The aim of this work is to investigate experimentally the detector size effect on narrow beam profile measurements. Polymer gel and magnetic resonance imaging dosimetry was used for this purpose. Profile measurements (Pm(s)) of a 5 mm diameter 6 MV stereotactic beam were performed using polymer gels. Eight measurements of the profile of this narrow beam were performed using correspondingly eight different detector sizes. This was achieved using high spatial resolution (0.25 mm) two-dimensional measurements and eight different signal integration volumes A X A X slice thickness, simulating detectors of different size. "A" ranged from 0.25 to 7.5 mm, representing the detector size. The gel-derived profiles exhibited increased penumbra width with increasing detector size, for sizes >0.5 mm. By extrapolating the gel-derived profiles to zero detector size, the true profile (Pt) of the studied beam was derived. The same polymer gel data were also used to simulate a small-volume ion chamber profile measurement of the same beam, in terms of volume averaging. The comparison between these results and actual corresponding small-volume chamber profile measurements performed in this study, reveal that the penumbra broadening caused by both volume averaging and electron transport alterations (present in actual ion chamber profile measurements) is a lot more intense than that resulted by volume averaging effects alone (present in gel-derived profiles simulating ion chamber profile measurements). Therefore, not only the detector size, but also its composition and tissue equivalency is proved to be an important factor for correct narrow beam profile measurements. Additionally, the convolution kernels related to each detector size and to the air ion chamber were calculated using the corresponding profile measurements (Pm(s)), the gel-derived true profile (Pt), and convolution theory. The response kernels of any desired detector can be derived, allowing the elimination of the errors associated with narrow beam profile measurements.

Small photon fields are increasingly used in modern radiotherapy and especially in IMRT and SRS/SRT treatments. The uncertainties related to small field profile measurements can introduce significant systematic errors to the overall treatment process. These measurements are challenging mainly due to the absence of charged particle equilibrium conditions, detector size and composition effects, and positioning problems. In this work four different dosimetric methods have been used to measure the profiles of three small 6 MV circular fields having diameters of 7.5, 15.0, and 30.0 mm: a small sensitive volume air ion chamber, a diamond detector, a novel silicon-diode array ͑DOSI͒, and vinyl-pyrrolidone based polymer gel dosimeter. The results of this work support the validity of previous findings, suggesting that ͑a͒ air ion chambers are not suitable for small field dosimetry since they result in penumbra broadening and require significant corrections due to severe charged particle transport alterations; ͑b͒ diamond detectors provide high resolution and rather accurate small field profile measurements, as long as positioning problems can be addressed and the necessary dose rate corrections are correctly applied; and ͑c͒ the novel silicon-diode array ͑DOSI͒ used in this study seems to be adequate for small field profile measurements overcoming positioning problems. Polymer gel data were assumed as reference data to which the other measurement data were compared both qualitatively and quantitatively using the gamma-index concept. Polymer gels are both phantom and dosimeter, hence there are no beam perturbation effects. In addition, polymer gels are tissue equivalent and can provide high-spatial density and high-spatial resolution measurements without positioning problems, which makes them useful for small field dosimetry measurements. This work emphasizes the need to perform beam profile measurements of small fields ͑for acceptance, commissioning, treatment planning systems data feed, and periodic quality assurance purposes͒ using more than one dosimetric method. The authors believe this to be a safe way towards the reduction of the overall uncertainty related to SRS/SRT treatments.

Second-harmonic generation (SHG) and two-photon excitation fluorescence (TPEF) are relatively new and promising tools for the detailed imaging of biological samples and processes at the microscopic level. By exploiting these nonlinear phenomena phototoxicity and photobleaching effects on the specimens are reduced dramatically. The main target of this work was the development of a compact inexpensive and reliable experimental apparatus for nonlinear microscopy measurements. Femtosecond laser pulses were utilized for excitation. We achieved high-resolution imaging and mapping of Caenorhabditis elegans (C. elegans) neurons and muscular structures of the pharynx, at the microscopic level by performing SHG and TPEF measurements. By detecting nonlinear phenomena such as SHG and TPEF it is feasible to extract valuable information concerning the structure and the function of nematode neurons.

Peripheral dose (PD) to critical structures outside treatment volume is of clinical importance. The aim of the current study was to estimate PD on a linear accelerator equipped with multileaf collimator (MLC). Dose measurements were carried out using an ionization chamber embedded in a water phantom for 6 and 18 MV photon beams. PD values were acquired for field sizes from 5 x 5 to 20 x 20 cm2 in increments of 5 cm at distances up to 24 cm from the field edge. Dose data were obtained at two collimator orientations where the measurement points are shielded by MLC and jaws. The variation of PD with the source to skin distance (SSD), depth, and lateral displacement of the measurement point was evaluated. To examine the dependence of PD upon the tissue thickness at the entrance point of the beam, scattered dose was measured using thermoluminescent dosemeters placed on three anthropomorphic phantoms simulating 5- and 10-year-old children and an average adult patient. PD from 6 MV photons varied from 0.13% to 6.75% of the central-axis maximum dose depending upon the collimator orientation, extent of irradiated area, and distance from the treatment field. The corresponding dose range from 18 MV x rays was 0.09% to 5.61%. The variation of PD with depth and with lateral displacements up to 80% of the field dimension was very small. The scattered dose from both photon beams increased with the increase of SSD or tissue thickness along beam axis. The presented dosimetric data set allows the estimation of scattered dose outside the primary beam.

The aim of this work was to investigate the dosimetric performance properties of the N-vinylpyrrolidone argon (VIPAR) based polymer gel as a dosimetric tool in clinical radiotherapy. VIPAR gels with a larger concentration of gelatin than the standard recipe were manufactured and irradiated up to 68 Gy using a 6 and 18 MV linear accelerator. Using MRI, the R2-dose response was recorded at different imaging sessions within a 34 day time period post-irradiation. The R2-dose response was found to be linear between 5 and 68 Gy. Although dose sensitivity did not show significant variation with time, the measured R2-dose values showed an increasing trend, which was less evident beyond 17 days. At one day post-irradiation, calculated dose standard uncertainties at 20 Gy and 56 Gy were 2.2% and 1.7%, providing a dose resolution of 0.45 Gy and 0.97 Gy, respectively. Although these values fulfilled the 2% limit of ICRU, when gels were imaged at one day post-irradiation, it was shown that the temporal evolution of the R2 values deteriorated the per cent standard uncertainty and the dose resolution by approximately 57%, when imaged 17 days post-irradiation. Variation in the coagulation temperature of the gels did not impact the R2-dose sensitivity. This study has shown that the VIPAR gel has the properties of a dosimetric tool required in clinical radiotherapy, especially in applications where a wide dose dynamic range is employed. For results with the lowest per cent uncertainty and the optimum dose resolution, the dosimetry gels used in this work should be MR scanned at one day post-irradiation. Furthermore, a preliminary study on the R2-dose response of a new normoxic N-vinylpyrrolidone-based polymer gel showed that it could potentially replace the traditional VIPAR gel formulation, while preserving the wide dynamic dose response inherent to that monomer.