam Representative Beam Data

Purpose: To compare the commissioning water tank data of the two TrueBeam type linacs with TrueBeam Representative Beam Data. Methods: Three flattening filtered (FF), 6MV, 10MV, and 15MV, and two flattening filtered free (FFF), 6FFF and 10FFF, photon beams of one TrueBeam and one TrueBeam STX Linacs (Varian Medical Systems) are compared to the TrueBeam Representative Beam Data (TRBD) as provided by Varian. All measurements are scanned in Blue Phantom (IBA) with cc04 ion chamber at SSD 100cm at the highest dose rate available for each beam. The TRBD set is the average values of three TrueBeams at a single institution acquired by Iba cc13 ion chamber. To compare the beam quality, the PDD at d10 and d20 and the corresponding PDD 10/20 ratio at field sizes of 3×3, 10×10 and 30×30 cm2 are compared. The flatness of 10×10 and 30×30 cm2 open field profiles at depth of dmax, 10cm and 20cm are also compared. The output factors for square fields are also compared. Results: The PDD difference at both d10 and d20 between all FF and FFF beams and TRBD are within 0.6% and 0.9% respectively. The average PDD ratio difference between all beams and TRBD is within 0.5%. The flatness of all the FF and FFF beams are within 0.4% and 1.0% of TRBD respectively. The output factor differences between the two TrueBeams and RTBD for all beams and field sizes are within 0.8%. Conclusion: All the beam parameters of both the FF and FFF beams of the two Truebeams are within 1% of TRBD. Research grant from Varian Medical Systems

SU‐E‐T‐71: An Investigation of Measurement Techniques for Small Field Dosimetry Using Commercially Available Detectors

PURPOSE To identify and quantify the pros and cons of currently available commercial detectors and measurement techniques for small fields. METHODS Measurements are made for a TrueBeam STX (Varian Medical System) for 6X, 6XFFF, and 15XFFF beams. Four ion chambers, Iba cc01, cc04, cc13, and Exradin A16, and two diodes, Iba stereotactic diode (SD) and photon diode (PD), are used with an IBA blue phantom. For ion chambers, the recombination and polarity effects are evaluated based on the PDDs, while the out-of field doses are compared with the diode measurements. The effects of finite detector size on penumbra are compared for field sizes from 3×3 to 10×10, while PDD are compared down to 1x1 cm2 . RESULTS Recombination affects the PDDs by up-to 0.5% and 3.1% at deeper depths for cc04 and cc13, respectively. Polarity effects for cc04 and cc13 are 2.0% and 0.3%, respectively for 40x40 cm2, while both cc01 and cc04 are within 0.5% for 10x10 cm2 .The 3x3 and 5x5 cm2 PDD of cc04, cc01, A16 and SD are within 1% of each other but cc01, A16 and diodes over-respond by 1- 2% for 10×10 cm2 compared to cc04. At 1×1 cm2, A16, cc01 and PD remains within 1% of each other while cc04 and SD show 1.3% over-respond and 3.0% under-respond at d20 relative to cc01. Relative to cc04, the relative out-of-field dose difference for PD, SD, cc01 and A16 are -0.4%, +0.8%, +0.5% and +0.3% respectively at dmax and 10×10 cm2. The penumbras measured by all the small detectors are between 0.6 to 1.0 mm sharper than cc04. CONCLUSION No single detector is accurate over the entire range of field size. The effects of detector size, recombination and polarity effects, and sensitivity to scatter can be minimized by appropriate mixes of detector and measurement techniques. Research grant from Varian Medical Systems.