Clinical experience in the use of 3D printing as a rapid replacement of traditional radiation therapy immobilization materials.

PURPOSE: Patient positioning and immobilization devices are commonly employed in radiation therapy. Unfortunately, cases can arise where the devices need to be reconstructed or improved. This work describes clinical processes to use a planning CT, to design and 3D print immobilization devices for reproducible patient positioning within a clinically feasible time frame when traditional methods can no longer be used or are insufficient. MATERIALS/METHODS: Three clinical cases required rapid 3D printing of an immobilization device mid-treatment due to the following: (1) a lost headrest cushion, (2) needed improvement in lumbar spine positioning, and (3) a partially deflated vacuum immobilization mattress. RESULTS: In the three cases, the 3D printed immobilization devices were clinically implemented successfully; two of the devices were fully designed and printed in 1 day. The 3D printed immobilization devices achieved a positioning accuracy sufficient to avoid the necessity to repeat the simulation and planning process. CONCLUSION: If traditional immobilization devices fail or are misplaced, it is feasible to have a 3D printed replacement within the time span of 1 day. The design and fabrication methods, as well as the experiences gained, are described in detail to assist clinicians to implement 3D printing for similar situations.

Evaluation of SrAl2 O4 :Eu, Dy phosphor for potential applications in thermoluminescent dosimetry.

PURPOSE: To evaluate the use of commercial-grade strontium aluminate phosphorescent powder as a thermoluminescent (TL) dosimeter for clinical radiotherapy beams. MATERIALS AND METHOD: Commercially available Eu2+ , Dy3+ co-doped strontium aluminate powder (SrAl2 O4 :Eu, Dy) was annealed and then irradiated using 20 × 20 cm2 field size, with 6-MV (PDD10  = 70.7) and 18-MV (PDD10  = 79.4) photon beams and and 9-MeV (R50  = 3.6), 15 MeV (R50  = 5.9) and 18-MeV (R50  = 7.2) electron beams. To calibrate the relationship between the TL readings and the irradiated doses, TL glow curves were acquired for doses up to 600 cGy at all beam energies. For the percentage depth dose (PDD) measurement, the SrAl2 O4 :Eu, Dy powder was sandwiched by solid water phantoms, with varying thickness of solid water placed above to determine the depth. PDDs were measured at four representative depths and compared against the commissioning depth dose data for each beam energy. RESULTS: Linear dose response models of doses up to 200 cGy were created for all beam energies. Superlinearity was observed with doses greater than 200 cGy. The PDD measurement acquired experimentally agrees well with the commissioning data of the medical linear accelerator. Trapping parameters such as order of kinetics, activation energy and frequency factor have been obtained via TL glow curve analysis. CONCLUSION: The linear dose response demonstrates that SrAl2 O4 :Eu, Dy is a potential TLD dosimeter for both electron beams and photon beams at different beam energies. The PDD measurements further support its potential use in quality assurance and radiation dosimetry.

Three-dimensional printing in radiation oncology: A systematic review of the literature.

PURPOSE/OBJECTIVES: Three-dimensional (3D) printing is recognized as an effective clinical and educational tool in procedurally intensive specialties. However, it has a nascent role in radiation oncology. The goal of this investigation is to clarify the extent to which 3D printing applications are currently being used in radiation oncology through a systematic review of the literature. MATERIALS/METHODS: A search protocol was defined according to preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Included articles were evaluated using parameters of interest including: year and country of publication, experimental design, sample size for clinical studies, radiation oncology topic, reported outcomes, and implementation barriers or safety concerns. RESULTS: One hundred and three publications from 2012 to 2019 met inclusion criteria. The most commonly described 3D printing applications included quality assurance phantoms (26%), brachytherapy applicators (20%), bolus (17%), preclinical animal irradiation (10%), compensators (7%), and immobilization devices (5%). Most studies were preclinical feasibility studies (63%), with few clinical investigations such as case reports or series (13%) or cohort studies (11%). The most common applications evaluated within clinical settings included brachytherapy applicators (44%) and bolus (28%). Sample sizes for clinical investigations were small (median 10, range 1-42). A minority of articles described basic or translational research (11%) and workflow or cost evaluation studies (3%). The number of articles increased over time (P < 0.0001). While outcomes were heterogeneous, most studies reported successful implementation of accurate and cost-effective 3D printing methods. CONCLUSIONS: Three-dimensional printing is rapidly growing in radiation oncology and has been implemented effectively in a diverse array of applications. Although the number of 3D printing publications has steadily risen, the majority of current reports are preclinical in nature and the few clinical studies that do exist report on small sample sizes. Further dissemination of ongoing investigations describing the clinical application of developed 3D printing technologies in larger cohorts is warranted.

A Patient-Specific correspondence model to track tumor location in thorax during radiation therapy.

PURPOSE: We present a patient-specific model to estimate tumor location in the thorax during radiation therapy using chest surface displacement as the surrogate signal. METHODS: Two types of data are used for model construction: Four-dimensional computed tomography (4D-CT) images of the patient and the displacement of two points on the patient's skin on the thoracic area. Principal component analysis is used to fit the correspondence model. This model incorporates the recorded surrogate signals during radiation delivery as input and delivers the 3D trajectory of the tumor as output. We evaluated the accuracy of the proposed model on a respiratory phantom and five lung cancer patients. RESULTS: For the respiratory phantom, the location of the center of the sphere during treatment was calculated in three directions: Left-Right (LR), Anterior-Posterior (AP) and, Superior-Inferior (SI). The error of localization was less than 1 mm in the LR and AP directions and less than 2 mm in the SI direction. The location of the tumor center for two of the patients, and the location of the apex of the diaphragm for the other three, was calculated in three directions. For all patients, the localization error in the LR and AP directions was less than 1.1 mm for two fractions and the maximum localization error in the SI direction was 6.4 mm. CONCLUSIONS: This work presents a feasibility study of utilizing surface displacement data to locate the tumor in the thorax during radiation treatment. Future work will validate the model on a larger patient population.

Idiopathic Pneumonitis Syndrome After Total Body Irradiation in Pediatric Patients Undergoing Myeloablative Hematopoietic Stem Cell Transplantation: A PENTEC Comprehensive Review.

PURPOSE: Pulmonary complications, especially idiopathic pneumonitis syndrome (IPS), are potentially life altering or fatal sequelae of hematopoietic cell transplantation (HCT). Total body irradiation (TBI) as part of the conditioning regimen has been implicated in IPS. A comprehensive PENTEC (Pediatric Normal Tissues in the Clinic) review was performed to increase our understanding of the role of TBI in the development of acute, noninfectious IPS. METHODS AND MATERIALS: A systematic literature search was conducted using the MEDLINE, PubMed, and Cochrane library databases for articles describing pulmonary toxicity in children treated with HCT. Data pertaining to TBI and pulmonary endpoints were extracted. Risk of IPS was analyzed in relation to patient age, TBI dose, fractionation, dose rate, lung shielding, timing, and type of transplant, with the goal to better understand factors associated with this complication in children undergoing HCT. A logistic regression model was developed using a subset of studies with comparable transplant regimens and sufficient TBI data. RESULTS: Six studies met criteria for modeling of the correlation of TBI parameters with IPS; all consisted of pediatric patients undergoing allogeneic HCT with a cyclophosphamide-based chemotherapy regimen. IPS was variably defined, but all studies that reported IPS were included in this analysis. The mean incidence of post-HCT IPS was 16% (range, 4%-41%). Mortality from IPS, when it occurred, was high (median, 50%; range, 45%-100%). Fractionated TBI prescription doses encompassed a narrow range of 9 to 14 Gy. Many differing TBI methods were reported, and there was an absence of 3-dimensional dose analysis of lung blocking techniques. Thus, a univariate correlation between IPS and total TBI dose, dose fractionation, dose rate, or TBI technique could not be made. However, a model, built from these studies based on prescribed dose using a normalized dose parameter of equivalent dose in 2-Gy fractions (EQD2), adjusted for dose rate, suggested correlation with the development of IPS (P = .0004). The model-predicted odds ratio for IPS was 24.3 Gy-1 (95% confidence interval, 7.0-84.3). Use of TBI lung dose metrics (eg, midlung point dose) could not be successfully modeled, potentially because of dosimetric uncertainties in the actual delivered volumetric lung dose and imperfections in our modeling process. CONCLUSIONS: This PENTEC report is a comprehensive review of IPS in pediatric patients receiving fractionated TBI regimens for allogenic HCT. IPS was not clearly associated with 1 single TBI factor. Modeling using dose-rate adjusted EQD2 showed a response with IPS for allogeneic HCT using a cyclophosphamide-based chemotherapy regimen. Therefore, this model suggests IPS mitigation strategies can focus on not just the dose and dose per fraction but also the dose rate used in TBI. More data are needed to confirm this model and to determine the influence of chemotherapy regimens and contribution from graft-versus-host disease. The presence of confounding variables (eg, systemic chemotherapies) that affect risk, the narrow range of fractionated TBI doses found in the literature, and limitations of other reported data (eg, lung point dose) may have prevented a more straightforward link between IPS and total dose from being observed.

3D printed copper-plastic composite material for use as a radiotherapy bolus.

The purpose of this work is to evaluate a commercially available copper-plastic composite material for use as a custom fit 3D printed bolus. Superficial dose under copper-plastic composite bolus was assessed for 0.4 mm, 0.6 mm, and 0.8 mm thicknesses. Superficial dose measurements were performed with an Attix parallel plate ionization chamber and radiochromic film. Additionally, a custom-fit bolus was designed for the temporal-frontal cranial region of an anthropomorphic phantom. A treatment plan with a tangential field arrangement was designed, and radiochromic film was used to measure the dose enhancement to the surface of the phantom from the bolus and compared to the calculated dose. It was shown that 3D printed copper-plastic composite bolus can provide the equivalent dose enhancement of thicker conventional bolus. Due to the limited thickness of the copper-plastic composite the bolus can remain flexible, which can aid in the placement of the bolus and improve patient comfort.

Dosimetric characterization of a novel 90Y source for use in the conformal superficial brachytherapy device.

PURPOSE: To characterize the dose distribution in water of a novel beta-emitting brachytherapy source for use in a Conformal Superficial Brachytherapy (CSBT) device. METHODS AND MATERIALS: Yttrium-90 (90Y) sources were designed for use with a uniquely designed CSBT device. Depth dose and planar dose measurements were performed for bare sources and sources housed within a 3D printed source holder. Monte Carlo simulated dose rate distributions were compared to film-based measurements. Gamma analysis was performed to compare simulated and measured dose rates from seven 90Y sources placed simultaneously using the CSBT device. RESULTS: The film-based maximum measured surface dose rate for a bare source in contact with the surface was 3.35 × 10-7 cGy s-1 Bq-1. When placed in the source holder, the maximum measured dose rate was 1.41 × 10-7 cGy s-1 Bq-1. The Monte Carlo simulated depth dose rates were within 10% or 0.02 cm of the measured dose rates for each depth of measurement. The maximum film surface dose rate measured using a seven-source configuration within the CSBT device was 1.78 × 10-7 cGy s-1 Bq-1. Measured and simulated dose rate distribution of the seven-source configuration were compared by gamma analysis and yielded a passing rate of 94.08%. The gamma criteria were 3% for dose-difference and 0.07056 cm for distance-to-agreement. The estimated measured dose rate uncertainty was 5.34%. CONCLUSIONS: 90Y is a unique source that can be optimally designed for a customized CSBT device. The rapid dose falloff provided a high dose gradient, ideal for treatment of superficial lesions. The dose rate uncertainty of the 90Y-based CSBT device was within acceptable brachytherapy standards and warrants further investigation.

Step and shoot IMRT to mobile targets and techniques to mitigate the interplay effect.

The purpose of this work is to evaluate a method to mitigate temporal dose variation due to the interplay effect as well as investigate the effect of randomly varying motion patterns. The multi-leaf collimator (MLC) settings from 5, 9 and 11 field step and shoot intensity modulated radiation therapy (IMRT) of non-small cell lung cancer (NSCLC) treatment plans with tumor motion of 1.53, 1.03 and 1.95 cm, respectively, were used. Static planar dose distributions were determined for each treatment field using the Planar Dose Module in the Pinnacle(3) treatment planning system. The MotionSIM XY/4D robotic diode array was used to recreate the tumor motion orthogonal to each treatment beam. Dose rate modulation was investigated as a method to mitigate temporal dose variation due to the interplay effect. Computer simulation was able to identify individual fields where interplay effects are greatest. Computer simulation and physical measurement have shown that temporal dose variation can be mitigated by the selection of the dose rate or by selective dose rate modulation within a given IMRT treatment field. Selective dose rate modulation within a given IMRT treatment field reduced temporal dose variation to levels comparable to whole field dose rate reduction, while also producing shorter radiation delivery times in six of the seven cases investigated. For the cases considered, the interplay effect did not appear to have a greater effect on hypofractionation compared to traditional fractionation even though fewer fractions were delivered. Randomized motion kernel variation was also considered. For this portion of the study, a nine field step and shoot IMRT configuration was considered with a 1.03 cm tumor motion rather than the five field case. In general, if the extent of the variant motion pattern was mostly contained within the target volume, limited impact on the temporal dose variation was observed. In cases where the variant motion kernels increasingly exceeded the target volume limits, increases in temporal dose variation were observed.

Dose calibration of Gafchromic EBT3 film for Ir-192 brachytherapy source using 3D-printed PLA and ABS plastics.

3D printing technology has allowed the creation of custom applicators for high dose rate (HDR) brachytherapy, especially for complex anatomy. With conformal therapy comes the need for advanced dosimetric verification. It is important to demonstrate how dose to 3D printed materials can be related to dose to water. This study aimed to determine dose differences and uncertainties using 3D printed PLA and ABS plastics for Radiochromic film calibration in HDR brachytherapy.Gafchromic EBT3 film pieces were irradiated in water with an Ir-192 source at calculated dose levels ranging from 0 to 800 cGy, to create the control calibration curve. Similarly, film was placed below 3D printed PLA and ABS blocks and irradiated at the same dose levels calculated for water, ranging from 0 to 800 cGy. After a 72-h development time, film pieces were scanned on a flatbed scanner and the median pixel value was recorded in the region of highest dose. This value was converted to net optical density (NOD). A rational function was used to fit a calibration curve in water that relates NOD to dose for red, green, and blue color channels. Based on this fitted curve, ABS and PLA NOD values were used to estimate dose in 3D printed plastics.From the fitted calibration curve, mean residual error between measured and planned dose to water was less than 1% for each color channel at high dose levels. At high dose levels, ABS and PLA mean residual errors were about 6.9 and 7.8% in the red channel, while 5.2 and 5.7% in the green channel. Combined uncertainties measured to be about 6.9% at high dose levels. This study demonstrated dose differences and uncertainties using 3D printed applicators for HDR Ir-192 brachytherapy.

Influence of Total Body Irradiation Dose Rate on Idiopathic Pneumonia Syndrome in Acute Leukemia Patients Undergoing Allogeneic Hematopoietic Cell Transplantation.

PURPOSE: To determine the relationship between dose rate and other factors in the development of idiopathic pneumonia syndrome (IPS) in patients with acute lymphoblastic leukemia or acute myeloid leukemia who are undergoing total body irradiation (TBI)-based myeloablative conditioning for allogeneic hematopoietic cell transplantation (HCT). METHODS AND MATERIALS: From 2006 to 2016, 202 patients with acute leukemia (111 acute lymphoblastic leukemia, 91 acute myeloid leukemia) ranging in age from 1 to 57 years (median, 25 years) underwent allogeneic HCT at University of Minnesota. Pretransplantation conditioning included cyclophosphamide (120 mg/kg) with (68%) or without fludarabine (75 mg/m2) followed by 13.2 Gy TBI given in 8 twice-daily fractions of 1.65 Gy over 4 days. Dose rate varied based on linear accelerator availability and ranged from 8.7 to 19.2 cGy/min. Patients were stratified by receipt of high-dose-rate (HDR; >15 cGy/min; 56%) or low-dose-rate (LDR; ≤15 cGy/min; 44%) TBI for all 8 fractions. IPS was defined as pulmonary injury based on clinical symptoms, radiographic evidence, or pulmonary function testing within 100 days of HCT in the absence of concurrent infection. RESULTS: IPS developed in 42 patients (21%) between 4 and 73 days (median, 16 days) after transplantation. HDR TBI was associated with a higher rate of IPS compared with LDR TBI (29% vs 10%; P < .01). On multiple regression analysis, HDR remained a significant predictor of IPS (hazard ratio, 2.6; 95% confidence interval, 1.2-5.3; P = .01), and this led to inferior 1-year overall survival (60% vs 76%; P = .01) and increased 1-year nonrelapse mortality (28% vs 15%; P = .02). CONCLUSIONS: TBI dose rates ≤15 cGy/min reduce the risk of posttransplantation IPS and improve overall survival. LDR TBI should be strongly considered as an easily implemented parameter to improve the safety of pretransplantation TBI-based conditioning.

Novel design and development of a 3D-printed conformal superficial brachytherapy device for the treatment of non-melanoma skin cancer and keloids.

BACKGROUND: Skin tumors are the most predominant form of cancer in the United States. Radiation therapy, particularly high dose-rate (HDR) brachytherapy, provides an effective form of cancer control when surgery is not possible or when surgical margins are incomplete. The treatment of superficial skin cancers on irregular surfaces, such as the nose, lips or ears, present challenges for treatment. To address this issue, we designed and constructed a novel conformal superficial brachytherapy (CSBT) device prototype to improve patient-specific treatment for complex sites. The device is mounted on an automated remote after-loader, providing limited radiation exposure to operating personnel, is inexpensive to construct, and offers a unique method of conformal surface radiation therapy. RESULTS: A prototype of the CSBT device was successfully manufactured. A computed tomography (CT) scan of a Rando phantom was used to plan the target treatment area. The CSBT device has a hexagonal lattice array of retractable rods with radioactive seeds placed at the tip of each rod. A 3D-printed conformal shape insert with a hexagonal array of cylindrical projections of varying length is driven into the rods by a single linear actuator. The rods are displaced to conform to the patient's skin. This elegant device design permits the delivery of radiation to complex targets using readily available beta-emitting radionuclides, such as Yttrium-90 (Y-90) or Strontium-90 (Sr-90). CONCLUSION: A working prototype of a novel CSBT device was built using 3D-printing technology that provides a safe and economically attractive means of improving radiation delivery to complex treatment sites.

Lung 4D-IMRT treatment planning: an evaluation of three methods applied to four-dimensional data sets.

PURPOSE: To compare 4D-dose distributions for IMRT planning on three data sets: a single 4D-CT phase, a 4D-CT phase with a density override to the tumor motion envelope (TME) volume, and the average intensity projection (AIP). METHODS: Eight planning cases were considered. IMRT inverse planning optimization was performed on each of the three data set types, for each case considered. The plans were then applied to all ten phases of the associated 4D-CT data set. The dose to the GTV in each breathing phase was compared to the TME dose from the optimized dose distribution, as well as the GTV dose determined from a model-based deformable registration algorithm. RESULTS: IMRT optimization on a single 3D data set resulted in a greater equivalent uniform dose (EUD) to the GTV when applied to a 4D-CT data set than the EUD for the TME in the optimized plan. The difference was up to 5.5Gy in one case. For all cases and planning techniques considered, a maximum difference of 0.3Gy in the NTDmean to the healthy lung throughout the breathing cycle was found. CONCLUSIONS: For tumors located in the periphery of the lung, optimization on the AIP image resulted in a more uniform GTV dose throughout the breathing cycle. Averages in GTV EUD and healthy lung NTDmean taken over all the breathing phases were found to be in agreement with the dose effect parameters obtained from model-based deformable registration algorithms. All planning methods yielded GTV EUD values that were larger than the prescribed dose when the full 4D data set was considered.

Workload implications for clinic workflow with implementation of three-dimensional printed customized bolus for radiation therapy: A pilot study.

Bolus is commonly used in radiation therapy to improve radiation dose distribution to the target volume, but commercially available products do not always conform well to the patient surface. Tumor control may be compromised, particularly for superficial tumors, if bolus does not conform well and air gaps exist between the patient surface and the bolus. Three-dimensional (3D) printing technology allows the creation of highly detailed, variable shaped objects, making it an attractive and affordable option for customized, patient-specific bolus creation. The use of 3D printing in the clinical setting remains limited. Therefore, the objective of this study was to assess the implications on time and clinical fit using a workflow for 3D printing of customized bolus in companion animals with spontaneous tumors treated with radiation therapy. The primary aim of this study was to evaluate the time required to create a clinical 3D printed bolus. The secondary aims were to evaluate the clinical fit of the bolus and to verify the skin surface dose. Time to segmentation and 3D printing were documented, while the clinical fit of the bolus was assessed in comparison to the bolus created in the treatment planner. The mean and median time from segmentation to generation of 3D printed boluses was 6.15 h and 5.25 h, respectively. The 3D printed bolus was significantly less deviated from the planned bolus compared to the conventional bolus (p = 0.0078) with measured dose under the bolus within 5% agreement of expected dose in 88% of the measurements. Clinically acceptable 3D printed customized bolus was successfully created for treatment within one working day. The most significant impact on time is the 3D printing itself, which therefore has minimal implications on personnel and staffing. Quality assurance steps are recommended when implementing a 3D printing workflow to the radiotherapy clinic.

On the dose to a moving target while employing different IMRT delivery mechanisms.

BACKGROUND AND PURPOSES: To compare the temporal uniformity in dose delivered to a moving target for various intensity modulation radiotherapy (IMRT) modalities: solid intensity modulator (SIM), segmented multi-leaf collimator (SMLC), and dynamic multi-leaf collimator (DMLC). MATERIALS AND METHODS: Two separate four-dimensional computed tomography data sets were obtained. Tumor motion kernels and motion envelopes were determined from composite positions of the tumor in various phases of the breathing cycle. Treatment plans were created for an unmodulated open field, SIM, SMLC, and DMLC. The motion envelope was treated as a static target volume. A robotic apparatus equipped with a diode array simulated the tumor motion in the plane of the beam's eye view (BEV). Radiation was delivered to the moving target over ten trials for each modality. The average coefficient of variation (CV) was determined for each beam angle. RESULTS: The CV ranged from 0.09% to 0.15%, 0.23% to 3.14%, 1.14% to 5.51%, and 3.83% to 8.25% for the unmodulated open field, SIM, SMLC, and DMLC modalities, respectively. With gating, the CV was 0.23% to 2.31%, 0.31% to 2.97%, and 0.7% to 4.67% for SIM, SMLC, and DMLC, respectively. CONCLUSION: SIM consistently provided the most temporally uniform dose to the moving target while DMLC provided the least. The SMLC and DMLC CV improved with gated delivery.

Quality assurance device for four-dimensional IMRT or SBRT and respiratory gating using patient-specific intrafraction motion kernels.

Emerging technologies such as four-dimensional computed tomography (4D CT) and implanted beacons are expected to allow clinicians to accurately model intrafraction motion and to quantitatively estimate internal target volumes (ITVs) for radiation therapy involving moving targets. In the case of intensity-modulated (IMRT) and stereotactic body radiation therapy (SBRT) delivery, clinicians must consider the interplay between the temporal nature of the modulation and the target motion within the ITV. A need exists for a 4D IMRT/SBRT quality assurance (QA) device that can incorporate and analyze customized intrafraction motion as it relates to dose delivery and respiratory gating. We built a 4D IMRT/SBRT prototype device and entered (X, Y, Z)(T) coordinates representing a motion kernel into a software application that 1. transformed the kernel into beam-specific two-dimensional (2D) motion "projections," 2. previewed the motion in real time, and 3. drove a recision X-Y motorized device that had, atop it, a mounted planar IMRT QA measurement device. The detectors that intersected the target in the beam's-eye-view of any single phase of the breathing cycle (a small subset of all the detectors) were defined as "target detectors" to be analyzed for dose uniformity between multiple fractions. Data regarding the use of this device to quantify dose variation fraction-to-fraction resulting from target motion (for several delivery modalities and with and without gating) have been recently published. A combined software and hardware solution for patient-customized 4D IMRT/SBRT QA is an effective tool for assessing IMRT delivery under conditions of intrafraction motion. The 4D IMRT QA device accurately reproduced the projected motion kernels for all beam's-eye-view motion kernels. This device has been proved to, effectively quantify the degradation in dose uniformity resulting from a moving target within a static planning target volume, and, integrate with a commercial respiratory gating system to ensure that the system is working effectively. Such a device is discussed as a potential tool to optimize the gating duty cycle to maximize delivery efficiency while minimizing dose variability.

Effect of lung and target density on small-field dose coverage and PTV definition.

We have studied the effect of target and lung density on block margin for small stereotactic body radiotherapy (SBRT) targets. A phantom (50 × 50 × 50cm(3)) was created in the Pinnacle (V9.2) planning system with a 23-cm diameter lung region of interest insert. Diameter targets of 1.6, 2.0, 3.0, and 4.0cm were placed in the lung region of interest and centered at a physical depth of 15cm. Target densities evaluated were 0.1 to 1.0g/cm(3), whereas the surrounding lung density was varied between 0.05 and 0.6g/cm(3). A dose of 100cGy was delivered to the isocenter via a single 6-MV field, and the ratio of the average dose to points defining the lateral edges of the target to the isocenter dose was recorded for each combination. Field margins were varied from none to 1.5cm in 0.25-cm steps. Data obtained in the phantom study were used to predict planning treatment volume (PTV) margins that would match the clinical PTV and isodose prescription for a clinical set of 39 SBRT cases. The average internal target volume (ITV) density was 0.73 ± 0.17, average local lung density was 0.33 ± 0.16, and average ITV diameter was 2.16 ± 0.8cm. The phantom results initially underpredicted PTV margins by 0.35cm. With this offset included in the model, the ratio of predicted-to-clinical PTVs was 1.05 ± 0.32. For a given target and lung density, it was found that treatment margin was insensitive to target diameter, except for the smallest (1.6-cm diameter) target, for which the treatment margin was more sensitive to density changes than the larger targets. We have developed a graphical relationship for block margin as a function of target and lung density, which should save time in the planning phase by shortening the design of PTV margins that can satisfy Radiation Therapy Oncology Group mandated treatment volume ratios.

Patient specific 3D printed phantom for IMRT quality assurance.

The purpose of this study was to test the feasibility of a patient specific phantom for patient specific dosimetric verification.Using the head and neck region of an anthropomorphic phantom as a substitute for an actual patient, a soft-tissue equivalent model was constructed with the use of a 3D printer. Calculated and measured dose in the anthropomorphic phantom and the 3D printed phantom was compared for a parallel-opposed head and neck field geometry to establish tissue equivalence. A nine-field IMRT plan was constructed and dose verification measurements were performed for the 3D printed phantom as well as traditional standard phantoms.The maximum difference in calculated dose was 1.8% for the parallel-opposed configuration. Passing rates of various dosimetric parameters were compared for the IMRT plan measurements; the 3D printed phantom results showed greater disagreement at superficial depths than other methods.A custom phantom was created using a 3D printer. It was determined that the use of patient specific phantoms to perform dosimetric verification and estimate the dose in the patient is feasible. In addition, end-to-end testing on a per-patient basis was possible with the 3D printed phantom. Further refinement of the phantom construction process is needed for routine use.

On correlated sources of uncertainty in four dimensional computed tomography data sets.

The purpose of this work is to estimate the degree of uncertainty inherent to a given four dimensional computed tomography (4D-CT) imaging modality and to test for interaction of the investigated factors (i.e., object displacement, velocity, and the period of motion) when determining the object motion coordinates, motion envelope, and the confomality in which it can be defined within a time based data series. A motion phantom consisting of four glass spheres imbedded in low density foam on a one dimensional moving platform was used to investigate the interaction of uncertainty factors in motion trajectory that could be used in comparison of trajectory definition, motion envelope definition and conformality in an optimal 4D-CT imaging environment. The motion platform allowed for a highly defined motion trajectory that could be as the ground truth in the comparison with observed motion in 4D-CT data sets. 4D-CT data sets were acquired for 9 different motion patterns. Multifactor analysis of variance (ANOVA) was performed where the factors considered were the phantom maximum velocity, object volume, and the image intensity used to delineate the high density objects. No statistical significance was found for three factor interaction for definition of the motion trajectory, motion envelope, or Dice Similarity Coefficient (DSC) conformality. Two factor interactions were found to be statistically significant for the DSC for the interactions of 1) object volume and the HU threshold used for delineation and 2) the object velocity and object volume. Moreover, a statistically significant single factor direct proportionality was observed between the maximum velocity and the mean tracking error. In this work multiple factors impacting on the uncertainty in 4D data sets have been considered and some statistically significant two-factor interactions have been identified. Therefore, the detailed evaluation of errors and uncertainties in 4D imaging modalities is recommended in order to assess the clinical implications of interaction among the various uncertainty factors.

A method to automate the segmentation of the GTV and ITV for lung tumors.

Four-dimensional computed tomography (4D-CT) is a useful tool in the treatment of tumors that undergo significant motion. To fully utilize 4D-CT motion information in the treatment of mobile tumors such as lung cancer, autosegmentation methods will need to be developed. Using autosegmentation tools in the Pinnacle(3) v8.1t treatment planning system, 6 anonymized 4D-CT data sets were contoured. Two test indices were developed that can be used to evaluate which autosegmentation tools to apply to a given gross tumor volume (GTV) region of interest (ROI). The 4D-CT data sets had various phase binning error levels ranging from 3% to 29%. The appropriate autosegmentation method (rigid translational image registration and deformable surface mesh) was determined to properly delineate the GTV in all of the 4D-CT phases for the 4D-CT data sets with binning errors of up to 15%. The ITV was defined by 2 methods: a mask of the GTV in all 4D-CT phases and the maximum intensity projection. The differences in centroid position and volume were compared with manual segmentation studies in literature. The indices developed in this study, along with the autosegmentation tools in the treatment planning system, were able to automatically segment the GTV in the four 4D-CTs with phase binning errors of up to 15%.