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Dosimetry for the Flattening Filter Free (FFF) X-rays

Dosimetry for the Flattening Filter Free (FFF) X-rays. Accelerator Physics, LLC. Taiwan, May, 2012. FFF (Flattening filter free) X-rays. 6X, 40cm profile at 10 cm depth. FFF. Flat. Flattening filter.

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Dosimetry for the Flattening Filter Free (FFF) X-rays

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  1. Dosimetry for the Flattening Filter Free (FFF) X-rays Accelerator Physics, LLC. Taiwan, May, 2012.

  2. FFF (Flattening filter free) X-rays 6X, 40cm profile at 10 cm depth FFF Flat Flattening filter • Traditionally, radiation beam profiles are made to be “flat” for ease of dose computations. • However, with the development of IMRT and RapidArc, the beam flatness is no longer necessary. • FFF X-ray is generated by removing flattening filter from beam path • advantage: increased dose rate (shorter treatment time with potential benefit in patient/organ motion management and patient comfort). • advantage: more efficient use of the X-ray from accelerator guide (lower leakage and “off-field” radiation per cGy delivered to target at the CAX). Taiwan, May, 2012.

  3. First, about the FFF X-rays: Comparison of profiles from conventional 6X vs. FFF relative intensities Beam profiles from a 6X FFF, 20X20 beam 1.5 5.0 10.0 20.0 30.0 distances from CAX Beam profiles from a conventional 6X, 20X20 beam relative intensities 1.5 5.0 10.0 20.0 30.0 distances from CAX

  4. Comparison of profiles from conventional 10X vs. FFF relative intensities Beam profiles from a 10X FFF, 20X20 beam 2.5 5.0 10.0 20.0 30.0 distances from CAX Beam profiles from a conventional 10X, 20X20 beam relative intensities 2.5 5.0 10.0 20.0 30.0 distances from CAX

  5. Conventional, 10X, 20X20 beam 10X FFF, 20X20 beam Side-by-side comparison of profiles from conventional 10X vs. FFF (normalized to the relative intensities at CAX). Notice the relative decrease of “off-field” radiation from the FFF X-ray beams.

  6. Comparison between Gold Beam and TrueBeam (with and without flattening filter) With flattening filter (TrueBEAM vs. Gold Beam) Flattening Filter Free The FFF beam has a lower “apparent energy” than the conventional 6X beam May, 2012.

  7. Comparison between Gold Beam and TrueBeam (with and without flattening filter) With flattening filter (TrueBEAM vs. Gold Beam) Flattening Filter Free The FFF beam has a lower “apparent energy” than the conventional 10X beam May, 2012.

  8. Motivations of the study • Small field dosimetry is critical for high resolution X-ray radiation therapy, • such as the highly conformal stereotactic radiosurgery (SRS) and stereotactic • body radiotherapy (SBRT). • Silicon diodes yield a high signal from a small detecting volume, which makes • them suitable for the use in small-field measurement and radiation fields with • high-dose gradient. • Purposes of this study: • to investigate the small field photon dosimetry of the FFF X-ray beams • using silicon diode • to compare it to the corresponding conventional flat X-ray beams. May, 2012.

  9. Methods: • Two treatment machines in our institution were used in this study: • Varian’s TrueBEAM (6X-FFF, 6X-flat X-rays) with 0.5 cm MLC. • Novalis TX (6X-flat X-rays) with 0.25 cm MLC. • Output factors measurements using a diode detector (edge detector – • SunNuclear, Melbourne, Florida) and solid water phantom • Field sizes were formed using MLC rather than collimator jaws • (more realistic and practical in clinical application): • field sizes by MLC = 0.5, 1.0, 2.0, 3.0, 4.0, 6.0, 10.0 cm • corresponding jaw size = 0.7, 1.2, 2.2, 3.2, 4.2, 6.0, 10.0 cm May, 2012.

  10. 0.55 cm We used an “edge-detector” in this study. It has an active area (detection area) of 0.08 X 0.08 cm. May, 2012.

  11. Methods (cont.): • Measurement setup: • SSD = 100 cm and SSD = 220 cm • Dmax = 1.5 cm build-up • All readings normalized to 10 X 10 cm field size. May, 2012.

  12. Results: TrueBEAM’s output factors: • Good repeatability of the measurements was demonstrated. • The standard and extended SSD results were similar, for field • sizes = 0.5, 1.0, 2.0, 3.0, 4.0, 6.0, 10.0 cm: • Output factors for conventional 6X are smaller than Output factors for 6X-FFF from TrueBEAM. • Output factors for conventional 6X from TrueBEAM (0.5 cm MLC leaf width) are smaller than output factors for conventional 6X from Novalis TX which as an HD-MLC (0.25 cm MLC leaf width). May, 2012.

  13. Results: TrueBEAM’s Output factors (cont.) Fig 1: Small field output factors of TrueBeam and Novalis TX for SSD=100cm (OF for 6X-flat is slightly smaller than 6X-FFF for TB; OF for 6X TB is slightly smaller than NTX). TrueBEAM => MLC-120 NTX => HD-MLC

  14. Results: TrueBEAM’s Output factors (cont.) Fig 2: Small field output factors of TrueBeam and Novalis TX for extended SSD = 220 cm (OF for 6X-flat is slightly smaller than 6X-FFF for TB; OF for 6X TB is slightly smaller than NTX). TrueBEAM => MLC-120 NTX => HD-MLC

  15. Comparison of output factors between Eclipse and measured values with SRS mode Measured (PinPoint) Relative output factors Eclipse Square field sizes (cm X cm)

  16. Results: TrueBEAM’s Output factors (cont.) From this side-by-side comparison of 100 cm and 220 cm SSD output factor data, one can conclude that the detector size and differences in collimator scatter (resulted from different SSDs) are NOT contributing factors of these measurements for TrueBEAM 6X and 6X-FFF. The slight increase of output factor for the NTX (0.5 cm field size, 220 cm SSD) may be a result of inter-leaf leakage of the HD-MLC. TrueBEAM => MLC-120 NTX => HD-MLC TrueBEAM => MLC-120 NTX => HD-MLC May, 2012.

  17. Results: TrueBEAM’s Output factors (cont.) Table 1(a) SSD=100cm Table 1(b) extended SSD=220 cm The output factors taken at 100 cm and 220 cm SSD are quite similar. This indicates that the detector used was not under the influence of the radiation field sizes for any given MLC setting. (Note: radiation field sizes were set using MLCs, not collimator jaws). May, 2012.

  18. Discussions • The regular and extended SSD results are very similar and they are within measurement accuracy ranges. This indicates that the detector size is appropriate for the field sizes measured. • The output factors of conventional 6X were slightly smaller than those of 6X-FFF for field sizes: 0.5 to10.0 cm. This tendency is consistent when compared with output factors for larger field sizes as shown in the next picture. • Because the more efficient use of the X-rays from the accelerator, the “off- • field” radiation is lower than the conventional X-ray beams. This is particularly • suitable for SRS and SBRT applications where large dose per fraction is • indicated. • Similarly, the output factors of the TrueBEAM’s conventional 6X beam were smaller than those of the Novalis TX’s conventional 6X beam. This is because the thinner leaves in Novalis TX could introduce larger inter-leaf leakage (leaf thickness of NTX: 0.25cm; TrueBEAM: 0.5cm). This is specially noticeable when the radiation field is small and the inter-leaf leakages play a more dominant role. May, 2012.

  19. Output factors for conventional 6X vs. 6X (FFF) beams (0.6 cc. Farmer chamber, 300 volt bias) Difference between TrueBEAM & Gold Beam lengths of square fields (cm) May, 2012.

  20. Conclusions • In general, the 6X-FFF X-rays have slightly higher output than the 6X-flat for field sizes of 0.5 cm to 10cm (formed by MLCs) at Dmax in the Varian TrueBEAM. • The 6X-flat X-rays have slightly lower output factor than NTX’s 6X-flat X-rays in small fields (0.5 cm to 10 cm). • Silicon diodes detector is appropriate for the small field dosimetry measurements in the conventional beams and the FFF X-ray beams. Note: In SRS or SBRT clinical applications, often field penumbra characteristics are more important than the absolute doses. Beam profile measurements are important but often over-looked by many. May, 2012.

  21. Differences in beam profile scans using 2 different types of detectors (solid state and micro-scanning ion-chamber) diode IBA scanning chamber Field size: 50 X 50 mm) May, 2012.

  22. Comparison of profiles between Eclipse and measured values with SRS mode 10 X 10

  23. 10 X 10 May, 2012.

  24. Comparison of profiles between Eclipse and measured values with SRS mode 4 X 4

  25. 4 X 4 May, 2012.

  26. Comparison of profiles between Eclipse and measured values with SRS mode 3 X 3

  27. 3 X 3 May, 2012.

  28. Comparison of profiles between Eclipse and measured values with SRS mode 2 X 2

  29. 2 X 2 Taiwan, May, 2012.

  30. Comparison of profiles between Eclipse and measured values with SRS mode 1 X 1

  31. 1 x 1 Taiwan, May, 2012.

  32. Comparison of PDDs measured with ion-chamber and diode One must also realize that there are are differences between data taken with different types of detectors. percentage depth dose 6X-FFF, field size: 30X30 and 40X40 mm depths (mm)

  33. Checking the dose integration linearity of monitor chambers in Varian TrueBEAM using an ionization chamber and diode. Data illustrate that the monitor chambers have excellent dose linearity characteristics over typical range of clinical doses, down to 1 MU with filtered and FFF beams. May, 2012.

  34. Ion-recombination effect (P-ion) of a Farmer type, 0.6 cc, measurement chamber is shown here. If differences in P-ion is not accounted for, systematic errors may result in PDD and profile measurements. May, 2012.

  35. 15X FFF Un-corrected corrected Note: 15X FFF is not a clinically release product May, 2012.

  36. corrected Un-corrected 15X FFF corrected 6X FFF Un-corrected Note: 15X FFF is not a clinically release product May, 2012.

  37. Future works • Comparing the TLD “microcubes” data of the small field dosimetry for the conventional and Flattening Filter Free X-ray beams with the corresponding silicon diodes data. • Measure TMR and profiles for small fields (formed with both 0.5 cm and 0.25 cm MLCs) using FFF beams. Acknowledgements: A portion of this work and travel expenses have been funded by Accelerator Physics, Melbourne, Florida, 32901. May, 2012.

  38. Reference: Das, et al., TG-106, Med Phys 35, 4186, 2008. Sauer, W., Med Phys 34, 1983, 2007. Das, et al., Radiosurgery 3, 177, 2000. May, 2012.

  39. AAPM/COMP 2011, Vancouver, Canada Surface Dose Study of the Flattening Filter Free (FFF) X-ray Photons Taiwan, May, 2012.

  40. Comparison between Gold Beam and TrueBeam (with and without flattening filter) With flattening filter (TrueBEAM vs. Gold Beam) Flattening Filter Free The FFF beam has a lower “apparent energy” than the conventional 6X beam Depths (mm)

  41. Comparison between Gold Beam and TrueBeam (with and without flattening filter) With flattening filter (TrueBEAM vs. Gold Beam) Flattening Filter Free The FFF beam has a lower “apparent energy” than the conventional 10X beam Depths (mm)

  42. Motivation of the study increasing interest in FFF X-rays substantial increase in dose rate : potential improvement in patient / organ motion management and patient comfort some clinical concern on the skin doseof the FFF X-rays because the lack of beam hardening from the flattening filter in the beam path There are large ranges of surface doses from FFF beams (from non-Varian machines) reported in the literature This study evaluates the skin dose from the FFF X-ray beams from a Varian TrueBEAM • * Vassiliev et al. Stereotatic radiotherapy for lung cancer using a flattening filter free clinac. J ApplClin Med Phys 2009; 10: 14-21 • ** Vassilliev et al. Dosimetric properties of photon beams from a flattening filter free clinical accelerator. Phys Med Biol 2006; 51: 1907-17. Taiwan, May, 2012.

  43. Methods: • A Varian TrueBEAM has been fully commissioned for clinical treatments • at MIMA Cancer Center since July, 2010: • Dose rates (MU / minutes) from Varian TrueBeam (TB) : • (A) convention X-ray beams: 6X, 10X, and 15X • Maximum dose rate: 600 MU / minute • (B) FFF X-ray beams: nominal 6X, and 10X • Maximum dose rate: (6X FFF) 1400 MU / minute • Maximum dose rate: (10X FFF) 2400 MU / minute Taiwan, May, 2012.

  44. Methods: • Entrance (surface) doses are measured using a parallel plate ion chamber • and solid water phantom • Buildup depths: • For 6X flat and FFF, d = 0, 1, 2, 4, 5, 7, 10, 15 cm • For 10X flat and FFF, d = 0, 1, 2, 4, 5, 7, 10, 15, 20, 25 cm • Field sizes (formed by MLCs - not collimator jaws): • Field sizes = 2.0, 3.0, 4.0, 6.0, 10.0 cm (by MLCs) • corresponding jaw sizes = 2.2, 3.2, 4.2, 6.2, 10.2 cm Taiwan, May, 2012.

  45. Methods (cont): • A 0.8 cm diameter parallel-plate ionization chamber (PTW 23342) was used for all • measurements. This instrument was chosen because its small diameter compared • with the field sizes measured. • All data were normalized to the 10 X 10 cm field size at Dmax. 0.8 cm diameter parallel plate (PTW N-23342) ionization chamber

  46. Results: Fig 1: Measured surface doses (no build-up) are shown below for 6X and 10X conventional (flat) and FFF X-ray photons, as a function field sizes, where: As expected, 6X FFF surface doses are higher than the conventional 6X (flat) beam. And, the surface doses from 10X beams are less than those from 6X beams. Relative surface doses (no buildup) plotted against field sizes 6X FFF 6X 10X FFF 10X

  47. Results (cont): Fig 2: Surface doses for 6X FFF beams are higher than conventional (flat) X-ray beams. Relative surface doses plotted against buildup thicknesses solid lines: FFF dashed lines: flat

  48. Results (cont): Fig 3: Surface doses for 10X FFF beams are higher than conventional (flat) X-ray beams. Solid lines: FFF Dashed lines: flat

  49. Results (cont): Tabulated data 6X

  50. Results (cont): Tabulated data 10X

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