ATOM Phantom Research Articles

Development of Female ATOM-MIRD Hybrid Voxel Model for Monte Carlo Dose Calculations

A hybrid-type female computational model was constructed by combining the tomographic voxel model of the ATOM adult female physical phantom with the mathematical organ models of the MIRD5 stylized phantom. The constructed model has 13 internal organs: 5 organs from the ATOM phantom and the other 8 organs from the MIRD5 phantom. The model was employed in Monte Carlo dose calculations with MCNPX to calculate organ doses, and the calculated values were compared with the values of Regina for reference external irradiation conditions.

MO‐EE‐A2‐06: Measurements and Comparison of Out‐Of‐Field Organ Doses From Varian Clinac IMRT Plans Using the Atom Phantom

Purpose: To quantify and analyze the unwanted out‐of‐field organ doses during different IMRT prostate treatments and make a systematical comparison of previously reported organ doses. Method and Materials: The measurement involved an ATOM Phantom, thermoluminescent dosimeters (TLD) and a Varian 2100C Clinac. The ATOM phantom was scanned using CT machine to create virtual images for treatment planning. We developed two 6‐MV prostate IMRT plans of 5‐ and 9‐field with the same dose prescriptions, constraints to organs‐at‐risk, and similar in‐field dose distributions. A total of 162 TLDs were placed in the predefined holes to measure the average organ doses of 7 different organs. The results of the two treatment plans were compared to estimate the difference of the out‐of‐field doses. Results: The measurements showed that dose decreases as the distances from the measurement point to the radiation field increases for all treatment plans. The average out‐of‐field organ doses ranged from a maximum of 30.92 cGy to the small intestines to a minimum of 4.40 cGy to the thyroid. The 9‐field IMRT plan resulted in an absorbed dose up to 30% higher than the 5‐field plan which is mainly due to the increased accumulative scatter and lower MU efficiency of the 9‐field IMRT plan. The results were also compared with other studies and the discrepancies were mainly due to the different distances from organs to treatment fields. Conclusion: A method to use TLD and the ATOM physical phantom to measure out‐of‐field organ doses for IMRT has been presented. The measurement results demonstrated that the 9‐field IMRT treatment delivered higher photon doses than the 5‐field IMRT treatment to the out‐of‐field organs. Findings from this study involving the prostate treatments express the need to implement the out‐of‐field dose estimation to the treatment planning in order to minimize the secondary cancer and other radiation risk.

SU-GG-T-198: Measurements and Evaluation of Doses Outside the Treatment Volume From Photon External Beam Radiation Therapy

Purpose: There has been increased concern about the amount of dose delivered outside the treatment volume from advanced radiation therapy techniques. It has been suggested that these treatments, such as IMRT, IGRT, and SBRT, present a potential impact on the induction of second malignancies due to a larger scattered and leakage radiation. Children have much higher risks to develop a radiation-induced cancer from radiotherapy. In this study, doses outside the treatment volume calculated from the Eclipse planning system were evaluated and compared with measurements from MOSFET and TL dosimeters. An anthropomorphic phantom and a 3-D polymer gel dosimeter were employed for measurements. Method and Materials: The “ATOM” anthropomorphic phantom was CT- scanned into the Eclipse-Helios system. An IMRT prostate plan was designed for the ATOM phantom. Both MOSFET detectors and TLD were calibrated based on ion chamber dosimetry. The MOSFET and TLD were precisely placed in positions corresponding to various internal organs, allowing point-dose measurements and comparison. BANG® polymer gel, prepared in a cylindrical container with a volume similar to that of a 10-year child, was placed next to the phantom head for 3D dose distribution measurement in a brain IMRT. The DVH in the gel cylinder, analyzed with an optical CT scanner, was compared with that from the planning system. Results: Our results show that the agreement between the MOSFET measurements and the calculated results are within 5% for points within the target. At medium-dose regions (2–50%), discrepancies are within 15%. At low dose regions (<2%) the measured results from MOSFET and TLD agree well, and can be 2–3 times larger than the planning results. DVH comparison between gels and the treatment planning will be presented. Conclusion: Anthropomorphic phantom with MOSFET and TL detectors as well as polymer gels can provide evaluation of doses outside the treatment volume.

WE‐C‐224A‐05: Patient Modeling and Organ Dose Calculations Using Monte Carlo Methods

The risk for a patient to develop secondary cancers from non‐target exposures can be assessed from the equivalent doses to various radiosensitive organs. To determine organ doses, a computational model or physical phantom that represents the whole patient anatomy must be used. Physical phantoms (such as the RANDO and ATOM phantoms) have tiny cavities on each of the tissue‐equivalent slices for inserting TL and MOSFET dosimeters. When beam is delivered according to a patient's treatment plan, the dosimeters are processed to provide organ dose information. The experimental procedures can be time‐consuming and expensive, especially when various patients and treatment plans are studies. Virtual patient models, on the other hand, can be combined with a Monte Carlo code to simulate the transport of radiation in the body. These models cover the entire body and typically contain a large number of defined organs. Coupled with a model of the accelerator, one can calculate detailed information about secondary dose distributions in the patient body. Whole‐body models are classified into three types: 1) Stylized models that are based on surface equations, 2) Tomographic models that are derived from medical images, and 3) Hybrid equation‐voxel models that describe organ boundaries using advanced primitives such as the NURBS for realtime deformation (4D simulations). To date, more than 20 tomographic models have been developed for radiation protection and nuclear medicine applications. An international consortium on computational human models (www.virtualphantoms.org) has been recently formed to promote research in this area. A team of researchers from Rensselaer, Vanderbilt, University of Florida, Massachusetts General Hospital and Johns Hopkins University is working on a project to standardize a library of age‐ and gender‐specific models.This lecture presents the current status of patient modeling and the application of various tools to study secondary doses from radiation treatment involving both photons and protons. Detailed information on the VIP‐Man model developed from the Visible Human Project is presented. Segmentation of more than 80 organs, including the red bone marrow, and the implementation of the VIP‐Man model into EGS, MCNP/X and GEANT4 are discussed. Finally, organ doses and effective doses for proton treatment plans using various adult and pediatric patient models are presented.Educational Objectives of this symposium are:1. Understanding new tools of Monte Carlo‐based patient and accelerator modeling for secondary dose studies.2. Understanding the effective dose data for various modalities.

SU‐FF‐T‐172: Verification of Whole‐Body Dosimetry in An IMRT Treatment Planning System

Purpose: IMRT has been widely used in radiation therapy, since this technique shows potential for further improving the therapeutic ratio and reducing complications. On the other hand, it has been suggested that IMRT presents a potential impact on the induction of second malignancies, because it can result in a higher whole‐body dose due to leakage radiation. In the routine treatment planning process, complete information on the whole‐body dose‐volume histogram is not available due to the limited patient body volume imaged in the CT treatment planning process. In addition, for IMRT, larger volumes of normal tissues are being exposed to low doses, and the dosimetric uncertainties of a treatment planning system at these doses are relatively large. In this study, whole‐body dosimetry calculated from the Eclipse‐Helios planning system was verified using a whole‐body anthropomorphic phantom and MOSFET detectors, as well as polymer gels. Method and Materials: The “ATOM” whole‐body anthropomorphic phantom was CT‐ scanned into the Eclipse‐Helios system. An IMRT prostate plan was designed for the ATOM phantom. Each MOSFET detector was calibrated at various angles based on ion chamber dosimetry. The MOSFET detectors were precisely placed in relocatable dosimeter positions corresponding to various internal organs, allowing point‐dose measurements and comparison. BANG® polymer gel, prepared in a cylindrical container, was placed at the phantom head position to measure the 3D dose distribution. The DVH in the gel cylinder, analyzed with an optical CT scanner, was compared with that from the planning system. Results: Preliminary results show that the agreement between the MOSFET measurements and the calculated results is within 5% for points within the target. At low‐dose regions (0.1–60%), discrepancies are larger but reasonable. DVH comparison between gels and the treatment planning will be presented. Conclusion: Anthropomorphic phantom with MOSFET detectors and polymer gels can provide whole‐body dosimetry verification for IMRT.