Comparison of Dosimetric Performance among Commercial Quality Assurance Systems for Verifying Pretreatment Plans of Stereotactic Body Radiotherapy Using Flattening-Filter-Free Beams.

The purpose of this study was to compare the performance of different commercial quality assurance (QA) systems for the pretreatment verification plan of stereotactic body radiotherapy (SBRT) with volumetric arc therapy (VMAT) technique using a flattening-filter-free beam. The verification for 20 pretreatment cancer patients (seven lung, six spine, and seven prostate cancers) were tested using three QA systems (EBT3 film, I'mRT MatriXX array, and MapCHECK). All the SBRT-VMAT plans were optimized in the Eclipse (version 11.0.34) treatment planning system (TPS) using the Acuros XB dose calculation algorithm and were delivered to the Varian TrueBeam® accelerator equipped with a high-definition multileaf collimator. Gamma agreement evaluation was analyzed with the criteria of 2% dose difference and 2 mm distance to agreement (2%/2 mm) or 3%/3 mm. The highest passing rate (99.1% for 3%/3 mm) was observed on the MapCHECK system while the lowest passing rate was obtained on the film. The pretreatment verification results depend on the QA systems, treatment sites, and delivery beam energies. However, the delivery QA results for all QA systems based on the TPS calculation showed a good agreement of more than 90% for both the criteria. It is concluded that the three 2D QA systems have sufficient potential for pretreatment verification of the SBRT-VMAT plan.

Radiobiological model-based bio-anatomical quality assurance in intensity-modulated radiation therapy for prostate cancer.

A bio-anatomical quality assurance (QA) method employing tumor control probability (TCP) and normal tissue complication probability (NTCP) is described that can integrate radiobiological effects into intensity-modulated radiation therapy (IMRT). We evaluated the variations in the radiobiological effects caused by random errors (r-errors) and systematic errors (s-errors) by evaluating TCP and NTCP in two groups: patients with an intact prostate (G(intact)) and those who have undergone prostatectomy (G(tectomy)). The r-errors were generated using an isocenter shift of ±1 mm to simulate a misaligned patient set-up. The s-errors were generated using individual leaves that were displaced inwardly and outwardly by 1 mm on multileaf collimator field files. Subvolume-based TCP and NTCP were visualized on computed tomography (CT) images to determine the radiobiological effects on the principal structures. The bio-anatomical QA using the TCP and NTCP maps differentiated the critical radiobiological effects on specific volumes, particularly at the anterior rectal walls and planning target volumes. The s-errors showed a TCP variation of -40-25% in G(tectomy) and -30-10% in G(intact), while the r-errors were less than 1.5% in both groups. The r-errors for the rectum and bladder showed higher NTCP variations at ±20% and ±10%, respectively, and the s-errors were greater than ±65% for both. This bio-anatomical method, as a patient-specific IMRT QA, can provide distinct indications of clinically significant radiobiological effects beyond the minimization of probable physical dose errors in phantoms.

Development of a novel quality assurance system based on rolled-up and rolled-out radiochromic films in volumetric modulated arc therapy.

PURPOSE: To develop a cylindrical phantom with rolled-up radiochromic films and dose analysis software in the rolled-out plane for quality assurance (QA) in volumetric modulated arc therapy (VMAT). METHODS: The phantom consists of an acrylic cylindrical body wrapped with radiochromic film inserted into an outer cylindrical shell of 5 cm thickness. The rolled-up films with high spatial resolution enable detection of specific dose errors along the arc trajectory of continuously irradiated and modulated beams in VMAT. The developed dose analysis software facilitates dosimetric evaluation in the rolled-up and rolled-out planes of the film; the calculated doses on the corresponding points where the rolled-up film was placed were reconstructed into a rectangular dose matrix equivalent to that of the rolled-out plane of the film. The VMAT QA system was implemented in 3 clinical cases of prostate, nasopharynx, and pelvic metastasis. Each calculated dose on the rolled-out plane was compared with measurement values by modified gamma evaluation. Detected positions of dose disagreement on the rolled-out plane were also distinguished in cylindrical coordinates. The frequency of error occurrence and error distribution were summarized in a histogram and in an axial view of rolled-up plane to intuitively identify the corresponding positions of detected errors according to the gantry angle. RESULTS: The dose matrix reconstructed from the developed VMAT QA system was used to verify the measured dose distribution along the arc trajectory. Dose discrepancies were detected on the rolled-out plane and visualized on the calculated dose matrix in cylindrical coordinates. The error histogram obtained by gamma evaluation enabled identification of the specific error frequency at each gantry angular position. The total dose error occurring on the cylindrical surface was in the range of 5%-8% for the 3 cases. CONCLUSIONS: The developed system provides a practical and reliable QA method to detect dosimetric errors according to the gantry angle. Film dosimetry based on rolled-up and rolled-out techniques leads to dose verification in the subspaces of the 3D dose volume. The system can be employed as an alternative tool to detect the pitfalls of planar dose verification.

Comparison of target coverage and dose to organs at risk between simultaneous integrated-boost whole-field intensity-modulated radiation therapy and junctioned intensity-modulated radiation therapy with a conventional radiotherapy field in treatment of nasopharyngeal carcinoma.

We have retrospectively investigated 15 nasopharyngeal carcinoma patients treated at our institution between March 2007 and August 2009. We used simultaneous integrated-boost whole field intensity-modulated radiation therapy (SIB WF-IMRT) to treat the entire planning target volume in the head and neck cancer. All of the SIB WF-IMRT plans were replanned by use of the junctioned intensity modulated radiation therapy (J-IMRT) technique for comparison. The effect on target coverage and sparing of organs at risk, including laryngeal sparing in the optimal SIB WF-IMRT plan was compared with that achieved with use of the J-IMRT technique. The mean larynx dose and standard deviation was 25.2 ± 5.8 Gy for SIB WF-IMRT and 19.8 ± 16.8 Gy for J-IMRT. A comparison between SIB WF-IMRT and the J-IMRT technique demonstrated that the larynx dose was increased in SIB WF-IMRT. However, when the strong dose constraint was applied to the larynx and the pseudo-volume was used for a steep dose fall-off immediately outside the target, the SIB WF-IMRT technique would have led to a larynx dose comparable to that achieved with J-IMRT. Therefore, in our current practice we use the SIB WF-IMRT technique, which does not have the problem of setup error at the match line for treatment of nasopharyngeal carcinoma.

An impact source localization technique for a nuclear power plant by using sensors of different types.

In a nuclear power plant, a loose part monitoring system (LPMS) provides information on the location and the mass of a loosened or detached metal impacted onto the inner surface of the primary pressure boundary. Typically, accelerometers are mounted on the surface of a reactor vessel to localize the impact location caused by the impact of metallic substances on the reactor system. However, in some cases, the number of accelerometers is not sufficient to estimate the impact location precisely. In such a case, one of useful methods is to utilize other types of sensor that can measure the vibration of the reactor structure. For example, acoustic emission (AE) sensors are installed on the reactor structure to detect leakage or cracks on the primary pressure boundary. However, accelerometers and AE sensors have a different frequency range. The frequency of interest of AE sensors is higher than that of accelerometers. In this paper, we propose a method of impact source localization by using both accelerometer signals and AE signals, simultaneously. The main concept of impact location estimation is based on the arrival time difference of the impact stress wave between different sensor locations. However, it is difficult to find the arrival time difference between sensors, because the primary frequency ranges of accelerometers and AE sensors are different. To overcome the problem, we used phase delays of an envelope of impact signals. This is because the impact signals from the accelerometer and the AE sensor are similar in the whole shape (envelope). To verify the proposed method, we have performed experiments for a reactor mock-up model and a real nuclear power plant. The experimental results demonstrate that we can enhance the reliability and precision of the impact source localization. Therefore, if the proposed method is applied to a nuclear power plant, we can obtain the effect of additional installed sensors.

Performance evaluation of field-in-field technique for tangential breast irradiation.

Conventional hard or dynamic wedge systems are commonly applied to reduce the dose inhomogeneity associated with whole breast irradiation. We evaluated the dosimetric benefits of the field-in-field (FIF) technique by comparing it with the electronic compensator (EC), Varian enhanced dynamic wedge (EW) and conventional hard wedge (HW) techniques. Data were obtained from 12 patients who had undergone breast-conserving surgery (six left-sided and six right-sided). For these patients, the average breast planning target volume (PTV) was 447.4 cm(3) (range, 211.6-711.8 cm(3)). For the experiments, a 6 MV photon beam from a Varian 21 EX was used, the HW and EW angles were applied from 15 to 45 degrees, while 40-50% isodose values were chosen to achieve the best dose distribution for electronic compensation. In applying the FIF technique, we used two or three subfields for each portal. To evaluate the performance for each planning technique, we analysed a dose-volume histogram (DVH) for the PTV and organs-at-risk (OARs). To evaluate the effects of these techniques on dose inhomogeneity, we defined the PTV Dose Improvement (PDI) index, which was derived from a PTV volume between 97-103% of the differential DVHs. In addition, we compared the average monitor units (MUs) for each technique. The average PDI index with FIF is 76.4%, while the PDI indices for other treatments were 65.8, 41.8 and 50.9% for EC, EW and HW, respectively. This study demonstrated an improved performance using the FIF technique compared with the conventional HW/EW system, as well as a new modality for EC. We demonstrated that FIF is a very useful technique for improving PTV conformity, while protecting the OARs from breast tangential irradiation.

Effects of static dosimetric leaf gap on MLC-based small-beam dose distribution for intensity-modulated radiosurgery.

The aim of the present study was to evaluate the effect of various specific dosimetric leaf gaps on the multileaf collimator (MLC)-based small-beam dose distribution. The dosimetric static leaf gap was determined by comparing the profiles of small MLC-based beams with those of small collimated fields (square fields of 1, 2, 3, and 4 cm). The results showed that an approximately 2-mm gap was optimal with the Millennium 120-leaf MLC (Varian Medical Systems, Palo Alto, CA) and a Varian 21EX 6-MV photon beam. We also investigated how much the leaf gap affects the planning results and the actual dose distribution. A doughnut-shaped planning target volume (PTV, 6.1 cm3) and inner organ at risk (OAR, 0.3 cm3) were delineated for delicate intensity-modulated radiosurgery test planning. The applied leaf gaps were 0, 1, and 2 mm. The measured dose distributions were compared with the dose distribution in the treatment planning system. The maximum dose differences at inside PTV, outside PTV, and inner OAR were, respectively, 22.3%, 20.2%, and 35.2% for the 0-mm leaf gap; 17.8%, 22.8%, and 30.8% for the 1-mm leaf gap; and 5.5%, 8.5%, and 6.3% for the 2-mm leaf gap. In a human head phantom (model 605: CIRS, Norfolk, VA) study, large dose differences of 1.3%-12.7% were noted for the measurements made using the MLC files generated by the three different leaf gaps. The planned results were similar, and measurements showed a large dose difference associated with the various leaf gaps. These results strongly suggest that plans generated by a commercial inverse planning system commissioned using general collimated field data will probably demonstrate discrepancies between the planned treatments and the measured results.

Self-organization of amide dendrons and their dendronized macromolecules.

A polymerizable methacryl unit was introduced at the focal moiety of the amide dendrons which have amide branches and alkyl periphery. Their dendronized polymers were also prepared by the radical polymerization of the methacryl units. The self-organization characteristics of dendrons and dendronized polymers were then investigated in both the organic and aqueous phases. The amide dendrons (1M and 2M) in which the focal carboxyl group was blocked with methacryl units did not form gel in organic media such as chloroform or THF, whereas amide dendrons with a free carboxyl group at the focal point form self-organized structures. In the aqueous phase, 1M and 2M formed spherical vesicular assemblies. The dendronized polymers with first and second generation dendrons, 1P and 2P, respectively, exhibited lamellar and columnar organization in toluene. In addition to hydrogen bonding between the dendritic amide branches and van der Waals interactions between the alkyl periphery, steric confinement of dendritic side groups along the polymer backbone played a key role in the packing process of the dendronized polymers. In aqueous phase, 1P and 2P showed spherical vesicular aggregates with persistent stability in the presence of Triton X-100.