B Lynch

Important considerations for radiochromic film dosimetry with flatbed CCD scanners and EBT GAFCHROMIC® film

In this study, we present three significant artifacts that have the potential to negatively impact the accuracy and precision of film dosimetry measurements made using GAFCHROMIC® EBT radiochromic film when read out with CCD flatbed scanners. Films were scanned using three commonly employed instruments: a Macbeth TD932 spot densitometer, an Epson Expression 1680 CCD array scanner, and a Microtek ScanMaker i900 CCD array scanner. For the two scanners we assessed the variation in optical density (OD) of GAFCHROMIC EBT film with scanning bed position, angular rotation of the film with respect to the scan line direction, and temperature inside the scanner due to repeated scanning. Scanning uniform radiochromic films demonstrated a distinct bowing effect in profiles in the direction of the CCD array with a nonuniformity of up to 17%. Profiles along a direction orthogonal to the CCD array demonstrated a 7% variation. A strong angular dependence was found in measurements made with the flatbed scanners; the effect could not be reproduced with the spot densitometer. An IMRT quality assurance film was scanned twice rotating the film 90° between the scans. For films scanned on the Epson scanner, up to 12% variation was observed in unirradiated EBT films rotated between 0° and 90°, which decreased to approximately 8% for EBT films irradiated to 300cGy. Variations of up to 80% were observed for films scanned with the Microtek scanner. The scanners were found to significantly increase the film temperature with repeated scanning. Film temperature between 18 and 33°C caused OD changes of approximately 7%. Considering these effects, we recommend adherence to a strict scanning protocol that includes: maintaining the orientation of films scanned on flatbed scanners, limiting scanning to the central portion of the scanner bed, and limiting the number of consecutive scans to minimize changes in OD caused by film heating.

Comparative analysis of 60Co intensity-modulated radiation therapy

In this study, we perform a scientific comparative analysis of using (60)Co beams in intensity-modulated radiation therapy (IMRT). In particular, we evaluate the treatment plan quality obtained with (i) 6 MV, 18 MV and (60)Co IMRT; (ii) different numbers of static multileaf collimator (MLC) delivered (60)Co beams and (iii) a helical tomotherapy (60)Co beam geometry. We employ a convex fluence map optimization (FMO) model, which allows for the comparison of plan quality between different beam energies and configurations for a given case. A total of 25 clinical patient cases that each contain volumetric CT studies, primary and secondary delineated targets, and contoured structures were studied: 5 head-and-neck (H&N), 5 prostate, 5 central nervous system (CNS), 5 breast and 5 lung cases. The DICOM plan data were anonymized and exported to the University of Florida optimized radiation therapy (UFORT) treatment planning system. The FMO problem was solved for each case for 5-71 equidistant beams as well as a helical geometry for H&N, prostate, CNS and lung cases, and for 3-7 equidistant beams in the upper hemisphere for breast cases, all with 6 MV, 18 MV and (60)Co dose models. In all cases, 95% of the target volumes received at least the prescribed dose with clinical sparing criteria for critical organs being met for all structures that were not wholly or partially contained within the target volume. Improvements in critical organ sparing were found with an increasing number of equidistant (60)Co beams, yet were marginal above 9 beams for H&N, prostate, CNS and lung. Breast cases produced similar plans for 3-7 beams. A helical (60)Co beam geometry achieved similar plan quality as static plans with 11 equidistant (60)Co beams. Furthermore, 18 MV plans were initially found not to provide the same target coverage as 6 MV and (60)Co plans; however, adjusting the trade-offs in the optimization model allowed equivalent target coverage for 18 MV. For plans with comparable target coverage, critical structure sparing was best achieved with 6 MV beams followed closely by (60)Co beams, with 18 MV beams requiring significantly increased dose to critical structures. In this paper, we report in detail on a representative set of results from these experiments. The results of the investigation demonstrate the potential for IMRT radiotherapy employing commercially available (60)Co sources and a double-focused MLC. Increasing the number of equidistant beams beyond 9 was not observed to significantly improve target coverage or critical organ sparing and static plans were found to produce comparable plans to those obtained using a helical tomotherapy treatment delivery when optimized using the same well-tuned convex FMO model. While previous studies have shown that 18 MV plans are equivalent to 6 MV for prostate IMRT, we found that the 18 MV beams actually required more fluence to provide similar quality target coverage.

High-precision GAFCHROMIC EBT film-based absolute clinical dosimetry using a standard flatbed scanner without the use of a scanner non-uniformity correction

To report a study of the use of GAFCHROMIC EBT radiochromic film (RCF) digitized with a commercially available flatbed document scanner for accurate and reliable all‐purpose two‐dimensional (2D) absolute dosimetry within a clinical environment. We used a simplified methodology that yields high‐precision dosimetry measurements without significant postirradiation correction. The Epson Expression 1680 Professional scanner and the Epson Expression 10000XL scanner were used to digitize the films. Both scanners were retrofitted with light‐diffusing glass to minimize the effects of Newton rings. Known doses were delivered to calibration films. Flat and wedge fields were irradiated with variable depth of solid water and 5 cm back scatter solid water. No particular scanner nonuniformity effect corrections or significant post‐scan image processing were carried out. The profiles were compared with CC04 ionization chamber profiles. The depth dose distribution was measured at a source‐to‐surface distance (SSD) of 100 cm with a field size of 10×10 cm2. Additionally, 22 IMRT fields were measured and evaluated using gamma index analysis. The overall accuracy of RCF with respect to CC04 was found to be 2%–4%. The overall accuracy of RCF was determined using the absolute mean of difference for all flat and wedge field profiles. For clinical IMRT fields, both scanners showed an overall gamma index passing rate greater than 90%. This work demonstrated that EBT films, in conjunction with a commercially available flatbed scanner, can be used as an accurate and precise absolute dosimeter. Both scanners showed that no significant scanner nonuniformity correction is necessary for accurate absolute dosimetry using the EBT films for field sizes smaller than or equal to 15×15 cm2. PACS number: 87.53.Bn

A high-speed scintillation-based electronic portal imaging device to quantitatively characterize IMRT delivery

We have developed an electronic portal imaging device (EPID) employing a fast scintillator and a high-speed camera. The device is designed to accurately and independently characterize the fluence delivered by a linear accelerator during intensity modulated radiation therapy (IMRT) with either step-and-shoot or dynamic multileaf collimator (MLC) delivery. Our aim is to accurately obtain the beam shape and fluence of all segments delivered during IMRT, in order to study the nature of discrepancies between the plan and the delivered doses. A commercial high-speed camera was combined with a terbium-doped gadolinium-oxy-sulfide (Gd2O2S:Tb) scintillator to form an EPID for the unaliased capture of two-dimensional fluence distributions of each beam in an IMRT delivery. The high speed EPID was synchronized to the accelerator pulse-forming network and gated to capture every possible pulse emitted from the accelerator, with an approximate frame rate of 360 frames-per-second (fps). A 62-segment beam from a head-and-neck IMRT treatment plan requiring 68 s to deliver was recorded with our high speed EPID producing approximately 6 Gbytes of imaging data. The EPID data were compared with the MLC instruction files and the MLC controller log files. The frames were binned to provide a frame rate of 72 fps with a signal-to-noise ratio that was sufficient to resolve leaf positions and segment fluence. The fractional fluence from the log files and EPID data agreed well. An ambiguity in the motion of the MLC during beam on was resolved. The log files reported leaf motions at the end of 33 of the 42 segments, while the EPID observed leaf motions in only 7 of the 42 segments. The static IMRT segment shapes observed by the high speed EPID were in good agreement with the shapes reported in the log files. The leaf motions observed during beam-on for step-and-shoot delivery were not temporally resolved by the log files.

Quantification of planning target volume margin when using a robotic radiosurgery system to treat lung tumors with spine tracking

PURPOSE The use of fiducial markers or direct tumor visualization allows for tumor tracking and ultimately smaller planning target volume (PTV) margins in the treatment of lung tumors, yet many patients are either not amenable to fiducial marker placement or their tumors are unable to be visualized on orthogonal-axis x-ray images. Spine tracking is an alternative method for tumor localization but is limited by the assumption that the location of the lung tumor relative to the spine is constant. The purpose of this study was to quantify the additional PTV margin needed when spine tracking is used to ensure the internal target volume (ITV) receives the prescription dose during treatment. METHODS AND MATERIALS Daily cone beam computed tomography images, registered based on tumor position, from 63 patients with lung cancer treated with stereotactic body radiation therapy were collected and analyzed. Rigid registrations were reperformed so that the position of the spine on the cone beam computed tomography image was aligned to its position on the planning computed tomography. Shifts from the treatment position to the new position were recorded, and per-patient mean shifts and standard deviations were calculated, as well as group systematic and random standard deviations. These data were used with van Herks margin recipe to determine the additional margin required to adequately treat the patient population if spine tracking were used instead of direct daily tumor imaging. A retrospective dosimetric analysis was also performed on 6 patients with lung cancer previously treated by CyberKnife using spine tracking to determine the potential decrease in target coverage attributable to insufficient margin on the ITV. This analysis was performed by shifting the PTV volume relative to the CyberKnife treatment geometry to simulate a setup error caused by tracking the spine as opposed to the tumor. RESULTS The additional margins calculated by van Herks margin recipe to adequately cover the ITV with the 95% isodose surface for 90% of the entire patient population in the vertical, longitudinal, and lateral directions were 6.4, 6.0, and 4.5 mm, respectively. The retrospective analysis showed a decrease in PTV coverage from 95.6% to 93.1% and an increase in new conformity index by 2.7% when the average shift data were used to simulate setup error. When the maximum shift data were used to simulate the worst possible outcome, PTV coverage decreased to 73.4% and the new conformity index increased by 26.8%. CONCLUSIONS Standard margins of 5 mm on the ITV for patients with lung cancer being treated with stereotactic body radiation therapy are insufficient and may result in geographic misses of the tumor when spine tracking is used to locate the position of the tumor in the lung. Therefore, we recommend the addition of 5-mm margins in all directions for a total of 10 mm to take into account the change in position of the tumor relative to the spine from the time of simulation to treatment.

Percent depth-dose distribution discrepancies from very small volume ion chambers.

As very small ion chambers become commercially available, medical physicists may be inclined to use them during the linear accelerator commissioning process to better characterize the beam in steep dose gradient areas. For this work, a total of eight different ion chambers (volumes from 0.007 cc to 0.6 cc) and four different scanning systems were used to scan PDDs at both +300V and −300V biases. We observed a reproducible, significant difference (overresponse with depth) in PDDs acquired when using very small ion chambers, with specific bias/water tank combinations — up to 5% at a depth of 25 cm in water. This difference was not observed when the PDDs were sampled using the ion chamber in static positions in conjunction with an external electrometer. This suggests noise/signal interference produced by the controller box and cable system assemblies, which can become relatively significant for the very small current signals collected by very small ion chambers, especially at depth as the signal level is even further reduced. Based on the results observed here, the use of very small active volume chambers under specific scanning conditions may lead to collection of erroneous data, introducing systematic errors into the treatment planning system. In case the use of such a chamber is required, we recommend determining whether such erroneous effect exists by comparing the scans with those obtained with a larger chamber. PACS numbers: 87.56.bd, 87.56.Fc, 87.56.Da

Imaging linear accelerator output using a high-speed scintillation based electronic portal imaging device (Hi-EPID)

We have developed a high-speed scintillation based electronic portal imaging device (Hi-EPID) to image the fluence output of a linear accelerator as it exits the head. The Hi-EPID consists of a high speed digital camera coupled with a fast terbium-doped gadolinium-oxy-sulfide (Gd2O2S:Tb) scintillator. The camera can record every single radiation pulse out of the accelerator, at up to 500 frames per second. Since the Hi-EPID allows us to simultaneously obtain both the fluence and the field shape in a delivery, it serves as an independent means of verifying treatment delivery. In this study, we used the Hi-EPID to analyze the characteristics of the fluence delivered from a commercial clinical linear accelerator (Synergy® S, Elekta). Both open field and multileaf collimator (MLC) based intensity modulated radiation therapy (IMRT) fields were evaluated. Fluence delivery irregularities were observed with the Hi-EPID device. Fluence was not constant over the duration of a segment, resulting in a varying dose rate. For the open field delivery, the fluence increased with time until it reached a plateau (in approximately 100 ms), remaining constant over the remainder of the delivery. However, in the case of a six segment IMRT field delivery, the fluence variation differed between segments. For some segments, the fluence increased in the beginning and then reached a plateau. For others, the fluence rose and fell sharply, followed by a slower rise to the plateau. The rapid fluence variation before a plateau was as high as 50% of the maximum. Though the total fluence was correctly delivered, this result is intriguing and calls for further investigation. An analysis of field shapes showed no unplanned motion of MLC during step and shoot delivery, as has been reported for another clinical linear accelerator.

SU‐FF‐T‐311: LINAC Dosimetry: Benchmark Data Set Uncertainty

Purpose: Determine sources of error in the collection of a Benchmark Data set for LINACdosimetry and provide methods of error correction that will ensure the highest possible accuracy of the dosimetric data. Method and Materials: Guided by the measurement requirements for the Benchmark Datasets, the sources of experimental error can be divided into 3 sources: 1) discretization and volume averaging errors; 2) Stochastic errors; and 3) Systematic or artifactual errors. Measurements will be made in a 3D water phantom scanning system and in a water‐equivalent solid phantom that will allow the insertion of heterogeneous components. Results: We present theoreticalanalyses of the expected errors associated with ion chamber, radiochromic film, and diode measurements and provide specific techniques that will enable high spatial resolution of dose gradients resulting from beam limiters and dose perturbations in and around heterogeneity interfaces, such as air/tissue, lung/tissue, bone/tissue. These techniques include de‐convolution of chamber response and a 3D correction matrix of a film scanner. Conclusion:Theoretical limits of spatial resolution in LINACdosimetry are achievable and demonstrable using dosimetric tools available today. These tools and methods will be used to collect accurate Benchmark Dosimetry data for future research in Monte Carlo techniques for treatment planning as well as automated software tools that will enable systematic QA of modern treatment planning systems. Conflict of Interest: This project has been funded in part with Federal funds from the National Cancer Institute, National Institutes of Health, Department of Health and Human Services, under Contract No, HHSN261200522014C, NCI grants R01‐CA‐100636, and by Sun Nuclear Corporation.

TU‐C‐224A‐08: A Scientific Comparison of Inverse Treatment Plan Quality Using a Convex Non‐Linear Programming Model as a Function of Beam Quality and Beam Number

Purpose: Recent advances in large scale fluence map optimizations for IMRT allow the use of large beam numbers that conform to the target to generate the desired target coverage while at the same time maintaining dose to critical organs below tolerance limits. Additionally, IMRT has effectively removed the need for high energy accelerator beams due to the excellent plan quality achievable with low beam quality. We investigate the diminishing returns in plan quality with increasing beam numbers and compare IMRTtreatment planning of 6MV and 60Co therapy dose models.Method and Materials: A convex non‐linear model was used to compare the plan quality, from dose volume histograms and fluence maps, for three treatment sites (H & N, CNS and prostate) for a 6MV and 60Co dose model. Plans were calculated for 5, 7, 9, 11, 17, 35 and 71 equidistant beam angles and quality assessed on target coverage (R95% > RRx) and organ sparing for each case. Results: Similar target coverage was achieved for 60Co as with 6MV and equivalent organ sparing was also observed for all three sites. Increasing the number of beams provided some improvement in organ sparing while maintaining target coverage conditions. Dose calculation times increased linearly with beam number and FMO calculations increased by up to 900% between 5 and 71 beams. Conclusion: We have demonstrated that IMRT plan quality using a 60Co dose model produces similar dose distributions to 6MV. We also show that plan quality does not show considerable improvement above 11 beams for IMRT and significant increases in the treatment planning times are observed extending the number of treatment beams to 71 beams. This work supported in part by NSF grant DMI‐0457394 and the State of Florida DOH Grant 04‐NIR03.

TH-C-19A-08: PDD Discrepancies at Opposite Biases From Very Small Volume Ion Chambers When Using Water Scanners

PURPOSE As more so-called micro ion chambers become commercially available, medical physicists may be inclined to use them during the linear accelerator commissioning process, in an attempt to better characterize the beam in steep dose gradient areas. The purpose of this work is to inform the medical physics community of a non-trivial, anomalous behavior observed when very small chambers are used in certain beam scanning configurations. METHODS A total of six ion chambers (0.007cc to 0.6cc) were used to scan PDDs from a 10×10cm2 field at both +300V and -300V biases. PDDs were scanned using three different water tank scanning systems to determine whether different scanners exhibit the same abnormality. Finally, PDDs were sampled using an external electrometer to bypass the internal electrometer of the scanner to determine the potential contributions of the scanner electronics to the abnormality observed. RESULTS We observed a reproducible, significant difference (over-response with depth) in PDDs acquired when using very small ion chambers with certain bias and watertank combinations, on the order of 3-5% at a depth of 25 cm in water. This difference was not observed when the PDDs were sampled using the ion chambers in conjunction with an external electrometer. This suggests a contribution of interference produced by the controller box and scanning system, which becomes significant for the very small signals collected by very small ion chambers, especially at depth, as the signal level is reduced even further. CONCLUSION Based on the results observed here, if currently available very small ion chambers are used with specific bias and scanning water-tank combinations, erroneous PDD data may be collected. If this data is used as input to the Treatment Planning System, systematic errors on the order of 3%-5% may be introduced into the treatment planning process.