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Commissioning and quality assurance of the Pinnacle³ radiotherapy treatment planning system for external beam photons

¹ Joint Department of Physics, The Institute of Cancer Research and the Royal Marsden NHS Trust, Downs Road, Sutton, Surrey SM2 5PT and ² National Physical Laboratory, Centre for Ionising Radiation Metrology, Dosimetry Group, Kaye Building, Queens Road, Teddington TW11 0LW, UK.

View larger version (29K): [in a new window]   Figure 1. Differences between depth dose curves measured with an ionization chamber and an energy-compensated photon diode for a 10 MV beam. Results are shown for 10 cm x 10 cm (bold line) 20 cm x 20 cm (feint line) and 40 cm x 40 cm (broken line) field sizes. The differences are percentages of maximum dose, and a positive difference indicates that the ionization chamber records a higher dose than the diode. Large differences are seen in the build-up region, but thereafter the differences are comparable with the magnitude of the noise in the respective measurements.

View larger version (15K): [in a new window]   Figure 2. (a) The energy spectra of the 6 MV and 10 MV beams of an Elekta SL15 linear accelerator (Elekta, Crawley, UK) as generated by Monte Carlo simulation, using energy bins all 0.1 MeV in width. (b) The spectra rebinned for use in Pinnacle3 (Philips Radiation Oncology Systems, Milpitas, CA), with the unequally spaced energy bins depicted along the top of the graph.

View larger version (26K): [in a new window]   Figure 3. The Monte Carlo generated primary fluence at the exit of the accelerator head for 6 MV fields of size 4 cm x 4 cm, 10 cm x 10 cm and 40 cm x 40 cm. The off-axis fluence increase can be represented by the straight lines shown. Results are shown for X- (heavy lines) and Y- (feint lines) directions.

View larger version (13K): [in a new window]   Figure 4. (a) Mean photon energy as a function of off-axis distance, calculated by Monte Carlo simulation for the 6 MV (solid line) and 10 MV (broken line) beams of an Elekta SL15 accelerator (Elekta, Crawley, UK). The energy reduction is approximately linear. (b) Electron contamination for the 6 MV beam of an Elekta SL15 accelerator, as modelled by Monte Carlo simulation. The solid curve is the electron contamination predicted by Monte Carlo simulation, while the broken curve represents the formula available in Pinnacle3 (Philips Radiation Oncology Systems, Milpitas, CA), with the parameters fitted to the Monte Carlo curve. The equation for the fitted curve is: , where Fd (d, s) is the electron contamination factor as a function of depth d and field size s, Fs (s) is the dependence of electron contamination on field size, k is a constant, and dme is the maximum depth at which electron contamination occurs. SF is the ratio of electron contamination at the surface to that described by the above equation at d=0. Fs (s)=0.06, SF=0.6, k=1.3, and dme=2.2 cm. At depths less than DF.dme, where DF=0.2, the curve is linear, such that the surface dose is Fs (s).

View larger version (70K): [in a new window]   Figure 5. Planar dose distribution from an irregular block aperture computed using the small-field beam model (broken lines) compared with film measurement (solid lines). The large grid squares represent 2 cm. Both distributions are normalized on the central axis at the measurement depth (5 cm in water-equivalent material). The block shape has been digitized from the 50% isodose contour on the film in order to remove the uncertainties due to block production and light/radiation field disagreement.

View larger version (45K): [in a new window]   Figure 6. Ratios between depth dose curves with and without heterogeneities for (a) lung, (b) 1.35 g cm-3 bone, and (c) 1.85 g cm-3 bone. The shaded region in each graph shows the position of the inhomogeneity. Measurements with film (squares) and ionization chamber (circles) are compared with calculated values (lines). All measurements are for a 5 cm x 5 cm 10 MV field at gantry angle 5°. (d) Ratios between doses with and without heterogeneities at various distances beyond a region of lung density. Measurements using an ionization chamber are compared with calculated values for 6 MV and 10 MV beams with gantry angle 0°.

View larger version (36K): [in a new window]   Figure 7. Differences between calculated and measured doses at the centre of the in-house inhomogeneity phantom for 6 MV and 10 MV open beams on three SL-series linear accelerators, LA 1, LA 2 and LA 3. Differences are shown for fields passing through 0.28 g cm-3 lung, 1.35 g cm-3 bone and 1.85 g cm-3 bone. A positive difference indicates that Pinnacle3 predicts a higher dose than is actually measured. The differences between Pinnacle3 and measurements in water have been subtracted from the results, so that the figure represents the differences due to inhomogeneities only, rather than differences due to the beam models.

View larger version (49K): [in a new window]   Figure 8. Calculated (broken lines) and measured (solid lines) isodoses for a diagonal multileaf collimator (MLC) field. The MLC leaves project downwards from the upper right hand corner of the figure. The isodoses are normalized to the central axis of the field. The graduations represent 1.0 cm.

View larger version (23K): [in a new window]   Figure 9. (a) Dose-volume histogram (DVH) for a dose distribution comprised of eight uniform regions, calculated with a voxel size of 4 mm x 4 mm x 4 mm. The region of interest is placed centrally (heavy line), or displaced 2 mm laterally and 2 mm posteriorly (feint line). (b) DVHs for spherical structures of 6 cm, 8 cm and 10 cm diameter, calculated using either a 4 mm x 4 mm x 4 mm grid spacing (heavy lines) or a 2 mm x 2 mm x 2 mm grid spacing (feint lines). (c) DVHs for spherical structures of 6 cm, 8 cm and 10 cm diameter, calculated using Pinnacle3 (heavy lines) and an independent program (feint lines). Grid size is 4 mm x 4 mm x 5 mm.

View larger version (49K): [in a new window]   Figure 10. TLD verification of (a) pelvic and (b) thoracic treatment plans using an anthropomorphic phantom. Differences between calculated and measured doses are given as percentages of the isocentric dose. A positive difference indicates that Pinnacle3 predicts a higher dose than is actually measured.

View larger version (19K): [in a new window]   Figure 11. Differences between calculated and measured doses for national trial phantoms. (a) The START trial phantom [29], showing results for reference, lung, medial and apex points. A positive difference indicates that Pinnacle3 predicts a higher dose than is actually measured. Position relative to the central slice increases superiorly. (b) The RT-01 trial phantom. The error bars represent the maximum and minimum doses calculated by Pinnacle3 within a region of radius 4 mm around the point of interest, indicating the possible influence of positioning errors on the results, particularly in the penumbra, e.g. points CRECT and IRPTV. The first letter of each abbreviation represents the superior/inferior position: SBLAD, superior bladder; SRPTV, superior right PTV; SRECT, superior rectum; SSVL, superior seminal vesicles; CREF, central reference point; CBLAD, central bladder; CRPTV, central right PTV; CLPTV, central left PTV; CRECT, central rectum; CFMLT, central left femoral head; IAPEX, inferior apex of prostate; IRPTV, inferior right PTV; IRECT, inferior rectum.