Significant efforts such as the Pediatric Proton/Photon Consortium Registry (PPCR) involving multiple proton therapy centers have been made to conduct collaborative studies evaluating outcomes following proton therapy. As a groundwork dosimetry effort for the late effect investigation, we developed a Monte Carlo (MC) model of proton pencil beam scanning (PBS) to estimate organ/tissue doses of pediatric patients at the Maryland Proton Treatment Center (MPTC), one of the proton centers involved in the PPCR. The MC beam modeling was performed using the TOPAS (TOol for PArticle Simulation) MC code and commissioned to match measurement data within 1% for range, and 0.3 mm for spot sizes. The established MC model was then tested by calculating organ/tissue doses for sample intracranial and craniospinal irradiations on whole-body pediatric computational human phantoms. The simulated dose distributions were compared with the treatment planning system dose distributions, showing the 3 mm/3% gamma index passing rates of 94%–99%, validating our simulations with the MC model. The calculated organ/tissue doses per prescribed doses for the craniospinal irradiations (1 mGy Gy−1 to 1 Gy Gy−1) were generally much higher than those for the intracranial irradiations (2.1 μGy Gy−1 to 0.1 Gy Gy−1), which is due to the larger field coverage of the craniospinal irradiations. The largest difference was observed at the adrenal dose, i.e. ∼3000 times. In addition, the calculated organ/tissue doses were compared with those calculated with a simplified MC model, showing that the beam properties (i.e. spot size, spot divergence, mean energy, and energy spread) do not significantly influence dose calculations despite the limited irradiation cases. This implies that the use of the MC model commissioned to the MPTC measurement data might be dosimetrically acceptable for patient dose reconstructions at other proton centers particularly when their measurement data are unavailable. The developed MC model will be used to reconstruct organ/tissue doses for MPTC pediatric patients collected in the PPCR.

Purpose: To demonstrate an on-demand and nearly automatic method for fabricating tissue-equivalent physical anthropomorphic phantoms for imaging and dosimetry applications using a dual nozzle thermoplastic three-dimensional (3D) printer and two types of plastic. Methods: Two 3D printing plastics were investigated: (a) Normal polylactic acid (PLA) as a soft tissue simulant and (b) Iron PLA (PLA-Fe), a composite of PLA and iron powder, as a bone simulant. The plastics and geometry of a 1-yr-old computational phantom were combined with a dual extrusion 3D printer to fabricate an anthropomorphic imaging phantom. The volumetric fill density of the 3D-printed parts was varied to approximate tissues of different radiographic density using a calibration curve relating the printer infill density setting to measured CT number. As a demonstration of our method we printed a 10 cm axial cross-section of the computational phantom's torso at full scale. We imaged the phantom on a CT scanner and compared HU values to those of a 1-yr-old patient and a commercial 5-yr-old physical phantom. Results: The phantom was printed in six parts over the course of a week. The printed phantom included 30 separate anatomical regions including soft tissue remainder, lungs (left and right), heart, esophagus, rib cage (left and right ribs 1 to 10), clavicles (left and right), scapulae (left and right), thoracic vertebrae (one solid object defining thoracic vertebrae T1 to T9). CT scanning of the phantom showed five distinct radiographic regions (heart, lung, soft tissue remainder, bone, and air cavity) despite using only two types of plastic. The 3D-printed phantom demonstrated excellent similarity to commercially available phantoms, although key limitations in the printer and printing materials leave opportunity for improvement. Conclusion: Patient-specific anthropomorphic phantoms can be 3D printed and assembled in sections for imaging and dosimetry applications. Such phantoms will be useful for dose verification purposes when commercial phantoms are unavailable for purchase in the specific anatomies of interest.

No abstract

Radiotherapy treatment planning systems are designed for the fast calculation of dose to the tumor bed and nearby organs at risk using x-ray computed tomography (CT) images. However, CT images for a patient are typically available for only a small portion of the body, and in some cases, such as for retrospective epidemiological studies, no images may be available at all. When dose to organs that lie out-of-scan must be estimated, a convenient alternative for the unknown patient anatomy is to use a matching whole-body computational phantom as a surrogate. The purpose of the current work is to connect such computational phantoms to commercial radiotherapy treatment planning systems for retrospective organ dose estimation. A custom software with graphical user interface, called the DICOM-RT Generator, was developed in MATLAB to convert voxel computational phantoms into the Digital Imaging and Communications in Medicine radiotherapy (DICOM-RT) format, compatible with commercial treatment planning systems. DICOM CT image sets for the phantoms are created via a density-to-Hounsfield unit conversion curve. Accompanying structure sets containing the organ contours are automatically generated by tracing binary masks of user-specified organs on each phantom CT slice. The software was tested on a library of body size-dependent phantoms, the International Commission on Radiological Protection reference phantoms, and a canine voxel phantom, taking only a few minutes per conversion. The resulting DICOM-RT files were tested on several commercial treatment planning systems. As an example application, a library of converted phantoms was used to estimate organ doses for members of the National Wilms Tumor Study cohort. The converted phantom library, in DICOM format, and a standalone MATLAB-compiled executable of the DICOM-RT Generator are available for others to use for research purposes (http://ncidose.cancer.gov).

No abstract

No abstract

For external irradiation, the variability in organ dose estimation found between computational phantom generations arises particularly from the differences in organ positioning. This work represents the first effort to quantify the differences in organ depth below the body surface between a stylized and voxel phantom series. Herein, the revised Oak Ridge National Laboratory stylized phantom series and the University of Florida/National Cancer Institute voxel phantom series were compared. Both series include whole-body models of the newborn; the 1-, 5-, 10-, and 15-year-old; and the adult human. Organ depths from eight different directions applicable to external irradiation geometries were computed: antero-posterior, postero-anterior, left and right lateral, rotational, isotropic, cranial and caudal directions. Organ depths in the stylized phantoms were computed using a ray-tracing technique available through Monte Carlo radiation transport simulations in MCNP6. Organ depths in the voxel phantom were found using phantom matrix manipulation. Resultant organ depths for both series were plotted as distributions; available are twenty-four organs and two bone tissue distributions for each of six phantom ages and in each of the eight directional geometries. Quantitative data descriptors (e.g. mean and median depths) were also tabulated. For demonstration purposes, a literature review of relevant stylized versus voxel comparison works was performed to explore where the quantification of organ depth differences can provide further insight or evidence to study conclusions. The entire dataset of organ depth distributions and their data descriptors can be found in online supplementary files.

Objective: We conducted a Monte Carlo study to comprehensively investigate the fetal dose resulting from proton pencil beam scanning (PBS) craniospinal irradiation (CSI) during pregnancy. Approach: The gestational-age dependent pregnant phantom series developed at the University of Florida (UF) were converted into DICOM-RT format (CT images and structures) and imported into a treatment planning system (TPS) (Eclipse v15.6) commissioned to a IBA PBS nozzle. A proton PBS CSI plan (prescribed dose: 36 Gy) was created on the phantoms. The TOPAS MC code was used to simulate the proton PBS CSI on the phantoms, for which MC beam properties at the nozzle exit (spot size, spot divergence, mean energy, and energy spread) were matched to IBA PBS nozzle beam measurement data. We calculated mean absorbed doses for 28 organs and tissues and whole body of the fetus at eight gestational ages (8, 10, 15, 20, 25, 30, 35, and 38 weeks). For contextual purposes, the fetal organ/tissue doses from the treatment planning CT scan of the mother’s head and torso were estimated using the National Cancer Institute dosimetry system for CT (NCICT, Version 3) considering a low-dose CT protocol (CTDIvol: 8.97 mGy). Main Results: The majority of the fetal organ/tissue doses from the proton PBS CSI treatment fell within a range of 3 to 6 mGy. The fetal organ/tissue doses for the 38-week phantom showed the largest variation with the doses ranging from 2.9 mGy (adrenals) to 8.2 mGy (eye lenses) while the smallest variation ranging from 3.2 mGy (oesophagus) to 4.4 mGy (brain) was observed for the doses for the 20-week phantom. The fetal whole-body dose ranged from 3.7 mGy (25 weeks) to 5.8 mGy (8 weeks). Most of the fetal doses from the planning CT scan fell within a range of 7 to 13 mGy, approximately 2-to-9 times lower than the fetal dose equivalents of the proton PBS CSI treatment (assuming a quality factor of 7). Significance: The fetal organ/tissue doses observed in the present work will be useful for one of the first clinically informative predictions on the magnitude of fetal dose during proton PBS CSI during pregnancy.