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SRS & SBRT - Unflattened Beam

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1.Stereotactic Radiosurgery (SRS)

SRS is a precise and focused delivery of a single, high dose of irradiation to a small and critically located intracranial volume while sparing normal structure

2.Stereotactic Body Radiation Therapy (SBRT)

SBRT is a treatment procedure similar to SRS, except that it deals extra-cranial radiosurgery

3.Flattening Filter Free (FFF) mode

FFF beam is produced without the use of flattening Filter

In the 1990s, several groups studied about FFF high-energy photon beams. The main interest for that, is to increase the dose rate for radiosurgery or the "physics interest”.

Need of increase in dose rate from traditional 300-600 to 1400-2400MU/min to overcome time-inefficiency and to improve patients comfort specially in SRS/SBRT
Flattening Filter Free (FFF) mode
FFF beam is produced without the use of flattening Filter

In the 1990s, several groups studied about FFF high-energy photon beams. The main interest for that, is to increase the dose rate for radiosurgery or the "physics interest”.

Need of increase in dose rate from traditional 300-600 to 1400-2400MU/min to overcome time-inefficiency and to improve patients comfort specially in SRS/SBRT

Published in: Health & Medicine
  • Light and its nature have caused a lot of ink to flow during these last decades. Its dual behavior is partly explained by (1)Double-slit experiment of Thomas Young - who represents the photon’s motion as a wave - and also by (2)the Photoelectric effect in which the photon is considered as a particle. A Revolution: SALEH THEORY solves this ambiguity and this difficulty presenting a three-dimensional trajectory for the photon's motion and a new formula to calculate its energy. More information on https://youtu.be/mLtpARXuMbM
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SRS & SBRT - Unflattened Beam

  1. 1. 11 October 2014
  2. 2. Outline SRS and SBRT History of SRS Recent advances in SRS and SBRT Advantage of Flattening Filter Free(FFF) beam Characteristic of Flattening Filter Free beam Recommendation of AERB AAPM TG 101
  3. 3. Introduction Stereotactic Radiosurgery (SRS) – SRS is a precise and focused delivery of a single, high dose of irradiation to a small and critically located intracranial volume while sparing normal structure Stereotactic Body Radiation Therapy (SBRT) – SBRT is a treatment procedure similar to SRS, except that it deals extra-cranial radiosurgery
  4. 4. SRS and SBRT Definition 1. High doses of radiation via multiple beams 2. Limited number of treatment session (1-5) 3. Image guided treatment (CT, PET, MRI) 4. Computer assisted robotic delivery 5. Real time respiratory motion accommodation
  5. 5. SRS and SBRT The challenge for SRS and SBRT is to accurately deliver conformal high dose radiation to the target and minimize normal tissue damage.
  6. 6. Differences between Conventional and Stereotactic
  7. 7. Historical Background The first one to combine stereotactic methodology with radiation therapy was the Swedish neurosurgeon Lars Leksell. Leksel performed the first treatment in 1951, at the Karolinska Institute, and called the new therapy approach radiosurgery (RS) Leksel continued his work and built the first isotope radiation machine, in 1968, the Gamma knife The stereotactic radiation therapy with LINAC started in the early 1980s: the Swedish physicist Larsson proposed to use the LINAC instead Co 60 or protons (Larsson et al. 1974)
  8. 8. Gamma Knife Proton Therapy Radiosurgery Machines Cyberknife Tomotherapy Brainlab Vero Varian-Truebeam
  9. 9. SBRT Work Flow Immobilization Simulation Motion Management Planning Delivery Motion Verification IGRT
  10. 10. How LINAC Radiosurgery Works The gantry of the LINAC rotates around the patient, producing an arc of radiation focused on the target. The couch in which the patient rests is then rotated in the horizontal plane, and another arc is performed. In this manner, multiple non-coplanar arcs of radiation intersect at the target volume and produce a high target dose, resulting in minimal radiation affecting the surrounding brain and normal tissue.
  11. 11. How Gamma Knife Radiosurgery Works The GammaKnife is used to treat brain tumors. The procedure begins with the patient receiving anesthesia and a frame is attached to the head to hold it in place. The patient lays on their back and moved head first into the machine, where 201 beams of cobalt – 60 radiation target the diseased tissue, without damaging the surrounding tissue.
  12. 12. Recent Advances in SBRT and SRS VMAT Volumetric Modulated Arc Therapy (VMAT) was first introduced in 2007 and described as a novel radiation technique VMAT is the simultaneous variation of three parameters during treatment delivery, i.e. gantry rotation speed, treatment aperture shape via movement of MLC leaves and dose rate
  13. 13. Recent Advances in SBRT and SRS Flattening Filter Free (FFF) mode FFF beam is produced without the use of flattening Filter In the 1990s, several groups studied about FFF high-energy photon beams. The main interest for that, is to increase the dose rate for radiosurgery or the physics interest”. Need of increase in dose rate from traditional 300-600 to 1400-2400MU/min to overcome time-inefficiency and to improve patients comfort specially in SRS/SBRT
  14. 14. Dosimetric advantages of FFF beams FFF has increased dose rate, e.g., 1400 MU/min for 6 MV, 2400 MU/min for 10 MV. FFF beams have less variation of off-axis beam hardening. FFF has less photon head scatter and thus less field size dependence. FFF has less leakage outside of beam collimation
  15. 15. Potential advantages of FFF beams Fast treatment for Stereotactic Radiotherapy (SRT) and SRT plans between FB and FFF beams should be similar for small fields. FFF is especially useful for SBRT, where respiration controlled treatment delivery is compromised by the large number of MU to delivery high fraction doses. Patient beam on time can be reduced for IMRT
  16. 16. Cyberknife Tomotherapy BrainLab Vero FFF Mode Machines Varian -Truebeam Elekta – Versa HD
  17. 17. Two Different FFF machines at RGCI Varian – Truebeam Dose rate : 1400MU/min 6MV FFF 2400MU/min 10MV FFF Siemens – Artiste Dose rate: • 2000MU/min - 6MV_FFF 120Leaf HD – MLC Center - 2.5 mm width x 32 pairs Peripheral 5.0 mm width x 28 pairs Modulation Area 22x40 cm2 Speed of MLC 2.5cm/sec 160Leaf MLC Resolution 5.0 mm, 40cm wide Modulation Area 40x40 cm2 Speed of MLC 4.0cm/sec
  18. 18. Comparison between 6X FB and FFF (Varian TrueBeamTM) - Profiles FB Profiles FFF Profiles
  19. 19. Comparison of PDD for FBFFF for 10cmX10cm 10XFB 10XFFF 6XFFF 6XFB
  20. 20. Variation of Output factor in air with field size 1.0500 1.0400 1.0300 1.0200 1.0100 1.0000 Head Scatter Factor ( Sc) 0.9900 0.9800 0.9700 0.9600 0.9500 0.9400 0 5 10 15 20 25 30 35 40 Field Size in cm2 6MV-FB Varian True Beam 6MV-FFF True Beam 10MV-FB Varian True Beam 10MV-FFF True Beam
  21. 21. Dosimetry concern of FFF • Due to the above changes, the Dosimetric parameters like field size definition, beam quality, surface dose, off axis ratio (OAR), flatness, symmetry, degree of un-flatness, penumbra and depth dose profiles differs from standard Linac with Flat beam. • There is no international standard/acceptance test protocol available for FFF beam, AERB constituted a Task Group to evolve the acceptance criteria for FFF beam
  22. 22. AERB Recommendations for FFF Treatment should be implemented with TPS through Record Verify system, Manual planning and calculation shall not be adopted in clinical use of FFF beam.
  23. 23. AERB Recommendations for FFF Measurements should cover Beam Energy: TPR20/10 for 10 cm x 10 cm Field Size for all FFF energies Surface dose: 10cm x 10cm and 20cm x 20cm compared with the corresponding nominal flat beam energy OAR At ±3 cm from central axis at the depth of 10 cm for 10 cm x 10 cm collimator setting shall be measured for all available FFF energies – •
  24. 24. AERB Recommendations for FFF Depth dose profiles Dose profile for field size 5cm x 5cm, 10cm x 10cm and 20cm x 20cm at depth of Dmax and 10cm shall be recorded for all available energies FS 10 cm x 10 cm, the Dosimetric parameters such as field size, penumbra, flatness, symmetry shall be measured and evaluated the methods applied for flat beam If flatness is ± 3%, the evaluation criteria of unflattend beam shall be adopted
  25. 25. AERB Recommendations for FFF Depth dose profiles Flatness: As per IEC 976 (IEC 60976), the flat region for field sizes less than 10cm x 10cm along major axes defined by subtracting 1cm from the beam profiles. Eg. For F.S 5cm x 5cm flat region is central 3cm.
  26. 26. AERB Recommendations for FFF Depth dose profiles Inflection Point: Inflection Point can be identified as per its mathematical definition. However, for practical purposes it can be approximated as the mid point on either side of the high gradient region (sharply descending part) of the beam profile. IP is located at h/2. Penumbra:Lateral Separation beween either side of profile will be measured for the penumbra
  27. 27. AERB Recommendations for FFF Degree of un-flatness: • To quantify the degree of un-flatness, lateral distance from the central axis at 90%, 75% and 60% dose points on either side of the beam profile shall be recorded along major axes.
  28. 28. Trubeam FB and FFF beam Stereotactic Plan comparison – Liver 6 MV_FFF 1400MU/M 6 MV_FB 600MU/M
  29. 29. Trubeam FB and FFF beam Stereotactic Plan comparison – Brain 6 MV_FB 600MU/M 6 MV_FFF 1400MU/M
  30. 30. Comparison of FFF and FB for SBRT Similar Dose distribution and DVH for FB and FFF Treatment plan strategies are similar between FB and FFF beams since the beam profile are similar for field size 4 cm
  31. 31. AAPM TG 101 Recommendation for SBRT SBRT Patient Selection Criteria: When appropriate protocols are not available, clinicians must decide whether they will treat patients in accordance with published guidelines or develop new SBRT guidelines. At a minimum, an institutional treatment protocol or set of guidelines should be developed by radiation oncologists and physicists. Simulation imaging: The simulation study should cover the target and all organs at risk to obtain geometric and Dosimetric information for the treatment setup Slice thickness: 3 mm near clinically important organs
  32. 32. AAPM TG 101 Recommendation for SBRT Planning Recommendation: The adequacy of target margins i.e., GTV, CTV, ITV, in SBRT should be based on from information in the current literature available Dose Calculation Algorithm: Algorithms that account for 3D scatter integration such as convolution/superposition have been found to perform adequately in most clinical situations, including in many cases circumstances where there is a loss of electronic equilibrium such as the lung tissue interface or tumor margin in low-density medium. Calculation algorithms accounting for better photon and electron transport such as Monte Carlo would be ideal for the most demanding circumstances, such as a small lesion entirely surrounded by a low-density medium.
  33. 33. AAPM TG 101 Recommendation for SBRT Special Dosimetry Recommendation: Due to the small dimensions and steep dose gradients of photon beams used in SRS/SBRT and IMRT, an appropriate dosimeter with a spatial resolution of approximately 1 mm or better stereotactic detectors is required to measure the basic dosimetry data, e.g., the total scatter factor or relative output factor, tissue maximum ratio, and off-axis ratios.
  34. 34. Accuracy of SRS SBRT depends on Linac Mechanical – Iso Accuracy of SRS frame Immobilization Positional accuracy Dosimetry accuracy
  35. 35. Conclusions Advanced treatment techniques such as SBRT and IMRT have stimulated interest in FFF beam, which provides higher dose rate, and reduced head scatter, leaf transmission, and head radiation leakage. Clinical utilities of FFF beam include a treatment time reduction for SRT, SBRT, and IMRT. More studies are needed for FFF beam to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance.
  36. 36. Conclusions Several aspects related to standardization, dosimetry, treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of FFF beams.
  37. 37. Our Publications Abstracts of FFF beam “Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini Phantom” – Journal of Medical Physics, July-September 2014 A Comparison of Out-Of-Field Dose and Its Constituent Components for 6MV Flattened and 7MV Unflattened – AAPM 55th Annual Meeting - 2013 Comparison of the Depth Dose in the Build-Up Region and Surface Dose for 6MV Flattened and 7MV – AAPM 55th Annual Meeting - 2013 Scatter Factors Comparison of 6MV Flattened and 7MV Unflattened Beams – AAPM 54th Annual Meeting - 2012 Effect of Surface Dose and Depth of Maximum Dose with Physical Wedge Filters for 6MV Flattened and 7MV – AAPM 54th Annual Meeting - 2012
  38. 38. Our Medical Physics team Dr. Girigesh Yadav Mr. Manindra Mishra Mr. T. Suresh Mr. Lalit Kumar Mr. Pavan Singh

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