The effect of secondary electron spread on the penumbra in high energy photon beam therapy

Day, Michelle; Lambert, G.; Locks, S M

doi:10.1259/0007-1285-63-748-278

Cited by 12 publications

(8 citation statements)

References 28 publications

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“…The penumbra width ranged from 7.8 mm to 11.0 mm for the 6 MV and 8.4 mm to 11.1 mm for the 10 MV beams, respectively. These values are in agreement with the values derived with the line spread and point spread function analysis of Day et al 32 for megavoltage beams. Figure 5 shows the plot of penumbra versus field size.…”

Section: Beam Profiles and Penumbrasupporting

confidence: 91%

Beam characteristics of a retrofitted double‐focused multileaf collimator

et al. 1998

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Multileaf collimators (MLCs) are generally believed to be convenient and cost-effective tools for intensity modulation and conformal therapy. They are becoming a standard feature on new accelerators; however, the older units can be retrofitted with modern MLCs. Before such a unit can be clinically used, the beam characteristics must be verified. In this study the beam characteristics of a Siemens double-focused MLC retrofitted to an MD2 linear accelerator are presented. The head leakage along with inter- and intra-leaf radiation transmission were measured using film. The collimator (Sc), phantom (Sp), total (Scp) scatter factors, central axis depth dose, beam profiles for off-axis ratios, penumbra, and surface dose were evaluated for square, rectangular, and irregularly shaped fields. The maximum head leakage was estimated to be < 0.05% in any plane at a distance of 1 m and maximum transmission through the MLC leaves was estimated to be < 1.4% and < 1.1% for the 10 MV and 6 MV beams, respectively. The maximum differences between pre- and post-MLC installation data for the Sc and Scp were < or = 0.7% and < or = 1.4%, respectively. Similarly, the percent depth dose data for all fields and both beam energies were within 1.5% of the original data. The beam profiles measured at various depths were also in agreement with those of the pre-MLC installation data. The measured beam penumbra (20%-80%) showed a range of 7.8 mm-11.0 mm for the 6 MV and 8.4 mm-11.1 mm for the 10 MV beams from smallest to largest fields. These ranges differ by less than a millimeter from those of the old data. The surface dose measurements were slightly lower than the conventional jaw values suggesting that MLC does not produce significant electron contamination. It is concluded that the retrofitted MLC maintains the integrity of the original beam and may provide a cost-effective conformal therapy.

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Section: Beam Profiles and Penumbrasupporting

confidence: 91%

Beam characteristics of a retrofitted double‐focused multileaf collimator

et al. 1998

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“…The two measurement setups offer a method to determine the precision of the detector positioning and the repeatability of jaw collimator positioning using the Full Width of Half Maximum (FWHM) and they are treated as type B uncertainties. Furthermore, for broad beams, there is an established relation between penumbra width and focal spot size . Therefore, lateral dose profiles were measured for 10 cm × 10 cm fields using the IBA PFD.…”

Section: Methodsmentioning

confidence: 99%

6‐MV small field output factors: intra‐/intermachine comparison and implementation of TRS‐483 using various detectors and several linear accelerators

et al. 2019

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Purpose: To investigate the applicability of output correction factors reported in TRS-483 on 6-MV small-field detector-reading ratios using four solid-state detectors. Also, to investigate variations in 6-MV small-field output factors (OF) among nominally matched linear accelerators (linacs). Methods: The TRS-483 Code of Practice (CoP) introduced and provided output correction factors to be applied to measured detector-reading ratios to obtain OFs for several small-field detectors. Detector readings for 0.5 cm 9 0.5 cm to 8 cm 9 8 cm fields were measured and normalized to that of 10 cm 9 10 cm field giving the detector-reading ratios. Three silicon diodes, IBA PFD, IBA EFD (IBA, Schwarzenbruck, Germany), PTW T60017, and one microdiamond, PTW T60019 (PTW, Freiburg, Germany), were used. Output correction factors from the CoP were applied to measured detector-reading ratios. Measurements were performed on six Clinac and six TrueBeam linacs (Varian Medical Systems, Palo Alto, USA). An investigation of the relationship between the size of small fields and corresponding detector-reading ratio among the linacs was performed by measuring lateral dose profiles for 0.5 cm 9 0.5 cm fields to determine the full width half maximum (FWHM). The relationship between the linacs' focal spot size and the small-field detector-reading ratio was investigated by measuring 10 cm 9 10 cm lateral dose profiles and determining the penumbra width reflecting the focal spot size. Measurement geometry was as follows: gantry angle = 0°, collimator angle = 0°, source-to surface distance (SSD) = 90 cm, and depth in water = 10 cm. Results: For a given linac and 0.5 cm 9 0.5 cm field, the deviations in detector-reading ratios among the detectors were 9%-15% for the Clinacs and 4%-5% for the TrueBeams. Use of output correction factors reduced these deviations to 6%-12% and 3%-4%, respectively. For field sizes equal to or larger than 0.8 cm 9 0.8 cm, the deviations were corrected to 1% using output correction factors for both Clinacs and TrueBeams. For a given detector and 0.5 cm 9 0.5 cm field, the deviations in detector-reading ratios among the linacs were 11%-17% for the Clinacs and 5-6% for the TrueBeams. For 1 cm 9 1 cm the deviations were 1%-2% for Clinacs and 1% for TrueBeams. For field sizes larger than 1 cm 9 1 cm the deviations were within 1% for both Clinacs and TrueBeams. No relationship between FWHMs and detector-reading ratios for 0.5 cm 9 0.5 cm was observed. For Clinacs, larger 10 cm 9 10 cm penumbra width yielded lower 0.5 cm 9 0.5 cm detector-reading ratio indicating an effect of the focal spot size. For TrueBeams, the spread of penumbra widths was lower compared to Clinacs and no similar relationship was observed. Conclusions: Output correction factors from the TRS-483 CoP are not sufficient for accurate determination of OF for 0.5 cm 9 0.5 cm fields but are applicable for 0.8 cm 9 0.8 cm to 8 cm 9 8 cm fields. Nominally matched Clinacs and TrueBeams show large differences in detectorreading ratios for fields smaller than 1 cm 9 1...

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“…2 For x-ray energies higher than 6 MV, the radiological penumbra from secondary electrons dominates the field size effect, resulting in larger penumbra even for broad beams. 3,4 Other factors impacting beam energy selection include inter-MLC leakage, neutron dose, and shielding cost, and lower x-ray energies are favored in these regards. Because of these considerations, 6 MV, the lowest energy available on modern commercial Linacs, has gained dominant utilization in intensity-modulated therapies that employ predominantly coplanar beams.…”

Section: Introductionmentioning

confidence: 99%