Matthew B. Podgorsak

Using an EPID for patient‐specific VMAT quality assurance

PURPOSE A patient-specific quality assurance (QA) method was developed to verify gantry-specific individual multileaf collimator (MLC) apertures (control points) in volumetric modulated arc therapy (VMAT) plans using an electronic portal imaging device (EPID). METHODS VMAT treatment plans were generated in an Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (aS1000) acquired in continuous acquisition mode were used for pretreatment QA. Each cine image file contains the grayscale image of the MLC aperture related to its specific control point and the corresponding gantry angle information. The TPS MLC file of this RapidArc plan contains the leaf positions for all 177 control points (gantry angles). In-house software was developed that interpolates the measured images based on the gantry angle and overlays them with the MLC pattern for all control points. The 38% isointensity line was used to define the edge of the MLC leaves on the portal images. The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement at each control point and the frequency in which each particular leaf mismatches for the entire arc. RESULTS Seven patients plans were analyzed using this method. The leaves with the highest mismatched rate were found to be treatment plan dependent. CONCLUSIONS This in-house software can be used to automatically verify the MLC leaf positions for all control points of VMAT plans using cine images acquired by an EPID.

EPID dosimetry for pretreatment quality assurance with two commercial systems

This study compares the EPID dosimetry algorithms of two commercial systems for pretreatment QA, and analyzes dosimetric measurements made with each system alongside the results obtained with a standard diode array. 126 IMRT fields are examined with both EPID dosimetry systems (EPIDose by Sun Nuclear Corporation, Melbourne FL, and Portal Dosimetry by Varian Medical Systems, Palo Alto CA) and the diode array, MapCHECK (also by Sun Nuclear Corporation). Twenty‐six VMAT arcs of varying modulation complexity are examined with the EPIDose and MapCHECK systems. Optimization and commissioning testing of the EPIDose physics model is detailed. Each EPID IMRT QA system is tested for sensitivity to critical TPS beam model errors. Absolute dose gamma evaluation (3%, 3 mm, 10% threshold, global normalization to the maximum measured dose) yields similar results (within 1%–2%) for all three dosimetry modalities, except in the case of off‐axis breast tangents. For these off‐axis fields, the Portal Dosimetry system does not adequately model EPID response, though a previously‐published correction algorithm improves performance. Both MapCHECK and EPIDose are found to yield good results for VMAT QA, though limitations are discussed. Both the Portal Dosimetry and EPIDose algorithms, though distinctly different, yield similar results for the majority of clinical IMRT cases, in close agreement with a standard diode array. Portal dose image prediction may overlook errors in beam modeling beyond the calculation of the actual fluence, while MapCHECK and EPIDose include verification of the dose calculation algorithm, albeit in simplified phantom conditions (and with limited data density in the case of the MapCHECK detector). Unlike the commercial Portal Dosimetry package, the EPIDose algorithm (when sufficiently optimized) allows accurate analysis of EPID response for off‐axis, asymmetric fields, and for orthogonal VMAT QA. Other forms of QA are necessary to supplement the limitations of the Portal Vision Dosimetry system. PACS numbers: 87.53.Bn, 87.53.Jw, 87.53.Kn, 87.55.Qr, 87.56.Fc, 87.57.uq

Nuclear magnetic relaxation characterization of irradiated Fricke solution

The spin-lattice relaxation rate R1(= T1(-1) of irradiated Fricke solution was studied as a function of the absorbed dose D. The R1 increases linearly with dose up to D approximately 400 Gy after which the response saturates. A model describing the R1 of a solution of either ferrous (Fe2+) or ferric (Fe3+) ions is presented; it is based on the spin relaxation of protons on water molecules in the bulk and protons on water molecules in the coordination shells of the ions with fast exchange occurring between the two water environments. All inherent relaxation parameters of the different proton groups are determined empirically at NMR frequencies of 9 and 25 MHz. An extension of the model is made to describe the spin-lattice relaxation behavior of irradiated Fricke solution. Good agreement between model predictions and experimental results is observed. The model relates the spin-lattice relaxation rate of a Fricke dosimeter to the chemical yield of ferric ion, thus potentially creating an absolute NMR dosimetry technique. Various practical aspects of the NMR-Fricke system, such as the optimal initial ferrous concentration and the NMR frequency dependence of the sensitivity, are described.

American Society for Therapeutic Radiology and Oncology (ASTRO) Emerging Technology Committee Report on Electronic Brachytherapy

The development of novel technologies for the safe and effective delivery of radiation is critical to advancing the field of radiation oncology. The Emerging Technology Committee of the American Society for Therapeutic Radiology and Oncology appointed a Task Group within its Evaluation Subcommittee to evaluate new electronic brachytherapy methods that are being developed for, or are already in, clinical use. The Task Group evaluated two devices, the Axxent Electronic Brachytherapy System by Xoft, Inc. (Fremont, CA), and the Intrabeam Photon Radiosurgery Device by Carl Zeiss Surgical (Oberkochen, Germany). These devices are designed to deliver electronically generated radiation, and because of their relatively low energy output, they do not fall under existing regulatory scrutiny of radioactive sources that are used for conventional radioisotope brachytherapy. This report provides a descriptive overview of the technologies, current and future projected applications, comparison of competing technologies, potential impact, and potential safety issues. The full Emerging Technology Committee report is available on the American Society for Therapeutic Radiology and Oncology Web site.

TECHNICAL MANAGEMENT OF A PREGNANT PATIENT UNDERGOING RADIATION THERAPY TO THE HEAD AND NECK

The fetal dose in a pregnant patient undergoing radiation therapy to the head and neck region was investigated. Implicit in this study was the design and evaluation of a shield used to minimize the fetal dose. To evaluate the fetal dose, a phantom was irradiated with the fields designed for this patients therapy. The peripheral dose was measured for each field individually, both without and with a custom shield designed to be placed about the patients abdominal and pelvic regions. The total dose at the location of the fetus over the course of this patients radiation therapy was then estimated from peripheral dose rate measurements made at several points within the simulated uterus. With no shielding, the total dose within the uterus of the patient would have ranged from 13.3 cGy at the cervix to 28 cGy at the fundus. With the shield applied, the uterine dose was significantly less: 3.3 cGy at the cervix to 8.6 cGy at the fundus. In fact, at every measurement point, the peripheral dose with the shield in place was 30% to 50% of the dose without the shield. Some data suggest that the rate of significant abnormalities induced by irradiation in utero increases with increasing dose within the range of total peripheral doses incurred during most radiation treatment courses. It is therefore prudent to make reasonable attempts at minimizing the dose to the lower abdominal and pelvic regions of any pregnant patient. The shield designed in this work accomplished this goal for this patient and is flexible enough to be used in the treatment of almost all tumor volumes.

Characterization of the response of commercial diode detectors used for in vivo dosimetry

The response of a commercially available diode-based in vivo dosimetry system was studied over a selection of clinically relevant photon beam setups. The dosimetry system consists of a dedicated multichannel electrometer with several diode detectors differing only in their equivalent wall buildup. Each detector is calibrated for a specific nominal beam energy and used clinically with that energy only. To study dosimeter response, a diode taped to the surface of a solid water phantom was irradiated simultaneously with an end-window chamber placed at a depth of dmax inside the same phantom. Photon beams with energies of Co-60, 6 and 18 MV were used. For each beam energy, the response of the diode relative to the given dose as measured by the end-window chamber was evaluated for open and wedged fields (0 degree to 60 degrees) with source-to-surface distances (SSDs) ranging from 75 to 120 cm and collimator settings from 5 x 5 to 40 x 40 cm2. It was found that diode response, i.e., diode reading per cGy of given dose, varies significantly with treatment beam setup. For example, increasing field size for a constant SSD causes a decrease of up to 15% in diode response relative to the given dose for 6 and 18 MV beams, while for Co-60 an increase in response of up to 5% results. Furthermore, increasing SSD for a fixed collimator setting results in decreased diode response (up to 10%) for all beams. The complicated dependence of diode response on beam setup necessitates the use of empirical response curves, similar to those evaluated in this work, to accurately convert clinical dosimeter reading to dose at depth.

VOLUMETRIC MODULATED ARC THERAPY VS. IMRT FOR THE TREATMENT OF DISTAL ESOPHAGEAL CANCER

Several studies have demonstrated that volumetric modulated arc therapy (VMAT) has the ability to reduce monitor units and treatment time when compared with intensity-modulated radiation therapy (IMRT). This study aims to demonstrate that VMAT is able to provide adequate organs at risk (OAR) sparing and planning target volume (PTV) coverage for adenocarcinoma of the distal esophagus while reducing monitor units and treatment time. Fourteen patients having been treated previously for esophageal cancer were planned using both VMAT and IMRT techniques. Dosimetric quality was evaluated based on doses to several OARs, as well as coverage of the PTV. Treatment times were assessed by recording the number of monitor units required for dose delivery. Body V(5) was also recorded to evaluate the increased volume of healthy tissue irradiated to low doses. Dosimetric differences in OAR sparing between VMAT and IMRT were comparable. PTV coverage was similar for the 2 techniques but it was found that IMRT was capable of delivering a slightly more homogenous dose distribution. Of the 14 patients, 12 were treated with a single arc and 2 were treated with a double arc. Single-arc plans reduced monitor units by 42% when compared with the IMRT plans. Double-arc plans reduced monitor units by 67% when compared with IMRT. The V(5) for the body was found to be 18% greater for VMAT than for IMRT. VMAT has the capability to decrease treatment times over IMRT while still providing similar OAR sparing and PTV coverage. Although there will be a smaller risk of patient movement during VMAT treatments, this advantage comes at the cost of delivering small doses to a greater volume of the patient.

The transit dose component of high dose rate brachytherapy: direct measurements and clinical implications

PURPOSE To measure the transit dose produced by a moving high dose rate brachytherapy source and assess its clinical significance. METHODS AND MATERIALS The doses produced from source movement during Ir-192 HDR afterloading were measured using calibrated thermoluminescent dosimeter rods. Transit doses at distances of 0.5-4.0 cm from an endobronchial applicator were measured using a Lucite phantom accommodating 1 x 1 x 6 mm thermoluminescent rods. Surface transit dose measurements were made using esophageal and endobronchial catheters, a gynecologic tandem, and an interstitial needle. RESULTS No difference was detected in thermoluminescent dosimeter rod responses to 4 MV and Ir-192 spectra (427 nC/Gy) in a range of dose between 2 and 300 cGy. The transit dose at 0.5 cm from an endobronchial catheter was 0.31 cGy/(Curie-fraction) and followed an inverse square fall-off with increasing distance. Surface transit doses ranged from 0.38 cGy/(Curie-fraction) for an esophageal catheter to 1.03 cGy/(Curie-fraction) for an endobronchial catheter. Source velocity is dependent on the interdwell distance and varies between 220-452 mm/sec. A numeric algorithm was developed to calculate total transit dose, and was based on a dynamic point approximation for the moving high dose rate source. This algorithm reliably predicted the empirical transit doses and demonstrated that total transit dose is dependent on source velocity, number of fractions, and source activity. Surface transit doses are dependent on applicator diameter and wall material and thickness. Total transit doses within or outside the desired treatment volume are typically < 100 cGy, but may exceed 200 cGy when using a large number of fractions with a high activity source. CONCLUSION Current high dose rate brachytherapy treatment planning systems calculate dose only from source dwell positions and assume a negligible transit dose. Under certain clinical circumstances, however, the transit dose can exceed 200 cGy to tissues within and outside the prescribed treatment volume. These additional, unrecognized doses could increase potential late tissue complications, as predicted by the linear quadratic model. To enhance the clinical safety and accuracy of high dose rate brachytherapy, total transit dose should be included in calculated isodose distributions. Significant transit doses to tissues outside the treatment volume should be documented.

A dosimetric evaluation of VMAT for the treatment of non‐small cell lung cancer

The purpose of this study was to demonstrate the dosimetric potential of volumetric‐modulated arc therapy (VMAT) for the treatment of patients with medically inoperable stage I/II non‐small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT). Fourteen patients treated with 3D CRT with varying tumor locations, tumor sizes, and dose fractionation schemes were chosen for study. The prescription doses were 48 Gy in 4 fractions, 52.5 Gy in 5 fractions, 57.5 Gy in 5 fractions, and 60 Gy in 3 fractions for 2, 5, 1, and 6 patients, respectively. VMAT treatment plans with a mix of two to three full and partial noncoplanar arcs with 5°–25° separations were retrospectively generated using Eclipse version 10.0. The 3D CRT and VMAT plans were then evaluated by comparing their target dose, critical structure dose, high dose spillage, and low dose spillage as defined according to RTOG 0813 and RTOG 0236 protocols. In the most dosimetrically improved case, VMAT was able to decrease the dose from 17.35 Gy to 1.54 Gy to the heart. The D2cm decreased in 11 of 14 cases when using VMAT. The three that worsened were still within the acceptance criteria. Of the 14 3D CRT plans, seven had a D2cm minor deviation, while only one of the 14 VMAT plans had a D2cm minor deviation. The R50% improved in 13 of the 14 VMAT cases. The one case that worsened was still within the acceptance criteria of the RTOG protocol. Of the 14 3D CRT plans, seven had an R50% deviation. Only one of the 14 VMAT plans had an R50% deviation, but it was still improved compared to the 3D CRT plan. In this cohort of patients, no evident dosimetric compromises resulted from planning SBRT treatments with VMAT relative to the 3D CRT treatment plans actually used in their treatment. PACS numbers: 87.50.‐a, 87.53.‐j, 87.55.‐x, 87.55.D‐, 87.55.dk, 87.55.de

Measurements of the electron dose distribution near inhomogeneities using a plastic scintillation detector

PURPOSE Accurate measurement of the electron dose distribution near an inhomogeneity is difficult with traditional dosimeters which themselves perturb the electron field. We tested the performance of a new high resolution, water-equivalent plastic scintillation detector which has ideal properties for this application. METHODS AND MATERIALS A plastic scintillation detector with a 1 mm diameter, 3 mm long cylindrical sensitive volume was used to measure the dose distributions behind standard benchmark inhomogeneities in water phantoms. The plastic scintillator material is more water equivalent than polystyrene in terms of its mass collision stopping power and mass scattering power. Measurements were performed for beams of electrons having initial energies of 6 and 18 MeV at depths from 0.2-4.2 cm behind the inhomogeneities. RESULTS The detector reveals hot and cold spots behind heterogeneities at resolutions equivalent to typical film digitizer spot sizes. Plots of the dose distributions behind air, aluminum, lead, and formulations for cortical and inner bone-equivalent materials are presented. CONCLUSION The plastic scintillation detector is suited for measuring the electron dose distribution near an inhomogeneity.