CyberKnife Xsight versus fiducial-based target-tracking: a novel 3D dosimetric comparison in a dynamic phantom.

BACKGROUND: The CyberKnife Xsight lung-tracking system (XLTS) provides an alternative to fiducial-based target-tracking systems (FTTS) for non-small-cell lung cancer (NSCLC) patients without invasive fiducial insertion procedures. This study provides a method for 3D independent dosimetric verification of the accuracy of the FTTS compared to the XLTS without relying on log-files generated by the CyberKnife system. METHODS: A respiratory motion trace was taken from a 4D-CT of a real lung cancer patient and applied to a modified QUASAR™ respiratory motion phantom. A novel approach to 3D dosimetry was developed using Gafchromic EBT3 film, allowing the 3D dose distribution delivered to the moving phantom to be reconstructed. Treatments were planned using the recommended margins for one and three fiducial markers and XLTS 2-view, 1-view and 0-view target-tracking modalities. The dose delivery accuracy was analysed by comparing the reconstructed dose distributions to the planned dose distributions using gamma index analysis. RESULTS: For the 3%/2 mm gamma criterion, gamma passing rates up to 99.37% were observed for the static deliveries. The 3-fiducial and 1-fiducial-based deliveries exhibited passing rates of 93.74% and 97.82%, respectively, in the absence of target rotation. When target rotation was considered, the passing rate for 1-fiducial tracking degraded to 91.24%. The passing rates observed for XLTS 2-view, 1-view and 0-view target-tracking were 92.78%, 96.22% and 76.08%, respectively. CONCLUSIONS: Except for the XLTS 0-view, the dosimetric accuracy of the XLTS was comparable to the FTTS under equivalent treatment conditions. This study gives us further confidence in the CyberKnife XLTS and FTTS systems.

Breakdown of Bragg-Gray behaviour for low-density detectors under electronic disequilibrium conditions in small megavoltage photon fields.

In small photon fields ionisation chambers can exhibit large deviations from Bragg-Gray behaviour; the EGSnrc Monte Carlo (MC) code system has been employed to investigate this 'Bragg-Gray breakdown'. The total electron (+positron) fluence in small water and air cavities in a water phantom has been computed for a full linac beam model as well as for a point source spectrum for 6 MV and 15 MV qualities for field sizes from 0.25 × 0.25 cm(2) to 10 × 10 cm(2). A water-to-air perturbation factor has been derived as the ratio of total electron (+positron) fluence, integrated over all energies, in a tiny water volume to that in a 'PinPoint 3D-chamber-like' air cavity; for the 0.25 × 0.25 cm(2) field size the perturbation factors are 1.323 and 2.139 for 6 MV and 15 MV full linac geometries respectively. For the 15 MV full linac geometry for field sizes of 1 × 1 cm(2) and smaller not only the absolute magnitude but also the 'shape' of the total electron fluence spectrum in the air cavity is significantly different to that in the water 'cavity'. The physics of this 'Bragg-Gray breakdown' is fully explained, making reference to the Fano theorem. For the 15 MV full linac geometry in the 0.25 × 0.25 cm(2) field the directly computed MC dose ratio, water-to-air, differs by 5% from the product of the Spencer-Attix stopping-power ratio (SPR) and the perturbation factor; this 'difference' is explained by the difference in the shapes of the fluence spectra and is also formulated theoretically. We show that the dimensions of an air-cavity with a perturbation factor within 5% of unity would have to be impractically small in these highly non-equilibrium photon fields. In contrast the dose to water in a 0.25 × 0.25 cm(2) field derived by multiplying the dose in the single-crystal diamond dosimeter (SCDDo) by the Spencer-Attix ratio is within 2.9% of the dose computed directly in the water voxel for full linac geometry at both 6 and 15 MV, thereby demonstrating that this detector exhibits quasi Bragg-Gray behaviour over a wide range of field sizes and beam qualities.

Using cavity theory to describe the dependence on detector density of dosimeter response in non-equilibrium small fields.

The dose imparted by a small non-equilibrium photon radiation field to the sensitive volume of a detector located within a water phantom depends on the density of the sensitive volume. Here this effect is explained using cavity theory, and analysed using Monte Carlo data calculated for schematically modelled diamond and Pinpoint-type detectors. The combined impact of the density and atomic composition of the sensitive volume on its response is represented as a ratio, Fw,det, of doses absorbed by equal volumes of unit density water and detector material co-located within a unit density water phantom. The impact of density alone is characterized through a similar ratio, Pρ -, of doses absorbed by equal volumes of unit and modified density water. The cavity theory is developed by splitting the dose absorbed by the sensitive volume into two components, imparted by electrons liberated in photon interactions occurring inside and outside the volume. Using this theory a simple model is obtained that links Pρ - to the degree of electronic equilibrium, see, at the centre of a field via a parameter Icav determined by the density and geometry of the sensitive volume. Following the scheme of Bouchard et al (2009 Med. Phys. 36 4654-63) Fw,det can be written as the product of Pρ -, the water-to-detector stopping power ratio [L[overline](Δ)/ρ](w)(det), and an additional factor Pfl -. In small fields [L[overline](Δ)/ρ](w)(det) changes little with field-size; and for the schematic diamond and Pinpoint detectors Pfl - takes values close to one. Consequently most of the field-size variation in Fw,det originates from the Pρ - factor. Relative changes in see and in the phantom scatter factor sp are similar in small fields. For the diamond detector, the variation of Pρ - with see (and thus field-size) is described well by the simple cavity model using an Icav parameter in line with independent Monte Carlo estimates. The model also captures the overall field-size dependence of Pρ - for the schematic Pinpoint detector, again using an Icav value consistent with independent estimates.

Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields.

The impact of density and atomic composition on the dosimetric response of various detectors in small photon radiation fields is characterized using a 'density-correction' factor, F(detector), defined as the ratio of Monte Carlo calculated doses delivered to water and detector voxels located on-axis, 5 cm deep in a water phantom with a SSD of 100 cm. The variation of F(detector) with field size has been computed for detector voxels of various materials and densities. For ion chambers and solid-state detectors, the well-known variation of F(detector) at small field sizes is shown to be due to differences between the densities of detector active volumes and water, rather than differences in atomic number. However, associated changes in the measured shapes of small-field profiles offset these variations in F(detector), so that integral doses measured using the different detectors are quite similar, at least for slit fields. Since changes in F(detector) with field size arise primarily from differences between the densities of the detector materials and water, ideal small-field relative dosimeters should have small active volumes and water-like density.

Monte Carlo modeling of small photon fields: quantifying the impact of focal spot size on source occlusion and output factors, and exploring miniphantom design for small-field measurements.

The accuracy with which Monte Carlo models of photon beams generated by linear accelerators (linacs) can describe small-field dose distributions depends on the modeled width of the electron beam profile incident on the linac target. It is known that the electron focal spot width affects penumbra and cross-field profiles; here, the authors explore the extent to which source occlusion reduces linac output for smaller fields and larger spot sizes. A BEAMnrc Monte Carlo linac model has been used to investigate the variation in penumbra widths and small-field output factors with electron spot size. A formalism is developed separating head scatter factors into source occlusion and flattening filter factors. Differences between head scatter factors defined in terms of in-air energy fluence, collision kerma, and terma are explored using Monte Carlo calculations. Estimates of changes in kerma-based source occlusion and flattening filter factors with field size and focal spot width are obtained by calculating doses deposited in a narrow 2 mm wide virtual "milliphantom" geometry. The impact of focal spot size on phantom scatter is also explored. Modeled electron spot sizes of 0.4-0.7 mm FWHM generate acceptable matches to measured penumbra widths. However the 0.5 cm field output factor is quite sensitive to electron spot width, the measured output only being matched by calculations for a 0.7 mm spot width. Because the spectra of the unscattered primary (psi(pi)) and head-scattered (psi(sigma)) photon energy fluences differ, miniphantom-based collision kerma measurements do not scale precisely with total in-air energy fluence psi = (psi(pi) + psi(sigma) but with (psi(pi)+ 1.2psi(sigma)). For most field sizes, on-axis collision kerma is independent of the focal spot size; but for a 0.5 cm field size and 1.0 mm spot width, it is reduced by around 7% mostly due to source occlusion. The phantom scatter factor of the 0.5 cm field also shows some spot size dependence, decreasing by 6% (relative) as spot size is increased from 0.1 to 1.0 mm. The dependence of small-field source occlusion and output factors on the focal spot size makes this a significant factor in Monte Carlo modeling of small (< 1 cm) fields. Changes in penumbra width with spot size are not sufficiently large to accurately pinpoint spot widths. Consequently, while Monte Carlo models based exclusively on large-field data can quite accurately predict small-field profiles and PDDs, in the absence of experimental methods of determining incident electron beam profiles it will remain necessary to measure small-field output factors, fine-tuning modeled spot sizes to ensure good matching between the Monte Carlo and the measured output factors.

Using a Monte Carlo model to predict dosimetric properties of small radiotherapy photon fields.

Accurate characterization of small-field dosimetry requires measurements to be made with precisely aligned specialized detectors and is thus time consuming and error prone. This work explores measurement differences between detectors by using a Monte Carlo model matched to large-field data to predict properties of smaller fields. Measurements made with a variety of detectors have been compared with calculated results to assess their validity and explore reasons for differences. Unshielded diodes are expected to produce some of the most useful data, as their small sensitive cross sections give good resolution whilst their energy dependence is shown to vary little with depth in a 15 MV linac beam. Their response is shown to be constant with field size over the range 1-10 cm, with a correction of 3% needed for a field size of 0.5 cm. BEAMnrc has been used to create a 15 MV beam model, matched to dosimetric data for square fields larger than 3 cm, and producing small-field profiles and percentage depth doses (PDDs) that agree well with unshielded diode data for field sizes down to 0.5 cm. For fields sizes of 1.5 cm and above, little detector-to-detector variation exists in measured output factors, however for a 0.5 cm field a relative spread of 18% is seen between output factors measured with different detectors-values measured with the diamond and pinpoint detectors lying below that of the unshielded diode, with the shielded diode value being higher. Relative to the corrected unshielded diode measurement, the Monte Carlo modeled output factor is 4.5% low, a discrepancy that is probably due to the focal spot fluence profile and source occlusion modeling. The large-field Monte Carlo model can, therefore, currently be used to predict small-field profiles and PDDs measured with an unshielded diode. However, determination of output factors for the smallest fields requires a more detailed model of focal spot fluence and source occlusion.

Advances in intensity-modulated radiotherapy delivery.

Fixed-field intensity-modulated treatments, delivered by conventional linac-plus-multileaf systems, have rapidly become the most common form of IMRT. Several innovative alternative IMRT options are also now available, including tomotherapy, CyberKnife and jaws-only linacs. These innovative approaches have divergent rationales. Jaws-only IMRT is being developed because it allows treatments to be delivered using conventional linacs without expensive multileaf collimators and not because it improves dose distributions. On the other hand, tomotherapy and CyberKnife systems have different geometric degrees of freedom, beam sizes and modulation techniques than those of conventional linacs, which may enable these innovative systems to deliver superior dose distributions to some treatment sites. Because conventional linacs are themselves finely honed machines, enhancement of one aspect of machine performance is sometimes accompanied by diminution of another. For example, tomotherapy systems possess an enhanced rotational IMRT capability but currently can only deliver coplanar radiation beams. Thus the various delivery systems may prove optimal for different types of treatments, specific machine designs excelling for specific disease sites. In practice, of course, IMRT delivery systems will be distinguished not just by the quality of the dose distributions they deliver but also by factors not discussed in this chapter, such as the efficiency of their treatment process, the integration of on-board imaging into that process, and their ability to measure, minimise and compensate for the effects of respiratory motion, a major detriment to accurate IMRT delivery.