Influence of focal spot on characteristics of very small diameter radiosurgical beams

Abstract

Percentage depth dose (PDD) distributions and beam profiles of very small diameter (1.5-5 mm) megavoltage radiosurgical beams calculated with Monte Carlo (MC) technique critically depend on the diameter of the circular focal spot used in the simulation: The smaller is the field diameter, the larger is the effect. Thus, in simulations of radiosurgical fields that have diameters of the order of the focal spot size, an accurate focal spot geometry should be used. We used a simplified moving slit technique in conjunction with a diode detector for evaluation of the focal spot size and shape of a megavoltage 6 MV linac as well as for determination of the equivalent focal spot diameter of the linac for use in MC simulations. The measured total diode signal contains three components: A direct focal spot signal, a background signal, and an extra-focal radiation signal. A single profile scan of the focal spot signal is Gaussian like in shape, and its full width at half maximum is used to define the focal spot dimension for this scan. The focal spot of our 6 MV linac is approximated with a Gaussian circle, and when the geometry of the effective focal spot circle is used in MC simulations, the agreement between MC-calculated and measured PDD distributions as well as beam profiles is good even for radiosurgical fields as small as 1.5 mm in diameter. Our results also confirm that matching the penumbral areas of accurately measured large-field beam profiles to the same areas of the MC-calculated beam profiles reliably leads to a realistic effective focal spot size for use in MC simulations of very small diameter beams.

Small field dosimetry: What have we learnt?

“…Czarnecki et al (51) provided conclusive data indicating that k Ω is relatively independent of focal spot of various detectors very similar to data as shown in Figure 7. Sam et al (52) provided extensive Monte Carlo data for very small fields with variable focal spot. The data (PDD, Profiles) are only different for extremely small fields and can be ignored for modern machines and within the accuracy needed.…”

An investigation into the use of MMCTP to tune accelerator source parameters and testing its clinical application

“…However, this value is plausible, as measurements of focal spot sizes ( 17 ) have shown that a FWHM of 0.05 cm radius is possible as a FWHM of between 0.12 and 0.14 cm diameter for the Varian Clinac 2100 has been recorded. Sham et al ( 15 ) also obtain a FWHM of 0.15 cm diameter for a 6 MV beam for a Varian Clinac. In addition, our values are only correct to within 0.05 cm, as this was considered sensitive enough for the purposes of this investigation.…”

“…Initially starting at large field ($30\times 30\hspace{0.17em}{\text{cm}}^2$) dose profiles and PDDs and progressing to examination of output factors for a range of field sizes, focusing particularly on smaller field sizes for finer tuning. Small fields are more sensitive to changes in the FWHM of the radius of the incident electron beam, ( 3 , 15 ) which allows tuning to be performed more quickly and accurately. It was decided to focus on the larger field sizes initially as these are more sensitive to changes in the mean energy of the initial electron source when looking at profiles at 1.5 cm depth ( 3 , 16 ) — again improving efficiency and precision of the tuning process being developed.…”

Estimation of focal and extra-focal radiation profiles based on Gaussian modeling in medical linear accelerators

“…Because the characteristics of the focalspot and extra-focal radiations differ from each other, many methods for determination of extra-focal radiation have been investigated based on (1) direct measurement [9][10][11], (2) indirect measurement [12][13][14][15][16], and (3) Monte Carlo simulations [7,17,18]. However, further studies are needed for investigation of the experimental methods for the total focal radiation and more accurate modeling for them.…”

“…Some researchers hypothesized that the total focal radiation consisted of focal-spot radiation and extra-focal radiation [9][10][11]. Because the characteristics of the focalspot and extra-focal radiations differ from each other, many methods for determination of extra-focal radiation have been investigated based on (1) direct measurement [9][10][11], (2) indirect measurement [12][13][14][15][16], and (3) Monte Carlo simulations [7,17,18].…”