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Purpose Two‐dimensional (2D) IMRT QA has been widely performed in Radiation Oncology clinic. However, concerns regarding its sensitivity in detecting delivery errors and its clinical meaning have been raised in publications. In this study, a robust methodology of three‐dimensional (3D) IMRT QA using fiducial registration and structure‐mapping was proposed to acquire organ‐specific dose information. Methods Computed tomography (CT) markers were placed on the PRESAGE dosimeter as fiducials before CT simulation. Subsequently, the images were transferred to the treatment planning system to create a verification plan for the examined treatment plan. Patient’s CT images were registered to the CT images of the dosimeter for structure mapping according to the positions of the fiducials. After irradiation, the 3D dose distribution was read‐out by an optical‐CT (OCT) scanner with fiducials shown on the OCT dose images. An automatic localization algorithm was developed in MATLAB to register the markers in the OCT images to those in the CT images of the dosimeter. SlicerRT was used to show and analyze the results. Fiducial registration error was acquired by measuring the discrepancies in 20 fiducial registrations, and thus the fiducial localization error and target registration error (TRE) was estimated. Results Dosimetry comparison between the calculated and measured dose distribution in various forms were presented, including 2D isodose lines comparison, 3D isodose surfaces with patient’s anatomical structures, 2D and 3D gamma index, dose volume histogram and 3D view of gamma failing points. From the analysis of 20 fiducial registrations, fiducial registration error was measured to be 0.62 mm and fiducial localization error was calculated to be 0.44 mm. Target registration uncertainty of the proposed methodology was estimated to be within 0.3 mm in the area of dose measurement. Conclusions This study proposed a robust methodology of 3D measurement‐based IMRT QA for organ‐specific dose comparison and demonstrated its clinical feasibility.

Purpose A novel radiochromic PRESAGE sheet (Heuris Inc.) with 3 mm thickness has been developed as a measurement tool for 2D dosimetry. Its inherent ability to conform to irregular surfaces makes this dosimeter advantageous for patient surface dosimetry. This study is a comprehensive investigation into the PRESAGE sheet’s dosimetric characteristic, accuracy and its potential use as a dosimeter for clinical applications. Methods The characterization of the dosimeter included evaluation of the temporal stability of the dose linearity, reproducibility, measurement uncertainties, dose rate, energy, temperature and angular dependence, lateral response artifacts, percent depth dose curve, and 2D dose measurement. Dose distribution measurements were acquired for regular square fields on a flat and irregular surface and an irregular modulated field on the smooth surface. All measurements were performed using an Epson 11000XL high‐resolution scanner. Results The examined dosimeters exhibit stable linear response, standard error of repeated measurements within 2%, negligible dose rate, energy, and angular dependence. The same linear dose response was measured while the dosimeter was in contact with a heated water surface. Gamma test and histogram analysis of the dose difference between PRESAGE and EBT3 film, PRESAGE and the treatment planning system (TPS) were used to evaluate the measured dose distributions. The PRESAGE sheet dose distributions showed good agreement with EBT3 film and TPS. A discrepancy smaller than the statistical error of the two dosimeters was reported. Conclusions This study established a full dosimetric characterization of the PRESAGE sheets with the purpose of laying the foundation for future clinical uses. The results presented here for the comparison of this novel dosimeter with those currently in use reinforce the possibility of using this dosimeter as an alternative for irregular surface dose measurements.

Purpose The precise dosimetric and geometric characteristics of small animal irradiators are essential to achieving reproducible dose delivery, especially in cases where image-guidance is utilized. Currently, radiochromic film is the established measurement tool used to evaluate beam characteristics for these systems. However, only 2D information can be acquired with film. This study characterized both the dosimetric and geometric properties of the small animal research radiation platform (SARRP, Xstrahl) for commissioning purposes using a 3D radiochromic dosimetry system with a submillimeter resolution optical computed tomography (OCT) scanner. Methods Like a modern clinical linear accelerator, the SARRP features both a beam delivery system and a cone beam computed tomography (CBCT) imaging system. Dosimetric and geometric characteristics of the SARRP were studied using EBT3 radiochromic film and 3D PRESAGE dosimeters. Dosimetric measurements included percent depth dose (PDD) curves and beam profiles. For geometric evaluation, the isocenter sizes of the treatment stage and gantry rotations as well as their coincidence were measured using star shot patterns. A commercial Epson Expression 11000XL flatbed scanner was used for readout of irradiated EBT3 films at 300 dpi resolution. Each irradiated PRESAGE dosimeter was scanned using a submillimeter resolution single laser beam OCT scanner. Acquired data were reconstructed with a resolution of 0.3 mm/pixel. Results PDD data measured from films and 3D dosimeters agree to within ±3% for depths up to 5 cm, for both 3 × 3 and 10 × 10 mm2 fixed collimation. Profiles were analyzed at 10, 20, and 30 mm depth for 3 × 3 mm2 and 10 × 10 mm2 fields. The FWHM measurements for both dosimeters agreed to within 0.01 mm, and the penumbras agreed to within 0.1 mm for 3 × 3 mm2 and 0.5 mm for 10 × 10 mm2 . Gantry and treatment stage isocenter sizes were determined to be 0.21 and 0.43 mm using EBT3 film, and 1.72 and 0.75 mm using PRESAGE dosimeters. Absolute isocenter shifts, evaluated with 3D phantoms, were 0.80 mm for the gantry rotation isocenter (treatment isocenter) with respect to the laser-defined setup isocenter, and 0.71 mm for the gantry rotation isocenter relative to treatment stage rotation isocenter (CBCT isocenter). The difference between CBCT isocenter and laser-defined setup isocenter was 0.68 mm. Conclusions This study demonstrated that 3D PRESAGE dosimeters can be used for verification of precise targeting for the SARRP. This 3D dosimetry system can be utilized to obtain information on both geometric and dosimetric properties, as well as acquire beam data parameters for the purpose of commissioning image-guided small animal irradiator systems.

In recent years, small animal image-guided irradiators have been widely utilized in preclinical studies involving rodent models. However, the dosimetry commissioning of such equipment involving kilovoltage small-field dosimetry has not received as much interest as the megavoltage small-field dosimetry used clinically. To date, a paucity of measured kilovoltage beam data, especially for field sizes less than 3 mm, can be found in the literature. For improvement of rodent treatments in the future, this work aims to provide comprehensive and accurate beam data for the small animal radiation research platform (SARRP, Xstrahl) using EBT3 Gafchromic films and Monte Carlo calculation, with submillimeter resolution and accuracy. This work includes three primary tasks: (1) establish an optimized film measurement protocol for small field dosimetry of kilovoltage photon beam. (2) Acquire dosimetric data including (a) depth dose curves from the surface to 6 cm depth (b) beam profiles, (c) penumbra, (d) cone factors and (e) 2D dose distribution. These tasks were undertaken for a 220 kVp photon beam with five different small field widths and 33 cm source to surface distance (0.5 mm and 1 mm circular fields, 3 × 3 mm2, 5 × 5 mm2, 10 × 10 mm2 square fields). Beam data was measured with EBT3 films. (3) Provide comparative dosimetry for film measurements, Monte Carlo calculations, and the dose calculations performed with the SARRP treatment planning system, Muriplan. For the majority of parameters, film measurement agreed with Monte Carlo simulation within 1%. There were, however, discrepancies between measured beam data and Muriplan treatment planning data. Specifically, for PDD, Muriplan underestimates the dose for field sizes of 0.5 mm and 1 mm. For beam profiles comparisons, the calculation from Muriplan predicts a smaller lateral distance between the 50% isodose lines compared to film measurement. There is a difference of 0.18, 0.72, 0.6 mm between Muriplan and film for field sizes of 3, 5, 10 mm, respectively. This work demonstrates that accurate and precise kilovoltage small-field dosimetry can be conducted using EBT3 Gafchromic film with an optimized protocol. In addition, discrepancies between measured beam data and Muriplan were identified.

Preclinical in vivo studies have drastically improved over the past decade with the development of cone beam computed tomography (CBCT) image-guided small animal irradiation systems. Such systems produce 220 or 225 kV x-rays with square and circular field sizes ranging from 0.5 to 10 mm. The dosimetry of such equipment involving kilovoltage small-field dosimetry has not received as much attention as the megavoltage small-field dosimetry. The dosimetry of megavoltage small fields can be challenging due to lateral charged particle disequilibrium, detector volume averaging effect, and high dose gradients. Clinically there has been a rapid increase in the use of small fields in modern radiotherapy techniques such as stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SBRT). This study presents dosimetric properties of image-guided small animal irradiation systems. Both EBT Gafchromic films and 3-D PRESAGE measured beam data were presented and compared with the calculated dose distribution from a commissioned planning system. For megavoltage small-field dosimetry, EBT3 films and PRESAGE dosimeters were used to measure the dose distributions for MLC-delimited 6 x 6 to 20 x 20 mm2 square fields, and for selected IMRT and VMAT plans with small field sizes or segments of 1-4 cm. A single-beam optical CT scanner was used as the readout mechanism of the radiation-induced 3-D information in the PRESAGE phantoms. Measured data sets were compared with calculated results from Eclipse Acuros XB. The results for kilovoltage small animal irradiator showed that PDD data measured from EBT films and PRESAGE dosimeters are in agreement within 3%; profiles and 2-D dose distributions measured from PRESAGE present a larger penumbra compared with those from EBT films. Discrepancies between measured beam data and treatment planning data were identified. For megavoltage small fields, the measured data percent depth dose, beam profiles, and dose distributions were found to agree within experimental uncertainties for EBT films and PRESAGE dosimeters. Beam profiles for MLC-delimited field sizes less than 10 mm reveal some discrepancies in the penumbra region between measured data and calculated results. Dose distributions from EBT film and PRESAGE measurements demonstrate that 3-D dosimetry measurement for small-field IMRT and VMAT QA may be necessary to ensure a complete verification of dose delivery and accuracy.

Performance analysis of a modified 3D dosimetry optical scanner based on the first generation optical CT scanner OCTOPUS is presented. The system consists of PRESAGE™ dosimeters, the modified 3D scanner, and a new developed in-house user control panel written in Labview program which provides more flexibility to optimize mechanical control and data acquisition technique. The total scanning time has been significantly reduced from initial 8 h to ∼2 h by using the modified scanner. The functional performance of the modified scanner has been evaluated in terms of the mechanical integrity uncertainty of the data acquisition process. Optical density distribution comparison between the modified scanner, OCTOPUS and the treatment plan system has been studied. It has been demonstrated that the agreement between the modified scanner and treatment plans is comparable with that between the OCTOPUS and treatment plans.

Purpose: A method to perform transit dosimetry with an electronic portal imaging device (EPID) by extending the commercial implementation of a published through-air portal dose image (PDI) prediction algorithm Van Esch et al.[Radiother. Oncol. 71, 223-234 (2004)] is proposed and validated. A detailed characterization of the attenuation, scattering, and EPID response behind objects in the beam path is used to convert through-air PDIs into transit PDIs. Methods: The EPID detector response beyond a range of water equivalent thicknesses (0-35 cm) and field sizes (3x3 to 22.2x29.6 cm{sup 2}) was analyzed. A constant air gap between the phantom exit surface and the EPID was utilized. A model was constructed that accounts for the beam's attenuation along the central axis, the presence of phantom scattered radiation, the detector's energy dependent response, and the difference in EPID off-axis pixel response relative to the central pixel. The efficacy of the algorithm was verified by comparing predicted and measured PDIs for IMRT fields delivered through phantoms of increasing complexity. Results: The expression that converts a through-air PDI to a transit PDI is dependent on the object's thickness, the irradiated field size, and the EPID pixel position. Monte Carlo derived narrow-beam linear attenuation coefficients are usedmore » to model the decrease in primary fluence incident upon the EPID due to the object's presence in the beam. This term is multiplied by a factor that accounts for the broad beam scatter geometry of the linac-phantom-EPID system and the detector's response to the incident beam quality. A 2D Gaussian function that models the nonuniformity of pixel response across the EPID detector plane is developed. For algorithmic verification, 49 IMRT fields were repeatedly delivered to homogeneous slab phantoms in 5 cm increments. Over the entire set of measurements, the average area passing a 3%/3mm gamma criteria slowly decreased from 98% for no material in the beam to 96.7% for 35 cm of material in the beam. The same 49 fields were delivered to a heterogeneous slab phantom and on average, 97.1% of the pixels passed the gamma criteria. Finally, a total of 33 IMRT fields were delivered to the anthropomorphic phantom and on average, 98.1% of the pixels passed. The likelihood of good matches was independent of anatomical site. Conclusions: A prediction of the transit PDI behind a phantom or patient can be created for the purposes of treatment verification via an extension of the Van Esch through-air PDI algorithm. The results of the verification measurements through phantoms indicate that further investigation through patients during their treatments is warranted.« less

Both polymer gels and PRESAGE radiochromic plastic dosimeter, in conjunction with optical CT scanning system, have been employed to measure 3-D dose distributions in radiotherapy. The 3-D dose maps obtained from these systems can provide a useful tool for dose verification on complex treatments such as IMRT, radiosurgery, and RapidArc. These complex treatments present high dose gradient regions in the boundaries between the target and the surrounding critical organs. Dose accuracy in these areas can be critical, and may affect the treatment. There is a pressing need for a dosimeter that allows for accurate determination of 3-D dose distribution with high spatial resolution. In this review paper, clinical applications of polymer gels and PRESAGE dosimeter with optical CT scanning are presented. Dosimetric performance is evaluated in terms of their sensitivity calibration, irradiation, optimization of scanning procedures, precision, and accuracy. The results demonstrate that an accurate 3-D dose distribution with high resolution can be implemented.

Purpose: Portal doseimages acquired with an amorphous silicon electronic portal imaging device(EPID) suffer from artifacts related to backscatteredradiation. The backscatter signal varies as a function of field size (FS) and location on the EPID. Most current portal dosimetry algorithms fail to account for the FS dependence. The ramifications of this omission are investigated and solutions for correcting the measured doseimages for FS specific backscatter are proposed. Methods: A series of open field doseimages were obtained for field sizes ranging from 2 × 2 to 30 × 40 cm 2 . Each image was analyzed to determine the amount of backscatter present. Two methods to account for the relationship between FS and backscatter are offered. These include the use of discrete FS specific correction matrices and the use of a single generalized equation. The efficacy of each approach was tested on the clinical dosimetricimages for ten patients, 49 treatment fields. The fields were evaluated to determine whether there was an improvement in the dosimetric result over the commercial vendor’s current algorithm. Results: It was found that backscatter manifests itself as an asymmetry in the measured signal primarily in the inplane direction. The maximum error is approximately 3.6% for 10 × 10 and 12.5 × 12.5 cm 2 field sizes. The asymmetry decreased with increasing FS to approximately 0.6% for fields larger than 30 × 30 cm 2 . The dosimetric comparison between the measured and predicted doseimages was significantly improved ( p ⪡ .001 ) when a FS specific backscatter correction was applied. The average percentage of points passing a 2%, 2 mm gamma criteria increased from 90.6% to between 96.7% and 97.2% after the proposed methods were employed. Conclusions: The error observed in a measured portal doseimage depends on how much its FS differs from the 30 × 40 cm 2 calibration conditions. The proposed methods for correcting for FS specific backscatter effectively improved the ability of the EPID to perform dosimetric measurements. Correcting for FS specific backscatter is important for accurate EPIDdosimetry and can be carried out using the methods presented within this investigation.

The dose response of the BANG 3 polymer gel dosimeter (MGS Research Inc., Madison, CT) was studied using the OCTOPUS laser CT scanner (MGS Research Inc., Madison, CT). Six 17 cm diameter and 12 cm high Barex cylinders, and 18 small glass vials were used to house the gel. The gel phantoms were irradiated with 6 and 10 MV photons, as well as 12 and 16 MeV electrons using a Varian Clinac 2100EX. Three calibration methods were used to obtain the dose response curves: (a) Optical density measurements on the 18 glass vials irradiated with graded doses from 0 to 4 Gy using 6 or 10 MV large field irradiations; (b) optical-CT scanning of Barex cylinders irradiated with graded doses (0.5, 1, 1.5, and 2 Gy) from four adjacent 4x4 cm{sup 2} photon fields or 6x6 cm{sup 2} electron fields; and (c) percent depth dose (PDD) comparison of optical-CT scans with ion chamber measurements for 6x6 cm{sup 2}, 12 and 16 MeV electron fields. The dose response of the BANG 3 gel was found to be linear and energy independent within the uncertainties of the experimental methods (about 3%). The slopes of the linearly fitted dose response curves (dose sensitivities)more » from the four field irradiations (0.0752{+-}3%, 0.0756{+-}3%, 0.0767{+-}3%, and 0.0759{+-}3% cm{sup -1} Gy{sup -1}) and the PDD matching methods (0.0768{+-}3% and 0.0761{+-}3% cm{sup -1} Gy{sup -1}) agree within 2.2%, indicating a good reproducibility of the gel dose response within phantoms of the same geometry. The dose sensitivities from the glass vial approach are different from those of the cylindrical Barex phantoms by more than 30%, owing probably to the difference in temperature inside the two types of phantoms during gel formation and irradiation, and possible oxygen contamination of the glass vial walls. The dose response curve obtained from the PDD matching approach with 16 MeV electron field was used to calibrate the gel phantom irradiated with the 12 MeV, 6x6 cm{sup 2} electron field. Three-dimensional dose distributions from the gel measurement and the Eclipse planning system (Varian Corporation, Palo Alto, CA) were compared and evaluated using 3% dose difference and 2 mm distance-to-agreement criteria.« less

Radiation therapy has been evolving with technological advances in accelerator, computer, and imaging. As the planning target volumes (PTVs) are made increasingly conformal with the advent of three-dimensional conformal therapy (3D CRT) and intensity-modulated radiation therapy (IMRT), the requirements of precise PTVs localization and its dosimetric coverage for each treatment become more stringent. In 3D CRT, based on 3D anatomic information, treatment planning is designed to deliver dose distribution that conforms as closely as possible to the target volume and minimizes the dose to the critical organs. The advantage of IMRT is to treat a patient from a number of beams of different directions (or continuous arcs) with nonuniform fluences, which have been optimized to deliver a high dose to the target volume and an acceptable low dose to the surrounding normal tissue. The treatment planning program divides each beam into a large number of beamlets and determines their optimum fluences. This optimization process involves inverse treatment planning. Prediction of normal tissue complication probability (NTCP) after external beam radiotherapy (EBRT) is an important issue in the optimization of a treatment plan and should be considered because during EBRT a considerable volume of normal tissues receives radiation dose along with the tumor. Normal tissue complication probability (NTCP), in a normal tissue, is a function of delivered dose and irradiated volume of the normal tissue. Image-guided radiation therapy (IGRT) procedures employ imaging technology to help patient setup and target localization before and during treatment. Difficulties and problems associated with target localization may arise from inter- and intracranial variations in patient setup and anatomy, including shapes and volumes of treatment target and surrounding normal tissues. Some of current IGRT image guidance technologies include portal (MV) and radiographic (kV) imagers, in-room CT scanner, kV cone-beam CT, MV cone-beam CT, helical tomotherapy MVCT, and ultrasound. Dose verification for complex treatment techniques such as IMRT, SRS/SBRT, and brachytherapy is crucial. These complex treatments present high dose gradient regions in the boundaries between the target and surrounding critical organs. Dose accuracy in these areas can be critical and may affect treatment outcome. A dose verification phantom designed for advanced technology in radiation therapy clinical trials can serve as a tool for quality assurance program. Respiratory motion affects all tumor sites in the thorax and abdomen, although the disease of most prevalence and relevance for radiotherapy is lung cancer. Studies have shown that lung tumors can move several centimeters in any direction during irradiation. If respiratory motion is not accounted for, it causes artifacts during image acquisition. These artifacts cause distortion of the target volume and incorrect positional and volumetric information [1, 2]. A promising solution for obtaining high-quality CT data in the presence of respiratory motion is 4D CT or respiration-correlated CT (conventional and cone-beam approaches) [3]. The 4D images are reconstructed from scans acquired at each respiratory phase of the breathing cycle. Four-dimensional data can be analyzed to determine the mean tumor position, tumor range of motion for treatment planning [4], and the relation of tumor trajectory to other organs and to a respiration monitor. Respiratory gating involves the administration of radiation within a particular portion of the patient's breathing cycle, commonly referred to as the “gate.” The position and width of the gate within a respiratory cycle are determined by monitoring the patient's respiratory motion, using either an external respiration signal or internal fiducial markers. Since the beam is not continuously delivered, gated procedures are longer than nongated procedures. Most radiation therapy treatment planning systems now incorporate three-dimensional anatomy information attained by computed tomography (CT) images for registration. These images can be fused with magnetic resonance imaging (MRI) or positron emission tomography (PET) images for better delineation of the target volume. MRI is considered superior to CT in soft-tissue discrimination such as central nervous system tumors and abnormalities in the brain. MRI is also used in imaging head and neck cancers, sarcomas, the prostate gland, and lymph nodes. On the other hand, CT imaging is more sensitive to bony structures and calcification. Stereotactic radiosurgery (SRS) is a single-fraction radiation therapy procedure for treating intracranial lesions or a target volume close to critical structure, using a stereotactic apparatus and multiple small-field beams. The same procedure when used for delivering hypofractionated radiation treatment is called stereotactic radiotherapy (SRT). SRS and SRT involve three-dimensional imaging to localize the lesion and delivering a concentrated dose to the target volume while sparing as much as possible the normal tissues. Two common SRS techniques are available: linac-based X-ray knife and the Co-60 gamma-knife. Light ion (ion species with an atomic number less than or equal to 10) beams have also been used for SRS and SRT [5]. Local recurrence remains a problem in a relatively large number of patients after radiotherapy. One emerging method for dose escalation to improve local results is stereotactic body radiation therapy (SBRT). SBRT refers to a stereotactic radiotherapy procedure for treating extracranial tumors with a high doses per fraction (6 to 30 Gy), with a treatment regimen of five or fewer fractions [6–9]. SBRT procedures require meticulous planning, patient immobilization, organ motion management, and state-of-the-art image guidance techniques for target localization and geometric verification. This special issue is focusing on the new development in cancer therapy, quality control, radiotherapy techniques, radiation dosimetry, and clinical outcome studies. This special issue included various topics which have been discussed from researchers of the following: clinical trials and outcome research; new technologies development and implementation; treatment delivery techniques; disease specific treatment discussion; radiation dosimetry analysis; radiation protection, shielding, and design; clinical therapy physics review and applications; molecular imaging application in radiation therapy; medical imaging; professional issues in medical, clinical, and biomedical physics; radiobiology; quality control and assurance; computing algorithm and optimization; quality of life analysis; radiation safety. This editorial provides comprehensive introduction to technological advancements in radiation therapy and many important topics associated with these treatment technologies. Clinical data on these new technologies are provided by many institutions. Tsair-Fwu Lee Jack Yang Cheng-Shie Wuu An Liu Fu-Min Fang Shyh-An Yeh

Dose distributions generated from intensity-modulated-radiation-therapy (IMRT)treatment planning present high dose gradient regions in the boundaries between the target and the surrounding critical organs. Dose accuracy in these areas can be critical, and may affect the treatment. With the increasing use of IMRT in radiotherapy, there is an increased need for a dosimeter that allows for accurate determination of three-dimensional (3D) dose distributions with high spatial resolution. In this study, polymergeldosimetry and an optical CT scanner have been employed to implement 3D dose verification for IMRT. A plastic cylinder of 17 cm diameter and 12 cm height, filled with BANG®3 polymergels (MGS Research, Inc., Madison, CT) and modified to optimal dose-response characteristics, was used for IMRTdose verification. The cylindrical gel phantom was immersed in a 24 × 24 × 20 cm water tank for an IMRT irradiation. The irradiated gel sample was then scanned with an optical CT scanner (MGS Research Inc., Madison, CT) utilizing a single He - Ne laser beam and a single photodiode detector. Similar to the x-ray CT process, filtered back-projection was used to reconstruct the 3D dose distribution. The dose distributions measured from the gel were compared with those from the IMRTtreatment planning system. For comparative dosimetry, a solid water phantom of 24 × 24 × 20 cm , having the same geometry as the water tank for the gel phantom, was used for EDR2 film and ion chamber measurements. Root mean square (rms) deviations for both dose difference and distance-to-agreement (DTA) were used in three-dimensional analysis of the dose distribution comparison between treatment planning calculations and the gel measurement. Comparison of planar dose distributions among geldosimeter, film, and the treatment planning system showed that the isodose lines were in good agreement on selected planes in axial, coronal, and sagittal orientations. Absolute point-dose verification was performed with ion chamber measurements at four different points, varying from 48% to 110% of the prescribed dose. The measured and calculated doses were found to agree to within 4.2% at all measurement points. For the comparison between the gel measurement and treatment planning calculations, rms deviations were 2%–6% for dose difference and 1 – 3 mm for DTA, at 60%–110% doses levels. The results from this study show that optical CT based polymergeldosimetry has the potential to provide a high resolution, accurate, three-dimensional tool for IMRTdose distribution verification.

Performance analysis of a commercial three-dimensional (3-D) dose mapping system based on optical CT scanning of polymer gels is presented. The system consists of BANG®3 polymer gels (MGS Research, Inc., Madison, CT), OCTOPUS™ laser CT scanner (MGS Research, Inc., Madison, CT), and an in-house developed software for optical CT image reconstruction and 3-D dose distribution comparison between the gel, film measurements and the radiation therapy treatment plans. Various sources of image noise (digitization, electronic, optical, and mechanical) generated by the scanner as well as optical uniformity of the polymer gel are analyzed. The performance of the scanner is further evaluated in terms of the reproducibility of the data acquisition process, the uncertainties at different levels of reconstructed optical density per unit length and the effects of scanning parameters. It is demonstrated that for BANG®3 gel phantoms held in cylindrical plastic containers, the relative dose distribution can be reproduced by the scanner with an overall uncertainty of about 3% within approximately 75% of the radius of the container. In regions located closer to the container wall, however, the scanner generates erroneous optical density values that arise from the reflection and refraction of the laser rays at the interface between the gel and the container. The analysis of the accuracy of the polymer gel dosimeter is exemplified by the comparison of the gel/OCT-derived dose distributions with those from film measurements and a commercial treatment planning system (Cadplan, Varian Corporation, Palo Alto, CA) for a 6cm×6cm single field of 6MV x rays and a 3-D conformal radiotherapy (3DCRT) plan. The gel measurements agree with the treatment plans and the film measurements within the “3%-or-2mm” criterion throughout the usable, artifact-free central region of the gel volume. Discrepancies among the three data sets are analyzed.

In this study BANG® polymer gels in conjunction with OCTOPUS™ optical CT scanner (MGS Research Inc., Madison, CT) was employed to measure the relative 3D dose distribution of an IMRT treatment. Measured relative dose distributions from the gel measurement were compared with those from treatment planning system calculations and EDR2 film measurements with regard to planar dose distributions in axial, coronal, and sagittal orientations.

We examined the fetal dose from irradiation of glioblastoma during pregnancy using intensity modulated radiation therapy (IMRT), and describe fetal dose minimization using mobile shielding devices. A case report is described of a pregnant woman with glioblastoma who was treated during the third trimester of gestation with 60 Gy of radiation delivered via a 6 MV photon IMRT plan. Fetal dose without shielding was estimated using an anthropomorphic phantom with ion chamber and diode measurements. Clinical fetal dose with shielding was determined with optically stimulated luminescent dosimeters and ion chamber. Clinical target volume (CTV) and planning target volume (PTV) coverage was 100 and 98 % receiving 95 % of the prescription dose, respectively. Normal tissue tolerances were kept below quantitative analysis of normal tissue effects in the clinic (QUANTEC) recommendations. Without shielding, anthropomorphic phantom measurements showed a cumulative fetal dose of 0.024 Gy. In vivo measurements with shielding in place demonstrated a cumulative fetal dose of 0.016 Gy. The fetal dose estimated without shielding was 0.04 % and with shielding was 0.026 % of the target dose. In vivo estimation of dose equivalent received by the fetus was 24.21 mSv. Using modern techniques, brain irradiation can be delivered to pregnant patients in the third trimester with very low measured doses to the fetus, without compromising target coverage or normal tissue dose constraints. Fetal dose can further be reduced with the use of shielding devices, in keeping with the principle of as low as reasonably achievable.

Purpose: The dosimetric merit of a permanent prostate implant relies on two factors: the quality of the plan itself, and the fidelity of its implementation. The former factor depends on source type and on source strength, while the latter is a combination of skill and experience. The purpose of this study is to offer criteria by which to select a source type ( 125 I or 103 Pd) and activity. Methods and Materials: Given a prescription dose and potential seed positions along needles, treatment plans were designed for a number of seed types and activities, specifically for 125 I with activities ranging from 0.3 to 0.7 mCi, and for 103 Pd with activities in the range of 0.8 to 1.6 mCi. To avoid human planner bias, an automated computerized planning system based on integer programming was used to obtain optimal seed configurations for each seed type and activity. To simulate the effect of seed-placement inaccuracies, random seed-displacement "errors" were generated for all plans. The displacement errors were assumed to be uniformly distributed within a cube with side equal to 2σ. The resulting treatment plans were assessed using two volumetric and two dosimetric indices. Results: For 125 I implants a coverage index (CI) of 98.5% or higher can be achieved for all activities (CI is the fraction of the target volume receiving the prescribed or larger dose). The external volume index (EI) (i.e., the amount of healthy tissue, as percentage of the target volume, receiving the prescribed or larger dose) increases from 13.9% to 20% as the activity increases from 0.3 to 0.7 mCi. For implants using 103 Pd, the external volume index increases from 10.2% to 13.9% whenever CI exceeds 98.5%. Volumetric and dosimetric indices (coverage index, external volume index, D90, and D80) are all sensitive to seed displacement, although the activity dependence of these indices is more pronounced for 125 I than for 103 Pd implants. Conclusions: For both isotopes, the lower activities studied systematically result in lower EIs. If seeds can be placed within approximately 0.5 cm of their intended position 103 Pd should be preferred because its EI is lower than that of 125 I. For all activities the coverage indices and D90 are within the required range. If seed placement uncertainties are larger than 0.5 cm, 125 I provides slightly better target coverage; however, in terms of external volume (healthy tissue) covered, 103 Pd is superior to 125 I.

Both (125)I and (103)Pd sources have been widely used in the permanent prostate implant. An important consideration for the choice of brachytherapy sources is the relative biological effectiveness (RBE) for the source/seed used in the implantation. As RBE is closely related to the microdosimetric parameter, it is desirable to calculate the dose mean lineal energies for both (125)I and (103)Pd at various radial distances to the seed surface. Monte Carlo simulation was performed for photons emitted from (125)I and (103)Pd. Energy depositions from photons and all their secondary electrons were tracked. Dose distributions of lineal energy, d(y), were calculated for spheres of 1 microm in diameter and at various radial distances to the seed surface. From the dose distribution of lineal energy, the dose mean lineal energy, y(D), was derived. The results showed that the radiation qualities are constant in the distance range from 0.5 to 5 cm. In this distance range, the quality factor, relative to gamma rays from (60)Co, is 2.2 for (125)I and 2.5 for (103)Pd.

Accurate isocenter positioning on the target through the treatment process is crucial for high precision stereotactic radiosurgery. This paper addresses the impact of isocenter positioning error on the dose distribution for both target and critical organs. To assess this impact, a polymer gel dosimeter and an optical CT scanner have been employed to implement 3-D dose distribution measurements.

Purpose To prospectively evaluate a 2-dimensional transit dosimetry algorithm's performance on a patient population and to analyze the issues that would arise in a widespread clinical adoption of transit electronic portal imaging device (EPID) dosimetry. Methods and Materials Eleven patients were enrolled on the protocol; 9 completed and were analyzed. Pretreatment intensity modulated radiation therapy (IMRT) patient-specific quality assurance was performed using a stringent local 3%, 3-mm γ criterion to verify that the planned fluence had been appropriately transferred to and delivered by the linear accelerator. Transit dosimetric EPID images were then acquired during treatment and compared offline with predicted transit images using a global 5%, 3-mm γ criterion. Results There were 288 transit images analyzed. The overall γ pass rate was 89.1% ± 9.8% (average ± 1 SD). For the subset of images for which the linear accelerator couch did not interfere with the measurement, the γ pass rate was 95.7% ± 2.4%. A case study is presented in which the transit dosimetry algorithm was able to identify that a lung patient's bilateral pleural effusion had resolved in the time between the planning CT scan and the treatment. Conclusions The EPID transit dosimetry algorithm under consideration, previously described and verified in a phantom study, is feasible for use in treatment delivery verification for real patients. Two-dimensional EPID transit dosimetry can play an important role in indicating when a treatment delivery is inconsistent with the original plan.

There has been much research effort in the development of an accurate and reliable threedimensional dose verification system, prompted by the advances of technologies in radiation treatment of cancer patients. New radiotherapy techniques such as intensity modulated radiation therapy (IMRT), stereotactic radiosurgery (SRS) and high dose rate (HDR) brachytherapy are aimed at dose deliveries that are highly localized within the tumor volumes. The dose distributions from these treatment methods are typically characterized with high dose gradients around the boundaries of the targets, which present certain challenges for one or two-dimensional dosimeters such as film, TLD, and ion chamber (Webb, 2001). In recent years, gel dosimeter has emerged as a promising candidate for 3D dosimetry (Baldock et al, 2010). Extensive studies have been done on the development of different types of gel formulas (Maryanski et al, 1996; Baldock et al, 1998; Pappas et al, 1999; Fong et al, 2001; Adamovics and Maryanski, 2006). Between the two classes of gel dosimeters that have been studied so far, the radiochromic Fricke gel is easier to make and handle than the polymer gel, but it suffers from a major error of diffusion. The blurring of the dose distribution image within time as short as an hour makes it inconvenient to be implemented into clinics. The polymer gel has the advantage of preserving the spatial accuracy of the dose distribution, but is sensitive to oxygen contamination and thus needs to be made freshly before each measurement. Furthermore, both types of gels suffer from a long-term drift of the baseline optical density, either due to self-oxidation of the ferrous ions in the Fricke gel or polymerization initiated by the redox reactions in the polymer gel. The readout of gel dosimeters was initially conducted by magnetic-resonance imaging (MRI) (Gore et al, 1984; Maryanski et al, 1993; Audet and Schreiner 1997; De Deene et al, 1998; Low et al, 1999; Lepage et al, 2002) and subsequently extended to other imaging modalities such as optical computed tomography (CT) (Gore et al, 1996; Kelly et al, 1998; Doran et al, 2001; Xu et al, 2003), X-ray CT (Hilts et al, 2000) and ultrasound (Mather and Baldock, 2003). The idea of optical CT was first introduced to gel dosimetry in 1996 (Gore et al, 1996) together with a new tissue equivalent polymer gel dosimeter (Maryanski et al, 1996). Since then, optical CT scanners in a variety of forms have been built for imaging gel phantoms irradiated with photon, electron, proton beams as well as brachytherapy radiation sources (Islam et al, 2003; Wuu et al, 2003; Oldham and Kim, 2004; Xu et al, 2004; Doran et al, 2006; DeJean et al, 2006

Both 3-D gels and radiochromic plastic dosimeters, in conjunction with dose image readout systems (MRI or optical-CT), have been employed to measure 3-D dose distributions in many clinical applications. The 3-D dose maps obtained from these systems can provide a useful tool for clinical dose verification for complex treatment techniques such as IMRT, SRS/SBRT, brachytherapy, and proton beam therapy. These complex treatments present high dose gradient regions in the boundaries between the target and surrounding critical organs. Dose accuracy in these areas can be critical, and may affect treatment outcome. In this review, applications of 3-D gels and PRESAGE dosimeter are reviewed and evaluated in terms of their performance in providing information on clinical dose verification as well as commissioning of various treatment modalities. Future interests and clinical needs on studies of 3-D dosimetry are also discussed.

In a high resolution image-guided small animal irradiation platform, a cone beam computed tomography (CBCT) is integrated with an irradiation unit for precise targeting. Precise quality assurance is essential for both imaging and irradiation components. The conventional commissioning techniques with films face major challenges due to alignment uncertainty and labour intensive film preparation and scanning. In addition, due to the novel design of this platform the mouse stage rotation for CBCT imaging is perpendicular to the gantry rotation for irradiation. Because these two rotations are associated with different mechanical systems, discrepancy between rotation isocenters exists. In order to deliver x-ray precisely, it is essential to verify coincidence of the imaging and the irradiation isocenters. A 3D PRESAGE dosimeter can provide an excellent tool for checking dosimetry and verifying coincidence of irradiation and imaging coordinates in one system. Dosimetric measurements were performed to obtain beam profiles and percent depth dose (PDD). Isocentricity and coincidence of the mouse stage and gantry rotations were evaluated with starshots acquired using PRESAGE dosimeters. A single PRESAGE dosimeter can provide 3 -D information in both geometric and dosimetric uncertainty, which is crucial for translational studies.

Angioplasty balloons inflated with a solution of the beta-emitter Re-188 have been used for intravascular brachytherapy to prevent restenosis. Coronary stents are in extensive clinical use for the treatment of de novo atherosclerotic stenoses. In this study, the effect of an interposed stent on the dose distribution has been measured for Re-188 balloon sources using the proprietary BANG polymer gel dosimeters and He-Ne laser-beam optical CT scanner. In polymer gels, after ionizing radiation is absorbed, free-radical chain-polymerization of soluble acrylic monomers occurs to form an insoluble polymer. The BANG polymer gel dosimeters used in these measurements allow high resolution, precise, and accurate three-dimensional determination of dosimetry from a given source. Re-188 liquid balloons, with or without an interposed metallic stent, were positioned inside thin walled tubes placed in such a polymer dosimeter to deliver a prescribed dose (e.g., 15 Gy at 0.5 mm). After removing the balloon source, each irradiated sample was mounted in the optical scanner for scanning, utilizing a single compressed He-Ne laser beam and a single photodiode. In the absence of a stent, doses at points along the balloon axis, at radial distance 0.5 mm from the balloon surface and at least 2.5 mm from the balloon ends, are within 90% of the maximum dose. This uniformity of axial dose is independent of the balloon diameter and length. Dose rate and dose uniformity for intravascular brachytherapy with Re-188 balloon are altered by the presence of stent. The dose reduction by the stent is rather constant (13%-15%) at different radial distances. However, dose inhomogeneity caused by the stent decreases rapidly with radial distance.

Conventional B-mode ultrasound currently is the standard means of imaging the prostate for guiding prostate biopsies and planning brachytherapy to treat prostate cancer. Yet B-mode images do not adequately display cancerous lesions of the prostate. Ultrasonic tissue-type imaging based on spectrum analysis of radiofrequency (rf) echo signals has shown promise for overcoming the limitations of B-mode imaging for visualizing prostate tumors. This method of tissue-type imaging utilizes nonlinear classifiers, such as neural networks, to classify tissue based on values of spectral parameter and clinical variables. Two- and three-dimensional images based on these methods demonstrate potential for guiding prostate biopsies and targeting radiotherapy of prostate cancer. Two-dimensional images are being generated in real time in ultrasound scanners used for real-time biopsy guidance and have been incorporated into commercial dosimetry software used for brachytherapy planning. Three-dimensional renderings show promise for depicting locations and volumes of cancer foci for disease evaluation to assist staging and treatment planning, and potentially for registration or fusion with CT images for targeting external-beam radiotherapy.

Brachytherapy using small implanted radioactive seeds is becoming an increasingly popular method for treating prostate cancer. Seeds are inserted into the prostate transperineally using ultrasound guidance. Dosimetry software determines the optimal placement of seeds for achieving the prescribed dose based on ultrasonic determination of the gland boundaries. However, because of prostate movement after planning images are acquired and during the implantation procedure, seeds commonly are not placed in the desired locations and the delivered dose may differ from the prescribed dose. Current methods of ultrasonic imaging do not adequately display implanted seeds for the purpose of correcting the delivered dose. We are investigating new methods of ultrasonic imaging that overcome limitations of conventional ultrasound. These methods include resonance, modified elastographic, and signature techniques. Each method shows promise for enhancing the visibility of seeds in ultrasound images. Combining the information provided by each method may reduce ambiguities in determining where seeds are present or absent. If successful, these novel imaging methods will enable correction of seed-misplacement errors during the implantation procedure, and hence will improve the therapeutic radiation dose delivered to target tissues.

Purpose: To describe our approach to intraoperative preplanning (INTRA-OP) for prostate implants and compare it to our standard method using a pre-implant volume study (STAND). Methods and Materials: Twenty patients (10 STAND, 10 INTRA-OP) were evaluated. Time required for each step of the INTRA-OP procedure was recorded. Overall procedure times and operating room times were obtained for all sessions. Postimplant dosimetry was CT-based. Results: Mean times required for each stage of the INTRA-OP procedure were as follows: Pre-implant TRUS/prostate stabilization, 26 min; image transfer, 4 min; volume outlining, 8 min; plan generation, 18 min; initial needle loading, 17 min; seed implantation, 57 min. Mean time for the implantation session was 150 min for the INTRA-OP and 120 min for the STAND groups ( p = 0.002). However, this difference is negated if the preplanning volume study is included. In addition, there was a trend toward a shorter time for the INTRA-OP patients when evaluating mean total operating room times (200 min vs. 220 min; p = 0.07). The mean postimplant %D80 for the INTRA-OP patients was 104.8% vs. 116.2% for the STAND group ( p = 0.1). The corresponding %D90 values were 85.3% and 94.6%, respectively ( p = 0.08). Conclusion: Intraoperative preplanning increased the time required for the implantation session, but appeared to decrease overall operating room time. The overall convenience of the procedure makes intraoperative preplanning an attractive technique for transperineal ultrasound-guided prostate brachytherapy.

Purpose: To determine the microdosimetric-derived relative biological (RBE) of 103 Pd, 125 I, 241 Am, and 192 Ir brachytherapy sources at low doses and/or low dose rates. Methods and Materials: The Theory of Dual Radiation Action can be used to predict expected RBE values based on the spatial distribution of energy deposition at microscopic levels from these sources. Single-event lineal energy spectra for these isotopes have been obtained both experimentally and theoretically. A grid-defined wall-less proportional counter was used to measure the lineal energy distributions. Unlike conventional Rossi proportional counters, the counter used in these measurements has a conducting nylon fiber as the central collecting anode and has no metal parts. Thus, the Z-dependence of the photoelectric effect is eliminated as a source of measurement error. Single-event spectra for these brachytherapy sources have been also calculated by: (a) the Monte Carlo code MCNP to generate the electron slowing down spectrum, (b) transport of monoenergetic electron tracks, event by event, with our Monte Carlo code DELTA, (c) using the concept of associated volume to obtain the lineal energy distribution f(y) for each monoenergetic electron, and (d) obtaining the composite lineal energy spectrum for a given brachtherapy source based on the electron spectrum caculated at step (a). Results: Relative to 60 Co, the RBE values obtained from this study are: 2.3 for 103 Pd, 2.1 for 125 I, 2.1 for 241 Am, and 1.3 for 192 Ir. Conclusions: These values are consistent with available data from invitro cell survival experiments. We suggest that, at least for these brachytherapy sources, microdosimetry may be used as a credible alternative to timeconsuming (and often uncertain) radiobiological experiments to obtain information on radiation quality and make reliable predictions of RBE in low dose rate brachytherapy.

Both polymer gels and PRESAGE radiochromic solid dosimeter, in conjunction with optical CT scanning system, have been employed to measure 3-D dose distribution. The 3-D dose maps obtained from these systems can provide a useful tool for dose verification on complex treatments such as IMRT, radiosurgery, and RapidArc. These complex treatments present high dose gradient regions in the boundaries between the target and the surrounding critical organs. Dose accuracy in these areas can be critical, and may affect the treatment. There is a pressing need for a dosimeter that allows for accurate determination of 3-D dose distribution with high spatial resolution. In this paper, performance of polymer gels and PRESAGE dosimeter with optical CT scanning is reviewed and evaluated in terms of their sensitivity calibration, irradiation, optimization of scanning procedures, precision, and accuracy. Clinical applications of optical-CT dosimetry are presented.

Preclinical studies for boron neutron capture therapy (BNCT) using epithermal neutrons are ongoing at several laboratories. The absorbed dose in tumor cells depends on the boron concentration, thermal neutron flux at depth, size of the cell, plus fast neutron and gamma contamination in the epithermal beam. Monte Carlo computer simulations can estimate the various dose components, but dosimetry and treatment planning for BNCT present unique difficulties. Dosimetry is complicated by the admixture of thermal, epithermal, and fast neutrons, plus gamma rays; and the array of secondary high linear energy transfer (LET) particles produced within the patient from neutron interactions. Absorbed dose and radiation quality will be difficult to determine, and microdosimetry may be a viable technique for determining these quantities. Only one set of microdosimetric data for BNCT has previously been reportedl. Spectra were measured in air for an epithermal beam, using a tissue equivalent (TE) proportional counter to assess the effects of beam filter designs. We report here the first set of in phantom microdosimetric measurements using paired TE and TE+boron chambers on an epithermal beam. Such measurements permit assessment of the dose enhancement factor, and lineal energy distributions of the boron capture reaction.

Thermoluminescent dosimeter-CaSO4/Teflon discs were prepared by the authors. Series of experiments were applied to investigate the photon energy dependence of these TLD discs. The results showed that the energy dependence of TLD discs is almost the same as that of TLD powder. The decrease of the proportion of TL phosphor embedded in teflon does reduce the sensitivity but not affect the energy dependence of TLD discs unless the grain size of TL phosphor is smaller than several microns. A personnel beta-gamma dosimeter in connection with proper filters was designed. The photon energy dependence can be corrected and the effects of beta and gamma can be distinguished in this dosimeter.