A novel angular dependency model for MatriXX response and its application to true composite dose verification for IMRT plans.

PURPOSE: This paper proposes a model for the angular dependency of MatriXX response and investigates whether MatriXX, with the angular-model-based approach can be applied to true composite dose verification for IMRT plans. METHOD: This model attributes the angular dependence of MatriXX response to dynamical translation of its effective measurement plane (EMP) due to the change of beam angle. Considering this mechanism, true composite dose verifications for IMRT plans specified in AAPM TG 119 report using both MatriXX and Gafchromic EBT3 films were undertook and compared to validate the applicability of MatriXX for patient specific QA of composite beam IMRT plans. Dose verifications using MatriXX with and without angular-model-based approach were performed. RESULTS: MatriXX with angular-model-based approach achieved gamma passing rates with 3%/3 mm and 3%/2 mm criteria better than 98.3% and 98.1% respectively for true composite dose verification of plans in AAPM TG 119 report. The 3%/3 mm and 3%/2 mm gamma passing rates using MatriXX without angular-model-based approach ranged from 85.8% to 98.2% and from 81.3% to 96.5%, respectively. The p-values from the single sided paired t-test indicated no statistical difference between the passing rates from MatriXX with angular-model-based approach and from films, and significant difference between the passing rates from uncorrected MatriXX and from films. CONCLUSION: The proposed model for angular dependent MatriXX response is necessary and effective. Dose verification using MatriXX with angular-model-based approach is acceptable for true composite beam IMRT plans with required accuracy to simplify patient specific QA.

Frequency-Coded Chipless RFID Tags: Notch Model, Detection, Angular Orientation, and Coverage Measurements.

This paper focuses on the frequency coded chipless Radio Frequency Identification (RFID) wherein the tag's information bits are physically encoded by the resonators' notch position which has an effect on the frequency spectrum of the backscattered or retransmitted signal of the tag. In this regard, the notch analytical model is developed to consider the notch position and quality factor. Besides, the radar cross section (RCS) mathematical representation of the tag is introduced to consider the incident wave's polarization and orientation angles. Hence, the influences of the incident wave's orientation and polarization mismatches on the detection performance are quantified. After that, the tag measurement errors and limitations are comprehensively explained. Therefore, approaches to measureing RCS- and retransmission-based tags are introduced. Furthermore, the maximum reading range is theoretically calculated and practically verified considering the Federal Communications Commission (FCC) Ultra Wideband (UWB) regulations. In all simulations and experiments conducted, a mono-static configuration is considered, in which one antenna is utilized for transmission and reception.

Characteristics of the Exradin W1 scintillator in the magnetic field.

To investigate the angular dependency of the W1 scintillator with and without a magnetic field, the beam incidence angles to the detector varied from 0° to 360° at intervals of 30° when the detector was pointed in both the craniocaudal and right-to-left directions. The beam incidence angles also varied from 0° to 360° at intervals of 45° when the W1 scintillator was in the anterior-to-posterior direction. To investigate the field size dependency of the W1 scintillator with and without a magnetic field, the doses by an identical beam-on time were measured at various square field sizes and the measured doses were normalized to the dose at the field of 10.5 cm × 10.5 cm (FS10.5). With and without a magnetic field, the deviations of the doses to the dose at the beam incident angle of 0° were always less than 1% regardless of the dosimeter positioning relative to the magnetic field direction. When the field sizes were equal to or less than FS10.5, the differences in the output factors with and without a magnetic field were less than 0.7%. However, those were larger than 1% at fields larger than FS10.5, and up to 3.1%. The W1 scintillator showed no angular dependency to the magnetic field. Differences larger than 1% in the output factors with and without a magnetic field were observed at field sizes larger than 10.5 cm × 10.5 cm.

Angular insensitivity of a gamma spectrometer for in-field applications.

In-field measurements have particular challenges as compared with those conducted under laboratory conditions. Besides unknown source shielding, the source-detector distance varies and the detector orientation relative to the incident radiation is not necessarily constant. The incoming flux facing a detector is a parallel beam at long source-detector distances (>1 m). The counting efficiency depends on the tilting angle relative to the beam facing the detector. In principle, a cylindrical detector with a height-diameter ratio of π/4 (H/D = 0.785) exhibits the lowest angular dependency (41% at low energies for a tilting angle of 45° as compared with the orientation of the detector end cap relative to the beam). However, Monte Carlo simulations of a germanium detector showed that this variability can be greatly improved by slightly increasing H/D (0.84) and introducing a copper cladding around the detector (1.1 mm). The counting efficiency of such a detector is almost independent of the direction of photons arriving to the detector. The maximum deviation of 10% takes place at 200 keV.

Robust Beam Selection Based on Water Equivalent Thickness Analysis in Passive Scattering Carbon-Ion Radiotherapy for Pancreatic Cancer.

Carbon-ion radiotherapy (CIRT) is one of the most effective radiotherapeutic modalities. This study aimed to select robust-beam configurations (BC) by water equivalent thickness (WET) analysis in passive CIRT for pancreatic cancer. The study analyzed 110 computed tomography (CT) images and 600 dose distributions of eight patients with pancreatic cancer. The robustness in the beam range was evaluated using both planning and daily CT images, and two robust BCs for the rotating gantry and fixed port were selected. The planned, daily, and accumulated doses were calculated and compared after bone matching (BM) and tumor matching (TM). The dose-volume parameters for the target and organs at risk (OARs) were evaluated. Posterior oblique beams (120-240°) in the supine position and anteroposterior beams (0° and 180°) in the prone position were the most robust to WET changes. The mean CTV V95% reductions with TM were -3.8% and -5.2% with the BC for gantry and the BC for fixed ports, respectively. Despite ensuring robustness, the dose to the OARs increased slightly with WET-based BCs but remained below the dose constraint. The robustness of dose distribution can be improved by BCs that are robust to ΔWET. Robust BC with TM improves the accuracy of passive CIRT for pancreatic cancer.

Beam angle optimization using angular dependency of range variation assessed via water equivalent path length (WEPL) calculation for head and neck proton therapy.

PURPOSE: To investigate angular sensitivity of proton range variation due to anatomic change in patients and patient setup error via water equivalent path length (WEPL) calculations. METHODS: Proton range was estimated by calculating WEPL to the distal edge of target volume using planning CT (pCT) and weekly scatter-corrected cone-beam CT (CBCT) images of 11 head and neck patients. Range variation was estimated as the difference between the distal WEPLs calculated on pCT and scatter-corrected CBCT (cCBCT). This WEPL analysis was performed every five degrees ipsilaterally to the target. Statistics of the distal WEPL difference were calculated over the distal area to compare between different beam angles. Physician-defined contours were used for the WEPL calculation on both pCT and cCBCT, not considering local deformation of target volume. It was also tested if a couch kick (10°) can mitigate the range variation due to anatomic change and patient setup error. RESULTS: For most of the patients considered, median, 75% quantile, and 95% quantile of the distal WEPL difference were largest for posterior oblique angles, indicating a higher chance of overdosing normal tissues at distal edge with these angles. Using a couch kick resulted in decrease in the WEPL difference for some posterior oblique angles. CONCLUSIONS: It was demonstrated that the WEPL change has angular dependency for the cohort of head and neck cancer patients. Selecting beam configuration robust to anatomic change in patient and patient setup error may improve the treatment outcome of head and neck proton therapy.