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.

New composition polymer gels, the N-vinylpyrrolidone argon (VIPAR) gels, were developed and investigated as MRI dosimeters. VIPAR gels were irradiated in the dose range of 0-12 Gy by a 6 MV x-ray linear accelerator and MR-scanned in a 1.5 T magnetic resonance imager. A linear relationship was found between absorbed dose and spin spin relaxation rate R2. The dose sensitivity was found to be approximately 0.1 s(-1) Gy(-1) for a gel composition of 4% w/w in N-vinylpyrrolidone, 4% w/w in N,N'-methylene-bisacrylamide, 5% w/w in gelatine type A and 87% w/w in water. This dose sensitivity was stable with time and did not deteriorate even when a boost radiation dose of 2.5 Gy was applied 15 days after the first irradiation. Good reproducibility of these results was observed when a new batch of gels was produced and used for corresponding measurements and analysis.

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.

In this work, polymer gel-MRI dosimetry (using VIPAR gels), radiographic film and a PinPoint ion chamber were used for profile measurements of 6 MV x-ray stereotactic beams of 5 and 10 mm diameter. The VIPAR gel-MRI method exhibited a linear dose response up to 32 Gy. VIPAR gels were found to resolve the penumbra region quite accurately, provided that the in-plane image resolution of the related T2-map is adequate (< or = 0.53 mm). T2-map slice thickness had no significant effect on beam profile data. VIPAR measurements performed with a spatial resolution of 0.13 mm provided penumbra widths (80%-20% distance) of 1.34 and 1.70 mm for the 5 and 10 mm cones respectively. These widths were found to be significantly smaller than those obtained with the film (2.23 mm for the 5 mm cone, 2.45 mm for the 10 mm cone) and PinPoint (2.25 mm for the 5 mm cone, 2.52 mm for the 10 mm cone) methods. Regarding relative depth dose measurements, good correlation between VIPAR gel and PinPoint data was observed. In conclusion, polymer gel-MRI dosimetry can provide relatively accurate profile data for very small beams used in stereotactic radiosurgery since it can overcome, to some extent, the problems related to the finite size of conventional detectors.

Following the clinical introduction of the Elekta Unity MR-linac, there is an urgent need for development of dosimetry protocols and tools, not affected by the presence of a magnetic field. This work presents a benchmarking methodology comprising 2D/3D passive dosimetry and involving on-couch adaptive treatment planning, a unique step in MR-linac workflows. Two identical commercially available 3D-printed head phantoms (featuring realistic bone anatomy and MR/CT contrast) were employed. One phantom incorporated a film dosimetry insert, while the second was filled with polymer gel. Gel dose-response characteristics were evaluated under the Unity irradiation and read-out conditions, using vials and a cubic container filled with gel from the same batch. Treatment plan for the head phantoms involved a hypothetical large C-shape brain lesion, partly surrounding the brainstem. An IMRT step-and-shoot 7-beam plan was employed. Pre-treatment on-couch MR-images were acquired in order for the treatment planning system to calculate the virtual couch shifts and perform adaptive planning. Absolute 2D and relative 3D measurements were compared against calculations related to both adapted and original plans. Real-time dose accumulation monitoring in the gel-filled phantom was also performed. Results from the vials and cubic container suggest that gel dose-response is linear in the dose range investigated and signal integrity is mature at the read-out timings considered. Head phantom 2D and 3D measurements agreed well with calculations with 3D gamma index passing rates above 90% in all cases, even with the most stringent criteria used (2 mm/2%). By exploiting the 3D information provided by the gel, comparison also involved DVHs, dose-volume and plan quality metrics, which also reflected the agreement between adapted and delivered plans within  ±4%. No considerable discrepancies were detected between adapted and original plans. A novel methodology was developed and implemented, suitable for QA procedures in Unity. TPS calculations were validated within the experimental uncertainties involved.

In this work the extent of the linear dose response and the dynamic dose range of N-vinylpyrrolidone-argon based (VIPAR) polymer gels were investigated. VIPAR gels were irradiated using a 6 MV linear accelerator up to 60 Gy and a Nucletron microSelectron 192Ir HDR brachytherapy source to much higher doses to cover a dose range of two orders of magnitude. They were then MR scanned at 1.5 T to obtain T2-maps. VIPAR gel measurements obtained from the two irradiation regimes were calibrated against ion chamber measurements and dose calculations derived using the AAPM TG-43 protocol respectively. A satisfying agreement between the calibration results derived using the 6 MV x-rays and the 192Ir source was found for doses up to 60 Gy, implying that the response of the VIPAR gels is independent of photon energy and dose rate. A linear R2 dose response up to approximately 40 Gy and a dynamic dose range up to at least approximately 250 Gy were observed. VIPAR gel dose measurements derived using the monoexponentially fitted brachytherapy calibration data were found to be quite accurate.

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology. In a patient’s planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution. Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as ‘bone’) and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%. Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

In this work, the utilization of polymer gel-MRI dosimetry for measurements at distances relevant to clinical brachytherapy and intravascular applications [i.e., in the mm range, where steep three-dimensional (3-D) dose gradients exist] is investigated using N-vinylpyrrolidone-based gels. Transverse axis radial dose distributions, dose distributions parallel to the source axis, and 2-D dose distributions around the commonly used microSelectron 192Ir HDR source are measured for single source dwell position irradiations. Experimental results are found in good agreement with verified Monte Carlo calculations, even for distances less than 3 mm from the source. The effect of various MRI parameters, such as slice thickness, slice mispositioning, and in-plane resolution, on the accuracy of the method is also investigated. Possible limitations of the method are discussed, and its' overall potential in brachytherapy dosimetry is evaluated. Experimental 2-D dose distributions for an intravascular application following the Paris irradiation protocol are compared to corresponding commercial treatment planning system calculations. Results suggest that polymer gel-MRI dosimetry is capable of experimentally verifying dose distributions in relevant clinical intravascular applications.