Preliminary study of the Craniofacial Pain and Disability Inventory-11: validation for patients with head and neck cancer.

BACKGROUND: Cancer involves numerous physical, psychological and emotional changes and has a negative impact on patients. Although there are a wide variety of questionnaires for general use in patients with cancer, very few are available that assess the pain, disability and craniomandibular functionality of patients with head and neck cancer (HNC) in a more specific manner. The purpose of this study is to present the preliminary behavior of the CF-PDI in its reduced version adapted for patients with HNC. MATERIAL AND METHODS: A total of 61 patients with HNC were included in a study to preliminarily analyze the internal consistency of the instrument, the convergent validity and the floor and ceiling effects. All the patients completed the informed consent document and a battery of 5 questionnaires: The Numerical Rating Scale (NRS), the Tampa Scale for Kinesiophobia for Temporomandibular Disorders (TSK-TMD), the Pain Catastrophizing Scale (PCS), the Quality of Life Questionnaire in patients with HNC (QLQ-HN) and the reduced version of the Craniofacial Pain and Disability Inventory (CF-PDI-11). Patients also performed 2 physical tests: measurements of the pain threshold on the masseter muscle and on the distal phalanx of the first finger; and the maximum mouth opening in neutral head position. RESULTS: Cronbach's α coefficient showed a very high internal consistency of 0.92. In terms of convergent validity, a statistically significant correlation was found between the CF-PDI-11 and the following variables: NRS, TSK-TMD, PCS, QLQ-HN, the threshold of pain in the distal phalanx of the first finger and the maximum interincisal opening. However, 21.3% of patients obtained the lowest possible score. The strongest correlation was found between the CF-PDI-11 and the QLQ-HN (r = 0.85, p <0.01). CONCLUSIONS: The preliminary version of the CF-PDI-11 shows that it could be a valid and reliable instrument to measure pain, disability and quality of life in patients with HNC.

Impact of charge collection efficiency and electronic noise on the performance of solid-state 3D microdetectors.

Microdosimetry has been traditionally performed through gaseous proportional counters, although in recent years different solid-state microdosimeters have been proposed and constructed for this task. In this paper, we analyze the response of solid-state devices of micrometric size with no intrinsic gain developed by CNM-CSIC (Spain). There are two major aspects of the operation of these devices that affect the reconstruction of the probability distributions and momenta of stochastic quantities related to microdosimetry. For micrometric volumes, the drift and diffusion of the charge carriers gives rise to a partial charge collection efficiency in the peripheral region of the depleted volume. This effect produces a perturbation of the reconstructed pulse height (i.e. imparted energy) distributions with respect to the actual microdosimetric distributions. The relevance of this deviation depends on the size, geometry and operating conditions of the device. On the other hand, the electronic noise from the single-event readout set-up poses a limit on the minimum detectable lineal energy when the microdosimeter size is reduced. This article addresses these issues to provide a framework on the physical constraints for the design and operation of solid-state microdosimeters.

Recombination in liquid-filled ionization chambers beyond the Boag limit.

PURPOSE: The high mass density and low mobilities of charge carriers can cause important recombination in liquid-filled ionization chambers (LICs). Saturation correction methods have been proposed for LICs. Correction methods for pulsed irradiation are based on Boag equation. However, Boag equation assumes that the charge ionized by one pulse is fully collected before the arrival of the next pulse. This condition does not hold in many clinical beams where the pulse repetition period may be shorter than the charge collection time, causing overlapping between charge carriers ionized by different pulses, and Boag equation is not applicable there. In this work, the authors present an experimental and numerical characterization of collection efficiencies in LICs beyond the Boag limit, with overlapping between charge carriers ionized by different pulses. METHODS: The authors have studied recombination in a LIC array for different dose-per-pulse, pulse repetition frequency, and polarization voltage values. Measurements were performed in a Truebeam Linac using FF and FFF modalities. Dose-per-pulse and pulse repetition frequency have been obtained by monitoring the target current with an oscilloscope. Experimental collection efficiencies have been obtained by using a combination of the two-dose-rate method and ratios to the readout of a reference chamber (CC13, IBA). The authors have also used numerical simulation to complement the experimental data. RESULTS: The authors have found that overlap significantly increases recombination in LICs, as expected. However, the functional dependence of collection efficiencies on the dose-per-pulse does not change (a linear dependence has been observed in the near-saturation region for different degrees of overlapping, the same dependence observed in the nonoverlapping scenario). On the other hand, the dependence of collection efficiencies on the polarization voltage changes in the overlapping scenario and does not follow that of Boag equation, the reason being that changing the polarization voltage also affects the charge collection time, thus changing the amount of overlapping. CONCLUSIONS: These results have important consequences for saturation correction methods for LICs. On one hand, the two-dose-rate method, which relies on the functional dependence of the collection efficiencies on dose-per-pulse, can also be used in the overlapping situation, provided that the two measurements needed to feed the method are performed at the same pulse repetition frequency (monitor unit rate). This result opens the door to computing collection efficiencies in LICs in many clinical setups where charge overlap in the LIC exists. On the other hand, correction methods based on the voltage-dependence of Boag equation like the three-voltage method or the modified two-voltage method will not work in the overlapping scenario due to the different functional dependence of collection efficiencies on the polarization voltage.

Study of the PTW microLion chamber temperature dependence.

The use of liquid ionization chambers in radiotherapy has grown during the past few years. While for air ionization chambers the k(TP) correction for air mass density due to pressure and temperature variations is well known, less work has been done on the case of liquid ionization chambers, where there is still the need to take into account the influence of temperature in the free ion yield. We have measured the PTW microLion isooctane-filled ionization chamber temperature dependence in a ~ ±10 °C interval around the standard 20 °C room temperature for three operation voltages, including the manufacturer recommended voltage, and two beam qualities, (60)Co and 50 kV x-rays. Within the measured temperature range, the microLion signal exhibits a positive linear dependence, which is around 0.24% K(-1) at 800 V with (60)Co irradiation. This effect is of the same order of magnitude as the T dependence found in air ionization chambers, but its nature is completely different and its sign opposite to that of an air chamber. Onsager theory has been used to model the results and is consistent with this linear behaviour. However, some inconsistencies in the modelling of the 50 kV x-ray results have been found that are attributed to the failure of Onsager's isolated pair assumption for such radiation quality.

Evaluation of chamber response function influence on IMRT verification using 2D commercial detector arrays.

This work is devoted to studying the influence of chamber response functions on the standard IMRT verification for the different detector technologies available on commercial devices. We have tested three of the most used 2D detector arrays for radiotherapy dosimetry verification, based on air-ionization chambers and diode detectors. The response function has been carefully simulated using the Monte Carlo method and measured through slit and pinhole collimators. Although the response function of air-ionization detectors is considerably different with respect to that of standard diodes, the impact on a verification based in the gamma function with tolerances 3 mm and 3% is quite limited. The results show that the standard air-ionization detector arrays perform in a similar way whenever the tolerances for the gamma function are not lowered below 1.5 mm and 1.5%. Additionally, the sensitivity of these devices to fluence perturbations was measured by intentionally modifying some leaf positions in the multileaf collimator. The wider response function of air-ionization chamber arrays made them slightly more sensitive to random fluence perturbations, although silicon diode arrays are more accurate to describe the dose distribution in a point by point basis.

A convolution model for obtaining the response of an ionization chamber in static non standard fields.

PURPOSE: This work contains an alternative methodology for obtaining correction factors for ionization chamber (IC) dosimetry of small fields and composite fields such as IMRT. The method is based on the convolution/superposition (C/S) of an IC response function (RF) with the dose distribution in a certain plane which includes chamber position. This method is an alternative to the full Monte Carlo (MC) approach that has been used previously by many authors for the same objective. METHODS: The readout of an IC at a point inside a phantom irradiated by a certain beam can be obtained as the convolution of the dose spatial distribution caused by the beam and the IC two-dimensional RF. The proposed methodology has been applied successfully to predict the response of a PTW 30013 IC when measuring different nonreference fields, namely: output factors of 6 MV small fields, beam profiles of cobalt 60 narrow fields and 6 MV radiosurgery segments. The two-dimensional RF of a PTW 30013 IC was obtained by MC simulation of the absorbed dose to cavity air when the IC was scanned by a 0.6 × 0.6 mm(2) cross section parallel pencil beam at low depth in a water phantom. For each of the cases studied, the results of the IC direct measurement were compared with the corresponding obtained by the C/S method. RESULTS: For all of the cases studied, the agreement between the IC direct measurement and the IC calculated response was excellent (better than 1.5%). CONCLUSIONS: This method could be implemented in TPS in order to calculate dosimetry correction factors when an experimental IMRT treatment verification with in-phantom ionization chamber is performed. The miss-response of the IC due to the nonreference conditions could be quickly corrected by this method rather than employing MC derived correction factors. This method can be considered as an alternative to the plan-class associated correction factors proposed recently as part of an IAEA work group on nonstandard field dosimetry.

SU-E-T-105: The LET Dependence of Liquid Ionization Chambers (LICs) in High-LET Beams.

PURPOSE: LICs are novel detectors for radiotherapy: the higher density of the medium allows to build them with a smaller sensitive volume, making them appealing in particle therapy. With RBE varying along the depth dose curve (DDC) and with the rising interest in dose/LET-painting, verifying the LET is becoming more important. Nevertheless, while the LET distributions for different ionizing particles have been calculated, they have never been directly measured in realistic therapeutic beams. Our interest in LICs is based on the characterization of the beam quality in terms of LET. It has been shown in earlier works that the extrapolation of the linear portion of the voltage curve yields an intercept with the x-axis that depends on LET. The quantitative establishment of this method, however, depends on how accurately recombination effects are taken into account. METHODS: Due to the higher density of charge carriers produced in the liquid, LICs have high recombination effects: general recombination effects, involving pairs belonging to different tracks (dose rate dependent), and initial recombination between ion-electron pairs belonging to the same incident particle event (LET dependent). To perform this study we propose a two-dimensional array of LICs, composed by a 16×8 matrix of 2×2 mm2 pixels, which gives a fine spatial resolution on the plane. RESULTS: Voltage curves have been measured for proton, carbon and oxygen beams available at the HIT facility in Heidelberg for different energies and dose rates. After correcting the curves for general recombination losses using the Three Voltage Method, we have indications of dose rate independence, indicating successful correction. CONCLUSIONS: Further investigations are foreseen to quantify the LET dependence along the DDC, where different LET values are expected. A comparison with simulated dose averaged LET values will give quantitative information about 2D LET distributions for different beam species.

Determining charge collection efficiency in parallel-plate liquid ionization chambers.

An accurate computation of the collection efficiency due to recombination has a capital importance in order to perform high-precision dose measurements with ionization chambers. The two-voltage method, developed to compute charge collection efficiency in gas chambers, cannot be directly applied to liquid-filled ionization chambers (LICs) due to the ionized charge strong dependence on the collection electric field, which is caused by initial recombination. It is shown that in order to apply the two-voltage method to parallel-plate LICs it is necessary to introduce explicitly the slope of the ionization charge yield with respect to the polarization voltage applied in the chamber. A three-voltage method is also proposed to avoid the introduction of the slope parameter. That method allows decoupling initial and general recombination. It leads to a nonlinear system of equations which can be solved using numerical methods.