Radiation-sensitive hydrogels offer the capability of verifying intricate dose distributions in three-dimensional (3D) space conveniently in a single measurement with sub-millimetre spatial resolution. In this study, a new radiochromic hydrogel called leuco crystal violet (LCV) micelle gel is introduced. Upon irradiation, LCV converts to crystal violet (CV(+)). Triton X-100 micelles are used to provide the required hybrid-interfacing environment to dissolve LCV. The diffusion coefficient of the LCV gel has been measured to be 0.036 +/- 0.001 mm(2) h(-1), which is a factor of 25 times less than the standard radiochromic ferrous xylenol-orange (FX) gel; LCV gels without Triton X-100 micelles have a diffusion coefficient of 0.33 +/- 0.02 mm(2) h(-1). The LCV gel formulation contains: 1 mM LCV, 25 mM trichloroacetic acid, 4 mM Triton X-100 and 4% w/w gelatin. The primary innovative feature of this 3D hydrogel is that the radiation-induced CV(+) dye is more soluble in the Triton X-100 micelles than in the surrounding water which consequently leads to more stable post-irradiation dose distributions. A dosimetric characterization revealed that the dose response is reproducible to within 1% over three separate batches, independent of energy, dose rate and dose fractionation but is affected by the temperature ( approximately 4% per degree C) during irradiation. LCV micelle gels scanned optically with a yellow light source are a promising system for 3D dose verification. They may prove to be, especially, useful for scanning large volume dosimeters (i.e. 20 cm) since they are easily manufactured, transparent and near colourless prior to irradiation.

PurposeNoninvasive frameless systems are increasingly being utilized for head immobilization in stereotactic radiosurgery (SRS). Knowing the head positioning reproducibility of frameless systems and their respective ability to limit intrafractional head motion is important in order to safely perform SRS. The purpose of this study was to evaluate and compare the intrafractional head motion of an invasive frame and a series of frameless systems for single fraction SRS and fractionated/hypofractionated stereotactic radiotherapy (FSRT/HF‐SRT).MethodsThe noninvasive PinPoint system was used on 15 HF‐SRT and 21 SRS patients. Intrafractional motion for these patients was compared to 15 SRS patients immobilized with Cosman‐Roberts‐Wells (CRW) frame, and a FSRT population that respectively included 23, 32, and 15 patients immobilized using Gill‐Thomas‐Cosman (GTC) frame, Uniframe, and Orfit. All HF‐SRT and FSRT patients were treated using intensity‐modulated radiation therapy on a linear accelerator equipped with cone‐beam CT (CBCT) and a robotic couch. SRS patients were treated using gantry‐mounted stereotactic cones. The CBCT image‐guidance protocol included initial setup, pretreatment and post‐treatment verification images. The residual error determined from the post‐treatment CBCT was used as a surrogate for intrafractional head motion during treatment.ResultsThe mean intrafractional motion over all fractions with PinPoint was 0.62 ± 0.33 mm and 0.45 ± 0.33 mm, respectively, for the HF‐SRT and SRS cohort of patients (P‐value = 0.266). For CRW, GTC, Orfit, and Uniframe, the mean intrafractional motions were 0.30 ± 0.21 mm, 0.54 ± 0.76 mm, 0.73 ± 0.49 mm, and 0.76 ± 0.51 mm, respectively. For CRW, PinPoint, GTC, Orfit, and Uniframe, intrafractional motion exceeded 1.5 mm in 0%, 0%, 5%, 6%, and 8% of all fractions treated, respectively.ConclusionsThe noninvasive PinPoint system and the invasive CRW frame stringently limit cranial intrafractional motion, while the latter provides superior immobilization. Based on the results of this study, our clinical practice for malignant tumors has evolved to apply an invasive CRW frame only for metastases in eloquent locations to minimize normal tissue exposure.

The dosimetry of small fields as used in stereotactic radiotherapy, radiosurgery and intensity-modulated radiation therapy can be challenging and inaccurate due to partial volume averaging effects and possible disruption of charged particle equilibrium. Consequently, there exists a need for an integrating, tissue equivalent dosimeter with high spatial resolution to avoid perturbing the radiation beam and artificially broadening the measured beam penumbra. In this work, radiochromic ferrous xylenol-orange (FX) and leuco crystal violet (LCV) micelle gels were used to measure relative dose factors (RDFs), percent depth dose profiles and relative lateral beam profiles of 6 MV x-ray pencil beams of diameter 28.1, 9.8 and 4.9 mm. The pencil beams were produced via stereotactic collimators mounted on a Varian 2100 EX linear accelerator. The gels were read using optical computed tomography (CT). Data sets were compared quantitatively with dosimetric measurements made with radiographic (Kodak EDR2) and radiochromic (GAFChromic EBT) film, respectively. Using a fast cone-beam optical CT scanner (Vista), corrections for diffusion in the FX gel data yielded RDFs that were comparable to those obtained by minimally diffusing LCV gels. Considering EBT film-measured RDF data as reference, cone-beam CT-scanned LCV gel data, corrected for scattered stray light, were found to be in agreement within 0.5% and -0.6% for the 9.8 and 4.9 mm diameter fields, respectively. The validity of the scattered stray light correction was confirmed by general agreement with RDF data obtained from the same LCV gel read out with a laser CT scanner that is less prone to the acceptance of scattered stray light. Percent depth dose profiles and lateral beam profiles were found to agree within experimental error for the FX gel (corrected for diffusion), LCV gel (corrected for scattered stray light), and EBT and EDR2 films. The results from this study reveal that a three-dimensional dosimetry method utilizing optical CT-scanned radiochromic gels allows for the acquisition of a self-consistent volumetric data set in a single exposure, with sufficient spatial resolution to accurately characterize small fields.

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Freshly prepared radiochromic ferrous xylenol-orange (FX) gels optically scanned with a light source exhibit a threshold dose response that is thermally and wavelength dependent. Correction for this threshold dose leads to accurate dose calibration and better reproducibility in multiple fraction radiation exposures. The objective of this study was to determine the cause of the threshold dose effect and to control it through improved dose calibration procedures. The results of a systematic investigation into the chemical cause revealed that impurities within the various FX gel constituents (i.e. xylenol-orange, gelatin, sulfuric acid and ferrous ammonium sulfate) were not directly responsible for the threshold dose. Rather, it was determined that the threshold dose response stems from a spectral sensitivity to different chemical complexes that are formed at different dose levels in FX gels between ferric (Fe(III)) ions and xylenol-orange (XO), i.e. Fe(III)i:XOj. A double Fe(III)2:XO1 complex preferentially absorbs at longer wavelengths (i.e. yellow), while at shorter wavelengths (i.e. green) the sensitivity is biased toward the single Fe(III)1:XO1 complex. As a result, when scanning with yellow light, freshly prepared FX gels require a minimum concentration of Fe(III) ions to shift the equilibrium concentration to favor the predominant production of the double Fe(III)2:XO1 complex at low doses. This can be accomplished via pre-irradiation of freshly prepared gels to a priming dose of approximately 0.5 Gy or allowing auto-oxidation to generate the startup concentration of Fe(III) ions required to negate the apparent threshold dose response.

In conformal radiation therapy, a high dose of radiation is given to a target volume to increase the probability of cure, and care is taken to minimize the dose to surrounding healthy tissue. The techniques used to achieve this are very complicated and the precise verification of the resulting three-dimensional (3D) dose distribution is required. Polyacrylamide gelatin (PAG) dosimeters with magnetic resonance imaging and optical computed tomography scanning provide the required 3D dosimetry with high spatial resolution. Many basic studies have characterized these chemical dosimeters that polymerize under irradiation. However, the investigation of the fundamental properties of the radiation-induced polymerization in PAG dosimeters is complicated by the presence of the background gelatin matrix. In this work, a gelatin-free model system for the study of the basic radiation-induced polymerization in PAG dosimeters has been developed. Experiments were performed on gelatin-free dosimeters, named aqueous polyacrylamide (APA) dosimeters, containing equal amounts of acrylamide and N,N'-methylene-bisacrylamide. The APA dosimeters were prepared with four different total monomer concentrations (2, 4, 6 and 8% by weight). Nuclear magnetic resonance (NMR) spin-spin and spin-lattice proton relaxation measurements at 20 MHz, and gravimetric analyses performed on all four dosimeters, show a continuous degree of polymerization over the dose range of 0-25 Gy. The developed NMR model explains the relationship observed between the relaxation data and the amount of crosslinked polymer formed at each dose. This model can be extended with gelatin relaxation data to provide a fundamental understanding of radiation-induced polymerization in the conventional PAG dosimeters.

A product available commercially for making dental impressions, Jeltrate®Plus, was evaluated as a tissue equivalent bolus material. Jeltrate®Plus was found to be tissue equivalent in 6 and 15 MV photon energy beams and 6, 12, and 20 MeV electron energy beams. As a first step, different preparations for making the bolus material were investigated and an optimal mixture was determined to be two parts Jeltrate®Plus powder to three parts water by weight. A suitable method for storing the material was found to be in a water filled plastic container. Since the product is fairly inexpensive and is easily and quickly made and moulded into different shapes, it is an excellent bolus material to use when treating irregular patient contours.PACS number(s): 87.53.–j, 87.66.–a

Introduction: Intact brain metastases tend to be small and spherical compared to postsurgery brain cavities, which tend to be large and irregular shaped and, as a result, a challenge with respect to treatment planning. The purpose of the present study is to develop guidelines for normal brain tissue dose and to investigate whether there is a dependence on target type for patients treated with hypofractionated volumetric modulated arc radiotherapy (HF-VMAT). Methods: Treatment plans from a total of 100 patients and 136 targets (55 cavity and 81 intact) were retrospectively reviewed. All targets were treated with HF-VMAT with total doses ranging between 20 and 30 gray (Gy) in 5 fractions. All plans met institutional objectives for organ-at-risk constraints and were clinically delivered. Dose falloff was quantified using gradient index (GI) and distance between the 100% and 50% isodose lines (R50). Additionally, the dose to normal brain tissue (brain contour excluding all gross tumor or clinical target volumes) was assessed using volume receiving specific doses (Vx) where x ranged from 5 to 30 Gy. Best-fit curves using power law relationships of the form y ¼ ax b were generated for GI, R50, and Vx (normal brain tissue) versus target volume. Results: There was a statistically significant difference in planning target volume (PTV) for cavities versus intact metastases with mean volumes of 37.8 cm 3 and 9.5 cm 3 , respectively (P < .0001). The GI and R50 were statistically different: 3.4 and 9.8 mm for cavities versus 4.6 and 8.3 mm for intact metastases (P < .0001). The R50 increased with PTV with power law coefficients (a, b) ¼ (6.3, 0.12) and (5.9, 0.15) for cavities and intact, respectively. GI decreased with PTV with coefficients (a, b) ¼ (5.9, À0.18) and (5.7, À0.14) for cavities and intact, respectively. The normal brain tissue Vx also exhibited power law relationships with PTV for x ¼ 20 to 28.8 Gy.In conclusion, target volume is the main predictor of dose falloff. The results of the present study can be used for determining target volume-based thresholds for dose falloff and normal brain tissue dose-volume constraints.Keywords hypofractionated VMAT, brain, stereotactic radiosurgery, treatment planning Abbreviations CBCT, cone beam computed tomography; CI, conformity index; CT, computed tomography; CTV, clinical target volume; GI, gradient index; GTV, gross tumor volume; Gy, gray; HF-VMAT, hypofractionated volumetric modulated arc radiotherapy; HI, heterogeneity index; IMRT, intensity modulated radiation therapy; MD, maximum dose; MLC, multileaf collimator; MU, monitor unit; OAR, organ at risk; PD, prescribed dose; PIV, prescription isodose volume; PIV half , volume of 50% of the prescription isodose; PTV, planning target volume; QA, quality assurance; R50, radial difference between equivalent spherical volumes corresponding to the 100% and 50% isodose lines; R Eq (V), radius of an equivalent sphere with volume V; RI, regularity index; RTOG, radiation therapy oncology group; SRS, stereotactic radiosurg...