Diana Adlienė

Discrepancy between absorbed dose estimated from phantom measurements and from estimates using data on individual head sizes in head CT examinations.

Patient doses during computed tomography (CT) scanning are relative high, as compared with doses received by patients during conventional X-ray examinations. Owing to this, every possible optimisation of CT examinations is of importance. Dose estimation in CT examinations of patients are normally based on measurements of CT dose index and dose length products in standardised CT phantoms, representing a standard person of 70 kg. Real patients may differ significantly from this standard, and hence the dose values calculated for a phantom are not relevant for the individual patient. In this investigation, the effective dose for 31 patients undergoing a CT examination of their head has been evaluated using measurements on a standard CT head phantom and the data corrected for the real size (perimeter) and exposed volume of the patients head. It is shown that the actual doses to patients were higher in 50 % cases of all performed head CT examinations, compared with doses estimated from phantom measurements. A dose calculation model for head CT patients based on the individual patients head size (perimeter) and exposed head volume is proposed and the possibility to optimise head CT examinations of patients is discussed.

Holograms recording with pulsed laser on diazonaphthoquinone-novolac-based photoresists and their nanocomposites

Abstract. Direct write digital holography technique (DWDH) using a single 440-nm pulsed laser exposure has been proposed to record master holograms on commercially available positive-tone photoresist systems based on a mixture of diazonaphthoquinone and novolac resin (DNQ-novolac) of different thicknesses. The DNQ-novolac nanocomposite doped with copper nanoparticles (CuNPs) films was also used. The method for numerical evaluation of hologram quality based on reflected beam diffraction intensity measurements was proposed and verified. It was found that all investigated photoresist nanocomposites were sensitive enough to record holographic structures at low single pulse laser exposures (from 3.3 to 18.0  mJ/cm2). Moreover, doping DNQ-novolac nanocomposite with CuNPs s increases its sensitivity to pulsed laser exposure by more than 30%. The potential of single pulsed laser exposures to record high quality master holograms on commercially available and metal nanoparticles doped photoresists with at least five times lower exposures values as compared to the continuous-wave laser exposures usually used to expose photoresist materials in holographic applications, opens the possibility to use pulsed lasers for quick master-originals origination for embossed holograms applying a DWDH technique or analog methods.

The role of shielding in superficial X-ray therapy.

Superficial X-ray therapy is applicable in the kilovoltage range for the treatment of the cancer. Pb shielding is used to protect radiation-sensitive organs since the doses are high, however the question about shielding efficiency is still open. The role of shielding was investigated in this work based on the results of dose measurements performed using a set of pencil dosemeters and thermoluminescent dosimetries. According to the measured angular dose distributions on the phantom surface during X-ray irradiation, the area near the applicator exposed to the waste irradiation was evaluated and Pb shielding of a corresponding size was chosen. It has been shown that the dose in the area of interest decreases non-linearly, however high shielding efficiency (~90 %) remains almost stable in the whole area. No significant contribution of secondary scattered photons from Pb has been observed. The role of Pb shielding in superficial X-ray therapy is discussed on the basis of the obtained results.

The resulting skin dose in two-view mammography examinations.

The entrance surface dose (ESD) (skin dose) and its variation due to overlapping radiation fields at mammography have been experimentally investigated on patients during two-view mammography screening examinations of both breasts and on phantoms of silicon gel. Measurements were done using multiple thermoluminescence dosemeters for mapping the variations in absorbed dose at the surface of the breast phantoms. It was found that the total resulting skin dose at the reference point on the breast surface, described in the European Protocol [Zoetelief, Fitzgerald, Leitz and Sabel (European protocol on dosimetry in mammography. EUR 16263 (Luxemburg: EC), 1996)] after multiple exposure of patients breasts during two-view, four exposures mammography screening examinations is approximately 50 % higher as compared with the skin dose at the same point measured during one single (CC) exposure. The dose distributions on the breast phantom surface were non-uniform and indicated areas of higher doses, which were related to the regions of the superimposed fields on the surface during the examinations. The dosimetric importance of the superposition of X-ray fields on the ESD is discussed.

Electrical properties of the diamond like carbon films irradiated with high energy photons

SiOx—containing amorphous diamond like carbon (DLC) is very attractive material for a number of practical applications. DLC films are possible candidates for the formation of passive layers in electronic devices and are used as protective coatings. Both applications are interesting for the construction of medical radiation detectors. Radiation induced structural changes and electrical properties of DLC: SiOx films (undoped and co-doped) synthesized at room temperature by means of direct ion beam deposition method were investigated after their irradiation with high energy (Emax = 15 MeV) X-ray photons. It was found that transparency of the irradiated DLC films was not changed significantly, as compared to initial samples, stating only small increase of the optical band gap in DLC films. However radiation induced changes were dependent on co-doping and film deposition conditions which were responsible for the sp3/sp2 ratio and hydrogen content in the investigated films. Analysis of U-I characteristics showed decreasing tendency of the leakage current in the range 0–20 V and especially high dependency of breakdown voltage on the deposited contact area in irradiated films as compared to initial samples. Possible mechanism of radiation induced changes in irradiated DLC films is discussed on the basis of the results of Raman and Infrared spectroscopy.

In vivo dose verification method in catheter based high dose rate brachytherapy

In vivo dosimetry is a powerful tool for dose verification in radiotherapy. Its application in high dose rate (HDR) brachytherapy is usually limited to the estimation of gross errors, due to inability of the dosimetry system/ method to record non-uniform dose distribution in steep dose gradient fields close to the radioactive source. In vivo dose verification in interstitial catheter based HDR brachytherapy is crucial since the treatment is performed inserting radioactive source at the certain positions within the catheters that are pre-implanted into the tumour. We propose in vivo dose verification method for this type of brachytherapy treatment which is based on the comparison between experimentally measured and theoretical dose values calculated at well-defined locations corresponding dosemeter positions in the catheter. Dose measurements were performed using TLD 100-H rods (6 mm long, 1 mm diameter) inserted in a certain sequences into additionally pre-implanted dosimetry catheter. The adjustment of dosemeter positioning in the catheter was performed using reconstructed CT scans of patient with pre-implanted catheters. Doses to three Head&Neck and one Breast cancer patient have been measured during several randomly selected treatment fractions. It was found that the average experimental dose error varied from 4.02% to 12.93% during independent in vivo dosimetry control measurements for selected Head&Neck cancer patients and from 7.17% to 8.63% - for Breast cancer patient. Average experimental dose error was below the AAPM recommended margin of 20% and did not exceed the measurement uncertainty of 17.87% estimated for this type of dosemeters. Tendency of slightly increasing average dose error was observed in every following treatment fraction of the same patient. It was linked to the changes of theoretically estimated dosemeter positions due to the possible patients organ movement between different treatment fractions, since catheter reconstruction was performed for the first treatment fraction only. These findings indicate potential for further average dose error reduction in catheter based brachytherapy by at least 2-3% in the case that catheter locations will be adjusted before each following treatment fraction, however it requires more detailed investigation.

SIMPLE SURFACE PLASMON RESONANCE-BASED DOSEMETER.

The interest to application of various surface plasmon resonance (SPR)-based sensors for the investigation of chemical and biological processes in thin layers deposited on the gratings surface/media is developing. Characterisation of processes as well as specimens features might be performed analysing variations in optical properties (refraction index) of these thin layers. SPR sensors by default are characterised by high resolution and small uncertainties, and measurements might be performed in situ High-resolution, low-cost, SPR-based dosemeter concept has been proposed and realised depositing dose-sensitive nPAG gel layer onto diffraction gratings surface. The experimental set-up and method for information read out from the sensor were developed and implemented. Obtained results show a potential application of SPR-based dosemeter for dose measurements/mapping in steep gradient fields and/or large area fields.

Development of 3D Printed Phantom for Dose Verification in Radiotherapy for the Patient with Metal Artefacts Inside

One of the problems performing IMRT dose planning or plan verification using anthropomorphic phantoms for head and neck cancer patients is the presence of possible artefacts (dental crowns, metal dental implants, dental restoration materials.) inside the oral cavity. In many cases these artefacts are not accounted but may cause deviations from patient treatment plans due to enhanced scattering dose from the metal artefacts. Exploiting 3D printing technologies 3D dosimetry phantom corresponding to patient-specific anatomic structures with precisely positioned artefacts can be produced. Application of such 3D phantom in radiation therapy may contribute to more accurate dose planning and thus more efficient patient treatment. In this work we propose newly developed patient-specific 3D printed phantom of lower jaw with teeth which can be used for patient specific QA in IMRT. Developing this phantom DICOM image of the real patient was used for 3D reconstruction of patient’s lower jaw with teeth. 3D shape of this anatomic structure was printed out using Zortrax M300 3D printer. High Impact Polystyrene (HIPS) which is characterized as having satisfactory bone tissue equivalency was used as printing material. Keeping in mind a real patient, possibility of covering of corresponding teeth with a metallic crown in the 3D printed jaw construction was foreseen. Prepared construction was fixed in dose gel matrix, thus forming dosimetry phantom for the evaluation of possible radiation treatment errors caused by artefacts located in this anatomic region of the patient. Investigation of 3D printed lower jaw phantom has shown its feasibility for the assessment of dose distortions related to the presence of metal artefacts in the mouth of the patient. Also potential for the application of 3D printed phantoms in radiotherapy quality assurance has been shown.

Evaluation of the Independent Dose Calculation Algorithm

The results of the implementation of Radiotherapy Verification Program based on the application of the completely independent from TPS algorithm for dose calculations in checking of patient’s treatment plans are presented in this paper. Algorithm includes calculation of the absorbed dose from photons scattered in the patient, calculation of the primary kerma transmission in the patient, calculation of the primary kerma variation in the air with collimator settings, and corrections for heterogeneities in the patient.