Implementation of a Stereoscopic Camera System for Clinical Electron Simulation and Treatment Planning.

PURPOSE: A 3-dimensinal (3D) stereoscopic camera system developed by .decimal was commissioned and implemented into the clinic to improve the efficiency of clinical electron simulations. Capabilities of the camera allowed simulations to be moved from the treatment vault into any room with a flat surface that could accommodate patient positioning devices, eliminating the need for clinical patient setup timeslots on the treatment machine. This work describes the process used for these simulations and compares the treatment parameters determined by the system to those used in delivery. METHODS AND MATERIALS: The Decimal3D scanner workflow consisted of: scanning the patient surface; contouring the treatment area; determining gantry, couch, collimator, and source-to-surface distance (SSD) parameters for en face entry of the beam with sufficient clearance at the machine; and ordering custom electron cutouts when needed. Transparencies showing the projection of in-house library cutouts at various clinical SSDs were created to assist in choosing an appropriate library cutout. Data from 73 treatment sites were analyzed to evaluate the accuracy of the scanner-determined beam parameters for each treatment delivery. RESULTS: Clinical electron simulations for 73 treatment sites, predominately keloids, were transitioned out of the linear accelerator (LINAC) vault using the new workflow. For all patients, gantry, collimator, and couch parameters, along with SSD and cone size, were determined using the Decimal3D scanner with 57% of simulations using library cutouts. Tolerance tables for patient setup were updated to allow differences of 10, 20, and 5° for gantry, collimator, and couch, respectively. Approximately 7% of fractions (N = 181 total fractions) were set up outside of the tolerance table based on physician direction during treatment. This reflects physician preference to adjust the LINAC rather than patient position during treatment setup. No scanner-derived plan was untreatable because of cutout shape inaccuracy or clearance issues. CONCLUSIONS: Clinical electron simulations were successfully transitioned out of the LINAC vault using the Decimal3D scanner without loss of setup accuracy, as measured through machine parameter determination and electron cutout shape.

Automatic treatment planning for VMAT-based total body irradiation using Eclipse scripting.

The purpose of this work is to establish an automated approach for a multiple isocenter volumetric arc therapy (VMAT)-based TBI treatment planning approach. Five anonymized full-body CT imaging sets were used. A script was developed to automate and standardize the treatment planning process using the Varian Eclipse v15.6 Scripting API. The script generates two treatment plans: a head-first VMAT-based plan for upper body coverage using four isocenters and a total of eight full arcs; and a feet-first AP/PA plan with three isocenters that covers the lower extremities of the patient. PTV was the entire body cropped 5 mm from the patient surface and extended 3 mm into the lungs and kidneys. Two plans were generated for each case: one to a total dose of 1200 cGy in 8 fractions and a second one to a total dose of 1320 cGy in 8 fractions. Plans were calculated using the AAA algorithm and 6 MV photon energy. One plan was created and delivered to an anthropomorphic phantom containing 12 OSLDs for in-vivo dose verification. For the plans prescribed to 1200 cGy total dose the following dosimetric results were achieved: median PTV V100% = 94.5%; median PTV D98% = 89.9%; median lungs Dmean = 763 cGy; median left kidney Dmean = 1058 cGy; and median right kidney Dmean = 1051 cGy. For the plans prescribed to 1320 cGy total dose the following dosimetric results were achieved: median PTV V100% = 95.0%; median PTV D98% = 88.7%; median lungs Dmean = 798 cGy; median left kidney Dmean = 1059 cGy; and median right kidney Dmean = 1064 cGy. Maximum dose objective was met for all cases. The dose deviation between the treatment planning dose and the dose measured by the OSLDs was within ±4%. In summary, we have demonstrated that scripting can produce high-quality plans based on predefined dose objectives and can decrease planning time by automatic target and optimization contours generation, plan creation, field and isocenter placement, and optimization objectives setup.

CBCT image quality QA: Establishing a quantitative program.

PURPOSE: Routine quality assurance (QA) of cone-beam computed tomography (CBCT) scans used for image-guided radiotherapy is prescribed by the American Association of Physicists in Medicine Task Group (TG)-142 report. For CBCT image quality, TG-142 recommends using clinically established baseline values as QA tolerances. This work examined how image quality parameters vary both across machines of the same model and across different CBCT techniques. Additionally, this work investigated how image quality values are affected by imager recalibration and repeated exposures during routine QA. METHODS: Cone-beam computed tomography scans of the Catphan 604 phantom were taken on four TrueBeam® and one Edge™ linear accelerator using four manufacturer-provided techniques. TG-142 image quality parameters were calculated for each CBCT scan using SunCHECK Machine™. The variability of each parameter with machine and technique was evaluated using a two-way ANOVA test on a dataset consisting of 200 CBCT scans. The impact of imager calibration on image quality parameters was examined for a subset of three machines using an unpaired Student's t-test. The effect of artifacts appearing on CBCTs taken in rapid succession was characterized and an approach to reduce their appearance was evaluated. Additionally, a set of baselines and tolerances for all image quality metrics was presented. RESULTS: All imaging parameters except geometric distortion varied with technique (P < 0.05) and all imaging parameters except slice thickness varied with machine (P < 0.05). Imager calibration can change the expected value of all imaging parameters, though it does not consistently do so. While changes are statistically significant, they may not be clinically significant. Finally, rapid acquisition of CBCT scans can introduce image artifacts that degrade CBCT uniformity. CONCLUSIONS: This work characterized the variability of acquired CBCT data across machines and CBCT techniques along with the impact of imager calibration and rapid CBCT acquisition on image quality.

Dosimetric feasibility of brain stereotactic radiosurgery with a 0.35 T MRI-guided linac and comparison vs a C-arm-mounted linac.

PURPOSE: MRI is the gold-standard imaging modality for brain tumor diagnosis and delineation. The purpose of this work was to investigate the feasibility of performing brain stereotactic radiosurgery (SRS) with a 0.35 T MRI-guided linear accelerator (MRL) equipped with a double-focused multileaf collimator (MLC). Dosimetric comparisons were made vs a conventional C-arm-mounted linac with a high-definition MLC. METHODS: The quality of MRL single-isocenter brain SRS treatment plans was evaluated as a function of target size for a series of spherical targets with diameters from 0.6 cm to 2.5 cm in an anthropomorphic head phantom and six brain metastases (max linear dimension = 0.7-1.9 cm) previously treated at our clinic on a conventional linac. Each target was prescribed 20 Gy to 99% of the target volume. Step-and-shoot IMRT plans were generated for the MRL using 11 static coplanar beams equally spaced over 360° about an isocenter placed at the center of the target. Couch and collimator angles are fixed for the MRL. Two MRL planning strategies (VR1 and VR2) were investigated. VR1 minimized the 12 Gy isodose volume while constraining the maximum point dose to be within ±1 Gy of 25 Gy which corresponded to normalization to an 80% isodose volume. VR2 minimized the 12 Gy isodose volume without the maximum dose constraint. For the conventional linac, the TB1 method followed the same strategy as VR1 while TB2 used five noncoplanar dynamic conformal arcs. Plan quality was evaluated in terms of conformity index (CI), conformity/gradient index (CGI), homogeneity index (HI), and volume of normal brain receiving ≥12 Gy (V12Gy ). Quality assurance measurements were performed with Gafchromic EBT-XD film following an absolute dose calibration protocol. RESULTS: For the phantom study, the CI of MRL plans was not significantly different compared to a conventional linac (P > 0.05). The use of dynamic conformal arcs and noncoplanar beams with a conventional linac spared significantly more normal brain (P = 0.027) and maximized the CGI, as expected. The mean CGI was 95.9 ± 4.5 for TB2 vs 86.6 ± 3.7 (VR1), 88.2 ± 4.8 (VR2), and 88.5 ± 5.9 (TB1). Each method satisfied a normal brain V12Gy  ≤ 10.0 cm3 planning goal for targets with diameter ≤2.25 cm. The mean V12Gy was 3.1 cm3 for TB2 vs 5.5 cm3 , 5.0 cm3 and 4.3 cm3 , for VR1, VR2, and TB1, respectively. For a 2.5-cm diameter target, only TB2 met the V12Gy planning objective. The MRL clinical brain plans were deemed acceptable for patient treatment. The normal brain V12Gy was ≤6.0 cm3 for all clinical targets (maximum target volume = 3.51 cm3 ). CI and CGI ranged from 1.12-1.65 and 81.2-88.3, respectively. Gamma analysis pass rates (3%/1mm criteria) exceeded 97.6% for six clinical targets planned and delivered on the MRL. The mean measured vs computed absolute dose difference was -0.1%. CONCLUSIONS: The MRL system can produce clinically acceptable brain SRS plans for spherical lesions with diameter ≤2.25 cm. Large lesions (>2.25 cm) should be treated with a linac capable of delivering noncoplanar beams.

Full automation of spinal stereotactic radiosurgery and stereotactic body radiation therapy treatment planning using Varian Eclipse scripting.

The purpose of this feasibility study is to develop a fully automated procedure capable of generating treatment plans with multiple fractionation schemes to improve speed, robustness, and standardization of plan quality. A fully automated script was implemented for spinal stereotactic radiosurgery/stereotactic body radiation therapy (SRS/SBRT) plan generation using Eclipse v15.6 API. The script interface allows multiple dose/fractionation plan requests, planning target volume (PTV) expansions, as well as information regarding distance/overlap between spinal cord and targets to drive decision-making. For each requested plan, the script creates the course, plans, field arrangements, and automatically optimizes and calculates dose. The script was retrospectively applied to ten computed tomography (CT) scans of previous cervical, thoracic, and lumbar spine SBRT patients. Three plans were generated for each patient - simultaneous integrated boost (SIB) 1800/1600 cGy to gross tumor volume (GTV)/PTV in one fraction; SIB 2700/2100 cGy to GTV/PTV in three fractions; and 3000 cGy to PTV in five fractions. Plan complexity and deliverability patient-specific quality assurance (QA) was performed using ArcCHECK with an Exradin A16 chamber inserted. Dose objectives were met for all organs at risk (OARs) for each treatment plan. Median target coverage was GTV V100% = 87.3%, clinical target volume (CTV) V100% = 95.7% and PTV V100% = 88.0% for single fraction plans; GTV V100% = 95.6, CTV V100% = 99.6% and PTV V100% = 97.2% for three fraction plans; and GTV V100% = 99.6%, CTV V100% = 99.1% and PTV V100% = 97.2% for five fraction plans. All plans (n = 30) passed patient-specific QA (>90%) at 2%/2 mm global gamma. A16 chamber dose measured at isocenter agreed with planned dose within 3% for all cases. Automatic planning for spine SRS/SBRT through scripting increases efficiency, standardizes plan quality and approach, and provides a tool for target coverage comparison of different fractionation schemes without the need for additional resources.

A practical method for quantifying dose in bone and lung using TLDs when using 6 and 15 MV photon beams.

This paper presents a practical method for converting dose measured with thermoluminescent dosimeters (TLD) to dose in lung and bone for 6 MV and 15 MV photon beams. Monte Carlo (MC) simulations and Burlin cavity theory calculations were performed to calculate [Formula: see text], the dose-to-TLD to dose-to-medium conversion factor. A practical method was proposed for converting TLD-measured-dose to dose-in-medium using the TLD dose calibration in water and [Formula: see text] dose-to-medium to dose-to-water conversion factor. Theoretical calculations for [Formula: see text] were performed using photon spectrum weighted parameters and were compared with MC simulations. Verification of the proposed method was done using phantoms having either bone or lung equivalent slabs stacked in between solid water slabs. Percent depth dose (PDD) curves were measured using 0.089 cm thick LiF:Mg,Ti (TLD-100) dosemeters placed at various depths within these phantoms. They were then corrected with [Formula: see text] factors using the proposed dose conversion method, and were compared with the MC simulations. For 6 MV beam, the MC calculated [Formula: see text] factors were 0.942 and 1.002 for bone and lung, and for 15 MV it was 0.927 and 1.005 for bone and lung, respectively. The difference between the MC simulated and spectrum weighted theoretical [Formula: see text] factors were within 3% for both lung and bone. The PDD curves measured with TLD-100 chips that were corrected using the proposed method agreed well within 1.5% of the MC simulated PDD curves for both the water/lung/water and water/bone/water (WBW) phantoms. The dose-to-medium correction using MC simulated [Formula: see text] is convenient, easy, and accurate. Therefore, it can be used instead of Burlin cavity theory, especially in media with high atomic numbers such as bone for accurate dose quantification.

Dosimetric Feasibility of Utilizing the ViewRay Magnetic Resonance Guided Linac System for Image-guided Spine Stereotactic Body Radiation Therapy.

INTRODUCTION: Spine stereotactic body radiation therapy (SBRT) achieves favorable outcomes compared to conventional radiotherapy doses/fractionation. The spinal cord is the principal dose-limiting organ-at-risk (OAR), and safe treatment requires precise immobilization/localization. Therefore, image guidance is paramount to successful spine SBRT. Conventional X-ray imaging and alignment to surrogate bony anatomy may be inadequate, whereas magnetic resonance imaging (MRI) directly visualizes the dose-limiting cord. This work assessed the dosimetric capability of the ViewRay (ViewRay Inc. Oakwood Village, OH) magnetic resonance (MR) guided linac (MR-Linac) for spine SBRT. METHODS: Eight spine SBRT patients without orthopedic hardware who were previously treated on a TrueBeam using volumetric modulated arc therapy (VMAT) were re-planned using MR-Linac fixed-field intensity-modulated radiation therapy (IMRT). Phantom measurements using film, ionization chamber, and a commercial diode-array assessed feasibility. Plans included a variety of prescriptions (30-50 Gy in 3-10 fractions). RESULTS: MR-Linac plans satisfied all clinical goals. Compared to VMAT plans, both entrance dose and heterogeneity increased (Dmax: 134±3% vs. 120±2%, p=0.0270), while conformality decreased (conformity index: 1.28±0.06 vs. 1.06±0.06, p=0.0005), and heterogeneity increased. However, while not statistically significant, MR-linac cord sparing improved (cord Dmax: 16.1±2.7Gy vs. 19.5±1.6Gy, p=0.2066; cord planning organ at risk volume (cord PRV) Dmax: 20.0±2.6Gy vs. 24.5±2.0Gy, p=0.0996). Delivery time increased but was acceptable (14.39±1.26min vs. 9.57±1.19min). Ionization chamber measurements agreed with planned dose to within 2.5%. Film and diode measurements demonstrated accurate/precise delivery of dose gradients between the target and the cord. CONCLUSION: Spine SBRT with the MR-Linac is feasible as verified via re-planning eight clinical cases followed by delivery verification in phantoms using film, diodes, and an ionization chamber. Real-time visualization of the dose-limiting cord during spine SBRT may enable cord-based gating, reduced margins, alternate dose schemas, and/or adaptive therapy.

Initial experience using superflab as intravaginal packing during interstitial brachytherapy for advanced gynecologic cancer.

PURPOSE: Interstitial brachytherapy implemented for locally advanced gynecologic cancer can result in toxicity due to the proximity of organs at risk (OAR). We report our experience using superflab bolus as vaginal packing to displace OAR during interstitial brachytherapy. MATERIAL AND METHODS: Twelve patients with stage IB-IVA gynecologic cancer were treated with definitive chemoradiation including interstitial brachytherapy. A Syed template was used for a computed tomography (CT)-based pre-plan with magnetic resonance imaging (MRI) fusion. A 1-2 cm superflab bolus was cut and sterilized. The tandem and obturator were placed, and superflab was then inserted into the vagina. Interstitial needles were then placed through the template and superflab as per the pre-plan under transabdominal ultrasound guidance. Prescription doses ranged from 85-90 Gy EQD2 including external beam radiation therapy (EBRT). 5-6 Gy per fraction was delivered biologically effective dose (BID) over 2-3 days in 1-2 implants. Toxicities were evaluated post-treatment, 1 month, and 3 months. RESULTS: The rectum, bladder, and sigmoid had significant average displacement from the prescription isodose line. The average reduction in D2cc between pre- and post-implant was 5.19 Gy per fraction (p < 0.0001), 7.19 Gy (p < 0.0004), and 1.78 Gy (p < 0.003) for the rectum, bladder, and sigmoid, respectively. The high-risk target volume (HR-TV) received a median D90 of 104% (range, 58-122%) of the prescription dose, and 92% (range, 71-131%) in the pre-/post-implant plans, respectively (p = 0.4). CONCLUSIONS: Our initial experience with superflab as vaginal packing demonstrates technical feasibility and dosimetric improvement for OAR.

Single-institution report of setup margins of voluntary deep-inspiration breath-hold (DIBH) whole breast radiotherapy implemented with real-time surface imaging.

PURPOSE: We calculated setup margins for whole breast radiotherapy during voluntary deep-inspiration breath-hold (vDIBH) using real-time surface imaging (SI). METHODS AND MATERIALS: Patients (n = 58) with a 27-to-31 split between right- and left-sided cancers were analyzed. Treatment beams were gated using AlignRT by registering the whole breast region-of-interest to the surface generated from the simulation CT scan. AlignRT recorded (three-dimensional) 3D displacements and the beam-on-state every 0.3 s. Means and standard deviations of the displacements during vDIBH for each fraction were used to calculate setup margins. Intra-DIBH stability and the intrafraction reproducibility were estimated from the medians of the 5th to 95th percentile range of the translations in each breath-hold and fraction, respectively. RESULTS: A total of 7269 breath-holds were detected over 1305 fractions in which a median dose of 200 cGy was delivered. Each fraction was monitored for 5.95 ± 2.44 min. Calculated setup margins were 4.8 mm (A/P), 4.9 mm (S/I), and 6.4 mm (L/R). The intra-DIBH stability and the intrafraction reproducibility were ≤0.7 mm and ≤2.2 mm, respectively. The isotropic margin according to SI (9.2 mm) was comparable to other institutions' calculations that relied on x-ray imaging and/or spirometry for patients with left-sided cancer (9.8-11.0 mm). Likewise, intra-DIBH variability and intrafraction reproducibility of breast surface measured with SI agreed with spirometry-based positioning to within 1.2 and 0.36 mm, respectively. CONCLUSIONS: We demonstrated that intra-DIBH variability, intrafraction reproducibility, and setup margins are similar to those reported by peer studies who utilized spirometry-based positioning.

Development and characterization of an interferometer for calorimeter-based absorbed dose to water measurements in a medical linear accelerator.

The quantity of relevance for external beam radiotherapy is absorbed dose to water (ADW). An interferometer was built, characterized, and tested to measure ADW within the dose range of interest for external beam radiotherapy using the temperature dependence of the refractive index of water. The interferometer was used to measure radiation-induced phase shifts of a laser beam passing through a (10 × 10 × 10) cm3 water-filled glass phantom, irradiated with a 6 MV photon beam from a medical linear accelerator. The field size was (7 × 7) cm2 and the dose was measured at a depth of 5 cm in the water phantom. The intensity of the interference pattern was measured with a photodiode and was used to calculate the time-dependent phase shift curve. The system was thermally insulated to achieve temperature drifts of less than 1.5 mK/min. Data were acquired 60 s before and after the irradiation. The radiation-induced phase shifts were calculated by taking the difference in the pre- and post-irradiation drifts extrapolated to the midpoint of the irradiation. For 200, 300, and 400 monitor units, the measured doses were 1.6 ± 0.3, 2.6 ± 0.3, and 3.1 ± 0.3 Gy, respectively. Measurements agreed within the uncertainty with dose calculations performed with a treatment planning system. The estimated type-A, k = 1 uncertainty in the measured doses was 0.3 Gy which is an order of magnitude lower than previously published interferometer-based ADW measurements.

Absolute measurement of LDR brachytherapy source emitted power: Instrument design and initial measurements.

PURPOSE: Energy-based source strength metrics may find use with model-based dose calculation algorithms, but no instruments exist that can measure the energy emitted from low-dose rate (LDR) sources. This work developed a calorimetric technique for measuring the power emitted from encapsulated low-dose rate, photon-emitting brachytherapy sources. This quantity is called emitted power (EP). The measurement methodology, instrument design and performance, and EP measurements made with the calorimeter are presented in this work. METHODS: A calorimeter operating with a liquid helium thermal sink was developed to measure EP from LDR brachytherapy sources. The calorimeter employed an electrical substitution technique to determine the power emitted from the source. The calorimeter's performance and thermal system were characterized. EP measurements were made using four (125)I sources with air-kerma strengths ranging from 2.3 to 5.6 U and corresponding EPs of 0.39-0.79 µW, respectively. Three Best Medical 2301 sources and one Oncura 6711 source were measured. EP was also computed by converting measured air-kerma strengths to EPs through Monte Carlo-derived conversion factors. The measured EP and derived EPs were compared to determine the accuracy of the calorimeter measurement technique. RESULTS: The calorimeter had a noise floor of 1-3 nW and a repeatability of 30-60 nW. The calorimeter was stable to within 5 nW over a 12 h measurement window. All measured values agreed with derived EPs to within 10%, with three of the four sources agreeing to within 4%. Calorimeter measurements had uncertainties ranging from 2.6% to 4.5% at the k = 1 level. The values of the derived EPs had uncertainties ranging from 2.9% to 3.6% at the k = 1 level. CONCLUSIONS: A calorimeter capable of measuring the EP from LDR sources has been developed and validated for (125)I sources with EPs between 0.43 and 0.79 µW.

Impact of the differential fluence distribution of brachytherapy sources on the spectroscopic dose-rate constant.

PURPOSE: To investigate why dose-rate constants for (125)I and (103)Pd seeds computed using the spectroscopic technique, Λ spec, differ from those computed with standard Monte Carlo (MC) techniques. A potential cause of these discrepancies is the spectroscopic technique's use of approximations of the true fluence distribution leaving the source, φ full. In particular, the fluence distribution used in the spectroscopic technique, φ spec, approximates the spatial, angular, and energy distributions of φ full. This work quantified the extent to which each of these approximations affects the accuracy of Λ spec. Additionally, this study investigated how the simplified water-only model used in the spectroscopic technique impacts the accuracy of Λ spec. METHODS: Dose-rate constants as described in the AAPM TG-43U1 report, Λ full, were computed with MC simulations using the full source geometry for each of 14 different (125)I and 6 different (103)Pd source models. In addition, the spectrum emitted along the perpendicular bisector of each source was simulated in vacuum using the full source model and used to compute Λ spec. Λ spec was compared to Λ full to verify the discrepancy reported by Rodriguez and Rogers. Using MC simulations, a phase space of the fluence leaving the encapsulation of each full source model was created. The spatial and angular distributions of φ full were extracted from the phase spaces and were qualitatively compared to those used by φ spec. Additionally, each phase space was modified to reflect one of the approximated distributions (spatial, angular, or energy) used by φ spec. The dose-rate constant resulting from using approximated distribution i, Λ approx,i, was computed using the modified phase space and compared to Λ full. For each source, this process was repeated for each approximation in order to determine which approximations used in the spectroscopic technique affect the accuracy of Λ spec. RESULTS: For all sources studied, the angular and spatial distributions of φ full were more complex than the distributions used in φ spec. Differences between Λ spec and Λ full ranged from -0.6% to +6.4%, confirming the discrepancies found by Rodriguez and Rogers. The largest contribution to the discrepancy was the assumption of isotropic emission in φ spec, which caused differences in Λ of up to +5.3% relative to Λ full. Use of the approximated spatial and energy distributions caused smaller average discrepancies in Λ of -0.4% and +0.1%, respectively. The water-only model introduced an average discrepancy in Λ of -0.4%. CONCLUSIONS: The approximations used in φ spec caused discrepancies between Λ approx,i and Λ full of up to 7.8%. With the exception of the energy distribution, the approximations used in φ spec contributed to this discrepancy for all source models studied. To improve the accuracy of Λ spec, the spatial and angular distributions of φ full could be measured, with the measurements replacing the approximated distributions. The methodology used in this work could be used to determine the resolution that such measurements would require by computing the dose-rate constants from phase spaces modified to reflect φ full binned at different spatial and angular resolutions.