T Nurushev

Dose delivered from Varian's CBCT to patients receiving IMRT for prostate cancer

With the increased use of cone beam CT (CBCT) for daily patient setup, the accumulated dose from CBCT may be significantly higher than that from simulation CT or portal imaging. The objective of this work is to measure the dose from daily pelvic scans with fixed technical settings and collimations. CBCT scans were acquired in half-fan mode using a half bowtie and x-rays were delivered in pulsed-fluoro mode. The skin doses for seven prostate patients were measured on an IRB-approved protocol. TLD capsules were placed on the patients skin at the central axis of three beams: AP, left lateral (Lt Lat) and right lateral (Rt Lat). To avoid the ring artefacts centred in the prostate, the treatment couch was dropped 3 cm from the patients tattoo (central axis). The measured AP skin doses ranged 3-6 cGy for 20-33 cm separation. The larger the patient size the less the AP skin dose. Lateral doses did not change much with patient size. The Lt Lat dose was approximately 4.0 cGy, which was approximately 40% higher than the Rt Lat dose of approximately 2.6 cGy. To verify this dose asymmetry, surface doses on an IMRT QA phantom (oval shaped, 30 cm x 20 cm) were measured at the same three sites using TLD capsules with 3 cm table-drop. The dose asymmetry was due to: (1) kV source rotation which always starts from the patients Lt Lat and ends at Lt Lat. Gantry rotation gets much slower near the end of rotation but dose rate stays constant and (2) 370 degrees scan rotation (10 degrees scan overlap on the Lt Lat side). In vivo doses were measured inside a Rando pelvic heterogeneous phantom using TLDs. The left hip (femoral head and neck) received the highest doses of approximately 10-11 cGy while the right hip received approximately 6-7 cGy. The surface and in vivo doses were also measured for phantoms at the central-axis setup. The difference was less than approximately 12% to the table-drop setup.

Image-Guided Localization Accuracy of Stereoscopic Planar and Volumetric Imaging Methods for Stereotactic Radiation Surgery and Stereotactic Body Radiation Therapy: A Phantom Study

PURPOSE To evaluate the positioning accuracies of two image-guided localization systems, ExacTrac and On-Board Imager (OBI), in a stereotactic treatment unit. METHODS AND MATERIALS An anthropomorphic pelvis phantom with eight internal metal markers (BBs) was used. The center of one BB was set as plan isocenter. The phantom was set up on a treatment table with various initial setup errors. Then, the errors were corrected using each of the investigated systems. The residual errors were measured with respect to the radiation isocenter using orthogonal portal images with field size 3 × 3 cm(2). The angular localization discrepancies of the two systems and the correction accuracy of the robotic couch were also studied. A pair of pre- and post-cone beam computed tomography (CBCT) images was acquired for each angular correction. Then, the correction errors were estimated by using the internal BBs through fiducial marker-based registrations. RESULTS The isocenter localization errors (μ ±σ) in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ExacTrac, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI CBCT. The registration angular discrepancy was 0.1 ± 0.2° between the two systems, and the maximum angle correction error of the robotic couch was 0.2° about all axes. CONCLUSION Both the ExacTrac and the OBI CBCT systems showed approximately 1 mm isocenter localization accuracies. The angular discrepancy of two systems was minimal, and the robotic couch angle correction was accurate. These positioning uncertainties should be taken as a lower bound because the results were based on a rigid dosimetry phantom.

Direct dose mapping versus energy/mass transfer mapping for 4D dose accumulation: fundamental differences and dosimetric consequences

The direct dose mapping (DDM) and energy/mass transfer (EMT) mapping are two essential algorithms for accumulating the dose from different anatomic phases to the reference phase when there is organ motion or tumor/tissue deformation during the delivery of radiation therapy. DDM is based on interpolation of the dose values from one dose grid to another and thus lacks rigor in defining the dose when there are multiple dose values mapped to one dose voxel in the reference phase due to tissue/tumor deformation. On the other hand, EMT counts the total energy and mass transferred to each voxel in the reference phase and calculates the dose by dividing the energy by mass. Therefore it is based on fundamentally sound physics principles. In this study, we implemented the two algorithms and integrated them within the Eclipse treatment planning system. We then compared the clinical dosimetric difference between the two algorithms for ten lung cancer patients receiving stereotactic radiosurgery treatment, by accumulating the delivered dose to the end-of-exhale (EE) phase. Specifically, the respiratory period was divided into ten phases and the dose to each phase was calculated and mapped to the EE phase and then accumulated. The displacement vector field generated by Demons-based registration of the source and reference images was used to transfer the dose and energy. The DDM and EMT algorithms produced noticeably different cumulative dose in the regions with sharp mass density variations and/or high dose gradients. For the planning target volume (PTV) and internal target volume (ITV) minimum dose, the difference was up to 11% and 4% respectively. This suggests that DDM might not be adequate for obtaining an accurate dose distribution of the cumulative plan, instead, EMT should be considered.

Comprehensive evaluation and clinical implementation of commercially available Monte Carlo dose calculation algorithm.

A commercial electron Monte Carlo (eMC) dose calculation algorithm has become available in Eclipse treatment planning system. The purpose of this work was to evaluate the eMC algorithm and investigate the clinical implementation of this system. The beam modeling of the eMC algorithm was performed for beam energies of 6, 9, 12, 16, and 20 MeV for a Varian Trilogy and all available applicator sizes in the Eclipse treatment planning system. The accuracy of the eMC algorithm was evaluated in a homogeneous water phantom, solid water phantoms containing lung and bone materials, and an anthropomorphic phantom. In addition, dose calculation accuracy was compared between pencil beam (PB) and eMC algorithms in the same treatment planning system for heterogeneous phantoms. The overall agreement between eMC calculations and measurements was within 3%/2 mm, while the PB algorithm had large errors (up to 25%) in predicting dose distributions in the presence of inhomogeneities such as bone and lung. The clinical implementation of the eMC algorithm was investigated by performing treatment planning for 15 patients with lesions in the head and neck, breast, chest wall, and sternum. The dose distributions were calculated using PB and eMC algorithms with no smoothing and all three levels of 3D Gaussian smoothing for comparison. Based on a routine electron beam therapy prescription method, the number of eMC calculated monitor units (MUs) was found to increase with increased 3D Gaussian smoothing levels. 3D Gaussian smoothing greatly improved the visual usability of dose distributions and produced better target coverage. Differences of calculated MUs and dose distributions between eMC and PB algorithms could be significant when oblique beam incidence, surface irregularities, and heterogeneous tissues were present in the treatment plans. In our patient cases, monitor unit differences of up to 7% were observed between PB and eMC algorithms. Monitor unit calculations were also preformed based on point‐dose prescription. The eMC algorithm calculation was characterized by deeper penetration in the low‐density regions, such as lung and air cavities. As a result, the mean dose in the low‐density regions was underestimated using PB algorithm. The eMC computation time ranged from 5 min to 66 min on a single 2.66 GHz desktop, which is comparable with PB algorithm calculation time for the same resolution level. PACS number: 87.55.K‐

Clinical Use of Dual Image-Guided Localization System for Spine Radiosurgery

The recently released Novalis TX linac platform provides various image guided localization methods including a stereoscopic X-ray imaging technique (ExacTrac) and a volumetric cone beam computed tomography (CBCT) imaging technique. The ExacTrac combined with the robotic six dimensional (6D) couch provides fast and accurate patient setup based on bony structures and offers “snap shot” imaging at any point during the treatment to detect patient motion. The CBCT offers a three dimensional (3D), volumetric image of the patients setup with visualization of anatomic structures. However, each imaging system has a separate isocenter, which may not coincide with each other or with the linac isocenter. The aim of this paper was to compare the localization accuracy between Exactrac and CBCT for single fraction spine radiosurgery treatments. The study was performed for both phantom and patients (96 clinical treatments of 57 patients). The discrepancies between the isocenter between the ExacTrac and CBCT in four dimensions (three translations and one rotation) were recorded and statistically analyzed using two-tailed t-test.

SU‐E‐T‐165: Systematic Evaluation of Uncertainties Associated with GAFCHROMIC EBT2 Film Dosimetry for 6MV Photon Beams

Method and Materials:Eight packets of films were exposed to 13.5cm ×13.5cm, 6MV radiation fields in a solid water phantom. Dose levels of 1.1, 3.2, 5.3, 7.4, and 9.5 Gy were delivered to five films in each packet. Films were scanned both before and after irradiation using an Epson flat‐bed scanner (24hr wait‐time for post‐irradiation coloration). Corresponding 2D dose distributions were measured with a detector‐array (MatriXX). Point dose comparisons were performed with an ion chamber. Digitized film images were registered to the 2D dose distribution to generate a correction map that compensated the scanner non‐uniform response as a function of dose. Optical density (OD) and net optical density (NetOD) values were calculated for all images. Dose response curves were established using mean values of a central 0.5cm × 0.5cm region‐of‐interest (ROI). Images were converted to dose, and error uncertainties (1SD) were measured in the central 8cm × 8cm ROI. Results: The overall dosimetric uncertainties (1SD) of the NetOD approach were 2.2%, 1.9%, and 3.5% for red, green, and blue channels, respectively. The corresponding uncertainties of OD were 2.7%, 3.1%, and 8.3%, respectively. For low dose range (<3 Gy), the green channel revealed higher uncertainty (SDgreen= 3.3%) than the red channel (SDred=2.6%). However, for high doses (3∼9 Gy), the green channel showed less variability (SDgreen=1.6%, SDred=2.9%). Minimum SDred and SDgreen were 1.6% at 5.3Gy and 1.3% at 7.4 Gy, respectively. Scanner non‐uniformity correction mitigated the irregular response of scanner detector elements observed initially. Conclusion: NetOD may be a more useful metric for benchmarking EBT2 than OD. We demonstrated that the lowest dose uncertainties were achieved using the red channel for low dose range, while the green channel was preferred for higher doses. Scanner non‐uniformity correction is necessary for higher precision dosimetry.

TH-D-VaIB-02: Skin and Body Dose Measurements for Varian Cone-Beam CT (CBCT) During IMRT for Prostate Cancer

Purpose: With the increased use of CBCT for daily patient setup, kV dose delivered to patient should be investigated. This study is to measure skin and body dose from Varian daily CBCT for prostate patients. Methods and Materials: CBCT scans were acquired in half‐fan and pulsed‐fluoro mode with a half bow‐tie mounted. A technical setting of 125kV, 80mA and 25ms was used. Skin and body doses were first measured for a Rando pelvic and an IMRT QA phantom, set centrally, with TLD and a cylindrical chamber. Then skin dose for 7 prostate patients undergoing daily CBCT was measured. To avoid the ring artifacts centered at prostate, the treatment couch was dropped 3cm from patients tattoo. TLD capsules were placed on patients skin at 3 sites: AP, Lt Lat and Rt Lat. Phantom measurement was also made for this setup. The absorbed dose was determined by the air‐kerma‐based calibration method recommended by TG61. Results: For phantoms set centrally, measured skin dose was ∼6 cGy, ∼5.6 cGy, ∼3.7cGy at AP, Lt Lat, and Rt Lat, respectively. Body dose at the center was ∼3–4 cGy. With table dropping (TD), only AP skin dose was increased ∼12%. Patient AP skin dose varied with separation, ranging 4–6 cGy for thicker patients (AP 23 – 33 cm) and 6 – 8 cGy for thinner patients. Minimum changes were observed on lateral dose for patients with different size. Lt Lat skin (4cGy) and bone (9cGy) doses were higher than Rt Lat skin (3cGy) and bone dose (6cGy) Conclusions: Daily CBCT provides better patient setup but it increases skin and body dose. The dose can range from 120 – 330 cGy for skin and 120 – 380 cGy for body during the 42 daily fractions delivered for IMRT prostate patients.

SU‐E‐T‐874: Investigation of the Interplay Effect Between MLC and Lung Tumor Motions Using 4DCT and RPM Profile Data

Purpose: To evaluate the dosimetric impact of the interplay effect between the MLC motion and respiratory‐induced tumor motion in the context of 4D planning of small lung lesions treated with SBRT. Methods: IMRT plan was generated using the ITV from 10 respiratory‐correlated CT datasets on the reference (50% phase, CT50) image. The PTV included a 3 mm uniform expansion of ITV; the dose was calculated with Varian Eclipse AAA algorithm and 2.5 mm grid size. To account for the interplay effect, the time‐stamped signal from respiratory surrogate (RPM marker trace) was used to correlate the MLC control points to the individual CT phase. The dose to each phase was then computed using the actual MLC apertures and MU numbers for the specific phase. Doses from individual phases were mapped and accumulated to the CT50 dataset using a Demons‐based deformable registration algorithm. A “standard” 4D plan was also generated in which the entire treatment plan was applied to each phase with appropriate weighting of the time spent at each phase, but without accounting for the correlation of the MLC sequence with the respiratory phases. The method was applied to two lungSBRT cases with tumor motion 1.2 and 2.0 cm in amplitude sup/inf. Results: For mean PTV dose, the discrepancy induced by interplay effect was −5.7% to 33.4% (with mean −+ SD of 1.3 −+ 8.7%) for individual phases. For the accumulated dose it was 0.0% and 0.1% for the two cases respectively. The maximum point dose discrepancy within PTV was 0.4 out of 48 Gy. The interplay effect reduced the minimum PTV and ITV dose by about 0.5%. Conclusions: This initial study showed that the dosimetric impact of interplay effect dimmed out as 4D doses were accumulated. Further assessment for wide spectrum of plan and delivery scenarios should be performed.

SU‐FF‐J‐79: Implementation of Four Different Image‐Guided Radiotherapy (IGRT) Systems in a Radiotherapy Department

Purposes: to implement and compare four newly developed image‐guidedradiotherapy systems (Varians Cone‐Beam CT, BrainLAB ExacTrac, Restitu Ultrasound (U/S)‐Sim and Guide, and in‐house stereovision) in one department. Methods and Materials: The cone‐beam CT(CBCT) and the ultrasound(US) systems provide volumetric images of the target at daily setup. The ExacTrac system acquires the biplanar radiographs at patient setup. Both the US and ExacTrac systems are integrated with infrared‐tracking systems for patient‐couch positioning. The in‐house stereovision system captures 3D surface images of the patient at the instants of daily patient setup and during individual beam irradiation. All of four IGRT systems have used treatment planning volumetric imaging information for target position verification and adjustment. Electronic portal images are routinely used for patient position verification. External markers and possible internal markers such as seeds or small cysts or calcifications can be localized and used for additional verification. Results: Emerging data from several institutional IRB‐approved clinical trials demonstrate that the target reposition error and dose delivery uncertainties can be significantly reduced by using such image‐guided systems, each of which may be most useful in specific clinical situations. Conclusions: Our customized stereovision system, which, like US, involves no radiation exposure, is extremely efficient (<2 minutes) and accurate (<2 millimeters) for superficial sites, such as breast cancer. The ExacTrac system appears ideal for lesions associated with bony structures, such as spine and skull. The US and CBCT may be most useful for deformable internal structures, such as prostate cancer. Special methods for dealing with imaging artifacts, such as ring patterns in CBCT, shadow casts and multiple reflections in stereovision and US, and patient motion in ExacTrac and stereovision will be presented.

Optimized source selection for intracavitary low dose rate brachytherapy.

A procedure has been developed for automating optimal selection of sources from an available inventory for the low dose rate brachytherapy, as a replacement for the conventional trial-and-error approach. The method of optimized constrained ratios was applied for clinical source selection for intracavitary Cs-137 implants using Varian BRACHYVISION software as initial interface. However, this method can be easily extended to another system with isodose scaling and shaping capabilities. Our procedure provides optimal source selection results independent of the user experience and in a short amount of time. This method also generates statistics on frequently requested ideal source strengths aiding in ordering of clinically relevant sources.