Aspects of chemotherapy and photon and proton radiotherapy in patients with gastric cancer

Gastric cancer remains a major health problem worldwide. The addition of chemotherapy alone or in combination with radiotherapy to surgery in local gastric cancer improves outcome. In more advanced stages, the optimal palliative chemotherapy remains unknown, as well as the effect of different regimens on the patients’ quality of life. The aim of this thesis was to explore a new concept in chemotherapy, i.e. the sequential approach, and a new modality in radiotherapy, i.e. proton therapy, in the treatment of patients with gastric cancer. Quality of life (QoL) in patients treated with chemotherapy, and target delineation in radiotherapy of gastric cancer, were also studied. In Paper I, we evaluated the efficacy of sequential chemotherapy in patients with locally advanced and/or metastatic gastric cancer, with alternating irinotecan and docetaxel in combination with infusion 5-Fu. Eighty-one patients were randomized. No differences favoring either arm were found with respect to response rate, overall survival (OS), or toxicity. The median OS of 11 months indicated that the sequential approach was effective and similar to triple combinations, with potentially less toxicity. In Paper II, we evaluated the effect of sequential chemotherapy on the QoL in the same cohort. It was measured before, during, and after treatment. There were no statistically significant differences in QoL scores between the two treatment arms and no changes in mean scores during treatment. During the last 8 weeks of treatment, a significantly larger portion of patients with radiological response reported sustained or better QoL scores than those with no radiological response. In Paper III, we investigated the effect of inter physician variation on the delineation of target volumes in gastric cancer patients treated with perioperative chemoradiotherapy (CRT). Despite the use of a delineation atlas, we found a large variation in CTV and PTV volumes. There was only a small variation in target coverage and doses to organs at risk (OARs) in the corresponding plans. In Paper IV, we compared proton therapy to modern photon radiotherapy with respect to doses to OARs in gastric cancer patients treated with perioperative CRT. Protons offered significantly lower doses to the left kidney, liver, and spinal cord, and statistically lower risks for all types and malignant secondary neoplasms compared to photons. In Paper V, we evaluated the importance of daily anatomical variations, i.e. intestinal gas filling, on the dose distribution of proton beam therapy. The effect of intestinal gas variations on the PTV/CTV coverage was large. The sparing effect of protons was, however, sustained or the dose to the OARs did not significantly exceed the dose delivered with photons. In conclusion, sequential chemotherapy and proton radiotherapy are attractive alternatives in the treatment of gastric cancer. Standardization of target definitions in CRT, e.g. by reducing the inter physician variation, is important and should also be further investigated

Dielectric mixtures -- electrical properties and modeling

In this paper, a review on dielectric mixtures and the importance of the numerical simulations of dielectric mixtures are presented. It stresses on the interfacial polarization observed in mixtures. It is shown that this polarization can yield different dielectric responses depending on the properties of the constituents and their concentrations. Open question on the subject are also introduced.Comment: 40 pages 12 figures, to be appear in IEEE Trans. on Dielectric

Field-dependent electric conductivities of silicone rubbers deduced from measured currents and surface potential decay characteristics

Measurements of DC electric conductivities of four types of silicone rubber-based polymers for use in high voltage insulation systems of power components are reported. The field dependences of the conductivities obtained by two different techniques, namely by utilizing steady-state currents through materials’ samples placed between metallic electrodes and by employing surface potential decay characteristics in an open circuit configuration, are compared and discussed. It is shown that the surface potential decay technique allows for a wider range of electric field strength and reduces the time span of the measurements

Thermally stimulated currents of Zn-Bi-O thin film arresters

Ceramic varistors based on zinc oxide have excellent properties as protection devices used in power industry. However, their breakdown voltage, dependent on number of grain boundaries, is too high for use in electronic applications. In this work, performance of micro-devices having varistor-type current-voltage characteristics with low breakdown voltage is reported. The thermally stimulated depolarisation current (TSDC) technique was used to study the dielectric relaxation of the Zn-Bi-0 thin-film arresters. The surface varistor layers were prepared by r.f. magnetron sputtering on nickel support. The TSDC measuring system and cryostat apparatus used in experiment have been described

Temperature and Field Induced Variations of Electric Conductivities of HTV Silicone Rubbers Derived from Measured Currents and Surface Potential Decay Characteristics

The temperature and field dependencies of electric conductivities of two types of silicone rubber-based polymers intended for use in high voltage direct current applications are presented and discussed. The conductivities obtained with the standard method by measuring a current through the material sample placed between metallic electrodes in response to the applied voltage are compared with those deduced from the measured potential decay on pre-charged material surface in an open circuit configuration. The measurements were conducted in the range of the applied electric field strength (0.5–5) kV/mm and temperatures ranging from 22 \ub0C to 70 \ub0C. It is shown that the values of the conductivities obtained by the two methods are in agreement and their temperature dependences obey Arrhenius law yielding similar activation energies

Charging and Discharge Currents in Low-Density Polyethylene and its Nanocomposite

Charging and discharge currents measured in low-density polyethylene (LDPE) and LDPE/Al2O3 nanocomposite are analyzed. The experiments were conducted at temperatures of 40–80 \ub0C utilizing a consecutive charging–discharging procedure, with the charging step at electric fields varying between 20 and 60 kV/mm. A quasi-steady state of the charging currents was earlier observed for the nanofilled specimens and it was attributed to the enhanced trapping process at polymer–nanofiller interfaces. An anomalous behavior of the discharge currents was found at elevated temperatures for both the studied materials and its occurrence at lower temperatures in the nanofilled LDPE was due to the presence of deeply trapped charges at polymer–nanofiller interfaces. The field dependence of the quasi-steady charging currents is examined by testing for different conduction mechanisms. It is shown that the space-charge-limited process is dominant and the average trap site separation is estimated at less than 2 nm for the pristine LDPE and it is at about 5–7 nm for the LDPE/Al2O3 nanocomposite. Also, location of the trapping sites in the band gap structure of the nanofilled material is altered, which substantially weakens electrical transport as compared to the unfilled counterpart

PD Properties when Varying the Smoothness of Synthesized Waveforms

The increased use of power electronic components in power systems makes it important to understand how rapidly rising voltages affect insulation systems. One vital aspect of this challenge is the measurement of partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper an approach is presented to measure PDs in a dielectrically insulated cavity when exposed to pulse width modulated (PWM) voltage shapes with different degree of smoothness. This is a continuation of our earlier investigations on the different behavior of PDs where voltages characterized by different rise times were applied. The present investigation shows that the PD amplitude decreases significantly already at a moderate level of PWM voltage smoothness to a magnitude that is about the same as for sinusoidal voltage shape. For the phase resolved PD (PRPD) pattern to become similar to the normal AC pattern it is required that the remains from PWM steps are lower than the extinction voltage. This work elucidates how PDs are affected by synthesized waveforms and limits for a sufficient smoothing level are found, which is of importance when designing insulation systems exposed to fast transients

Partial Discharges in Motor Wires at PWM Voltages of Different Smoothness

Author’s recent work has been focused on exploring possibilities for measuring partial discharges (PDs) under the action of pulse width modulated waveforms (PWM), which resulted in a development of an electrical measurement method adopted for such voltages. This solution allows a more flexible PD analysis since voltage shapes appearing in service can be utilized. In this paper, results are presented where several waveforms are applied to quantify the PD properties for motor wire test objects, starting from a non-filtered PWM waveform and continuing with gradually smoothened ones towards an AC voltage shape. The results suggest that non-smoothed PWM voltage introduces considerably more PDs and with larger magnitudes, which most probably influences the lifetime of the insulation system in all tested cases. Additionally, above a certain level of filtering, the use of lower carrier frequencies implies higher PD exposure, which suggests that the filters used should be evaluated together with the carrier frequency to ensure a longer lifetime of the insulation. It was also observed that the changes in duty cycle reduce the PD density at higher frequencies. This suggests that only applying square voltage waveforms with 50 % duty cycle for PD testing may fail to capture the actual stress inflicted

Electrical Detection of Degradation in Specimens of HVDC Cable Insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In service, the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances are blocked from entering the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments. This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle-plane type and the needle injection process is semi-automated to ensure specimen repeatability

Charge Transport in LDPE Nanocomposites Part I—Experimental Approach

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials