Photon beam dose distributions in 2D

Transcription

1 Photon beam dose distributions in 2D Sastry Vedam PhD DABR Introduction to Medical Physics III: Therapy Spring 2014 Acknowledgments! Narayan Sahoo PhD! Richard G Lane (Late) PhD 1

2 Overview! Evaluation of photon beam characteristics! Isodose lines! Photon beam characteristics! Percent depth dose, Profiles, Umbra, Penumbra! Beam modifiers! Flattening filter, Blocks, Wedges, Compensators! Planning! Single field! Parallel opposed fields! Multiple beams! Wedges and arcs! Junctions! Field matching Isodose lines/curves! Family of curves/lines made by connecting points receiving identical dose! Typical curves pass through CAX a PDDs of 90%, 80%, 70%, 60% 10%! Employed to understand:! Beam flatness! Penumbra! Penetration! D max 2

3 Orthovoltage (Penetration)! 200 kvp, 50 cm SSD, 10x10 cm! Max dose on surface! Forward peaked! Rapid dose fall-off! Significant side scatter! 50% ~ 7 cm From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins 2003 Megavoltage (Penetration) 60 Co, 80 cm SSD 10x10 cm FS 4 MV, 100 cm SSD 10x10 cm FS 10 MV, 100 cm SSD 10x10 cm FS From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins

4 Megavoltage (Penetration) Student Exercise 60 Co, 80 cm SSD 10x10 cm FS 4 MV, 100 cm SSD 10x10 cm FS! Identify some salient features of each of the above beams.! Compare the beams based on parameters:! Across profile! Along depth From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins MV, 100 cm SSD 10x10 cm FS Percent depth dose (PDD) From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins

5 PDD vs Depth and Energy From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins 2003 PDD The Build up region From Metcalfe, Kron, Hoban, The Physics of Radiotherapy X-Rays from Linear Accelerators, Medical Physics Publishing

6 PDD Beyond D max From Metcalfe, Kron, Hoban, The Physics of Radiotherapy X-Rays from Linear Accelerators, Medical Physics Publishing 2002 Beam characterization/quality kv! By HVL! 100 kvp 3 mm Al! 250 kvp 3 mm Cu MV! 10 cm depth! 6 MV 67%! 10 MV 73%! 15 MV 77%! 18 MV 79%! 25 MV 83% Question: Why the difference in characterization? 6

7 Beam profile! Measurement of dose away from central axis! Normalized to dose on central axis at a depth of interest! Normalized to dose at D max on central axis Beam profile! Dose variation across field at constant depth measured in single vertical plane containing the central axis of the beam! Field size defined as the lateral distance between the 50% isodose lines at a reference depth From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins

8 Beam profile: Beams-eye View (BEV)! Provides flatness and symmetry information! Measurement using film placed in a phantom perpendicular to central axis (CAX) From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins 2003 Beam profiles at D max and 10cm depth From Metcalfe, Kron, Hoban, The Physics of Radiotherapy X-Rays from Linear Accelerators, Medical Physics Publishing

9 Family of beam profiles! Dose CAX! Dose beam edge! Horns common near surface of accelerator beams! Dose fall of near beam edge! Geometric penumbra! Reduced scatter From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins 2003 Physical Penumbra! beam edge where a point of interest is not irradiated by the entire radiation source! Lateral distance between two isodose specified depth! Such as between the 80% and 20% isodose 10 cm depth 9

10 Physical Penumbra Calculating geometric penumbra GEOMETRIC PENUMBRA Geometric Penumbra = s(ssd + d SDD) SDD s = source size ( 60 Co is 1 2 cm) (L.A. is 2 3 mm) SSD = source to skin distance ( cm) d = Depth in patient SDD = source to diaphragm distance ( 60 Co is cm) (L.A. is cm) 10

11 Geometric penumbra! Radiation penumbra: result of! Irradiation geometry and scatter! Geometric penumbra NOT energy dependent! Scatter penumbra IS energy dependent Geometric penumbra! Increases with:! Increasing source size! Increasing SSD! Increasing depth! Decreases with! Increasing SDD 11

12 Penumbra trimmer Sahani et al., Tech. Cancer Res Treat. 12, Typical Geometric Penumbra Parameters! Unit s SSD D SDD Penumbra (cm) (cm) (cm) (cm) (cm)! Co ! 4MV ! 6MV lower jaws! 6MV upper jaws 12

13 Beyond penumbra! Outside geometric limits of penumbra, dose continues to decrease slowly due to reduced contributions from:! Internal field scatter! Collimator scatter! Leakage from treatment head! Remains fairly constant for several cm beyond field edge Beam flatness and symmetry! Measures of beam uniformity! Related to dose homogeneity within the treatment volume 13

14 Flat ter to treat? Spatial distribution of x-rays around a thin target From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins 2003 Flattening filter From ARRO resident From Khan F, The Physics of Radiation Therapy, Third Edition, Lippincott Williams and Wilkins

15 Flattening filter shape! Linac photon beams! Intensity decreases away from CAX! Energy decreases away from Deeper depths, lower peripheral dose! Reduced scatter near field edge! Reduced beam energy! Flatness usually 10 cm depth! To within ± 3% over central 80% of beam profile Symmetry! Within 80% FWHM S =100! Symm = Area L/ Area A (Ratio) S =100 Area left Area right Area left + Area right 15

16 Asymmetric Beam Flatness and Symmetry Scan 16

17 Wedge field isodose distributions Wedge angle! Isodose angle produced by wedge a nominated depth e.g., 10 cm! Angle between a specific isodose curve and normal to CAX! 50% (impractical sometimes )! 80%! Current recommendation - 10 cm for all photon beams! Isodose angle can differ with depth! Different scatter conditions! Angle of tilt decreases with increasing depth 17

18 Wedge angles Wedge transmission/wedge factor! Ratio of dose with and without specified depth in phantom/water.! Usually, this depth is somewhere suitably beyond d max. e.g., 10 cm! Measured for all field sizes and all depths of clinical interest.! The average of measurements made at collimator angles of ± 90 reduce the effect of errors in chamber placement. 18

19 Wedge Factor Can Be Applied Independently of Isodose Curves Wedge Factor Can Be Included in the Isodose Curves 19

20 Wedge Filter Systems! Physical Wedge! Universal Wedge! Motorized Wedge! Dynamic or Virtual Wedge Wedge Types 20

21 Physical wedge Universal Wedge 21

22 Effect of Beam Energy on Wedged Dose Distributions! For Cobalt-60 beams there is no difference in depth dose or TAR when using wedges Effect of Beam Energy on Wedged Dose Distributions! For linear accelerators, the wedge acts to filter the beam.! Beam hardening takes place because of this filtering.! For high energy beams (> 10MV) the wedge produces little effect on beam parameters. 22

23 Effect of Beam Energy on Wedged Dose Distributions! At 6MV, the wedge does increase the mean beam energy.! At depths less than 10cm there is little change in percent depth dose.! For greater depths and smaller fields the change can be significant.! At a depth of 25cm and field size of 6x6cm this change can be 5% or greater. Typical Wedge Factors for 6MV Beams 23

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26 Typical Wedge Factors for 18MV Beams 26

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28 Off-Axis Wedge Factors! The wedge factor only applies to calculations done for points on the central axis of the beam.! For calculations at points off axis, corrections to the wedge factor must be applied. 28

29 Dynamic wedge/virtual wedge! Wedge profile achieved through simultaneous computerized control of one of the jaws (Y) and dose rate 29

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33 Wedge Field Dose Distributions! Wedge fields as compensators! ( anterior chest, breast tangents, vocal cord )! Wedge field dose to fill beyond Dmax of enface open field! ( nasopharynx, 3-field rectum )! Wedge pair! ( maxilla, antrum, brain ) 33

34 Wedge Fields as Compensators Wedge Field Plans 34

35 Wedge Field Dose Fill-In Wedge Pair Fields 35

36 Wedge Angle versus Hinge Angle! Wedge angle (θ) Hinge angle (φ)! θ = 90 - φ /2! This is the ideal relationship if 2 wedges are used.! Wedge angle = 90 hinge angle / 2! Hinge angle = 2 (90 wedge angle) Wedge Angle versus Hinge Angle! This is a starting point in treatment planning.! For a more uniform distribution adjust the angle slightly.! To cover the tumor or to spare critical structures adjust the angle slightly.! Wedge angle (θ) Hinge angle (φ)! ! ! 45 90!

37 Wedge Pair 37

38 Basics of Treatment Planning 1. Criteria for treating with Enface Fields 2. Characteristics of Parallel Opposed Fields 3. Patient Thickness vs. Beam Uniformity 4. Combinations of Multiple Fields 5. Role of Rotational Therapy 6. Wedged Field Pairs 7. Tools of Treatment Planning Criteria for Using Single Enface Treatment Fields 1) Dose distribution within the tumor volume is reasonably uniform (+ 5%) 2) Maximum dose is not excessive, not more than 110% of prescribed dose 3) Critical structures are kept below tolerances Examples of enface fields: a) s clav b) internal mammary c) spinal cord compression 38

39 Enface Beam Enface Treatment of Larynx 39

40 Enface Parotid Electrons Only vs Photons+Electrons PARALLEL OPPOSED BEAMS Characteristics of parallel opposed fields are as follows: 1. Hour glass shape of the 100% isodose curve 2. A uniform distribution at the patient midline 40

41 Isodose Curve Summatio n What is the exit dose of a field in Plan (A)? What is the depth dose at midplane from entrance field? 41

42 Advantages of PARALLEL OPPOSED BEAMS 1.) Simple to set up 2.) Homogeneous dose to tumor 3.) Less chance for geometrical miss (compared to angled beams) Disadvantages of PARALLEL OPPOSED BEAMS Excessive dose to structures above and below the tumor. 42

43 Patient Thickness versus Dose Uniformity! Parallel opposed beams give a uniform dose distribution across the patient.! Dose Uniformity depends on thickness, energy, and beam flatness! Dmax dose increases as either! thickness increases! energy decreases Dose uniformity varies with beam energy and patient thickness. There is a high dose region near the surface for Co-60 and 4MV. There is exaggerated skin sparing for 10 MV and 25 MV. 43

44 Patient Thickness versus Dose Uniformity! Maximum peripheral dose compared to midpoint dose can be plotted as a function of patient thickness for various energy beams Maximum Peripheral Dose to Mid-plane Dose 44

45 Patient Thickness versus Dose Uniformity! Maximum peripheral dose of 105% occurs as follows:! for opposed Co-60 at 16cm! for opposed 4MV beams at 18 cm! for opposed 10 MV beams at 21cm! for opposed 25 MV beams at 28 cm Patient Thickness versus Dose Uniformity! Lateral Tissue Damage or Edge Effect! Treating one field per day produces greater damage to superficial tissues than treating both fields each day! Even though the total dose is identical, the biological effect is greater when receiving alternate high and low doses. 45

46 Patient Thickness versus Dose Uniformity! The effect of treating only one field per day is most severe with larger field separations and lower energies! The dose per fraction to the subcutaneous tissue can be prohibitively high! Example: For a dose of 200cGy per fraction at the 50% depth dose, the given dose (dose at Dmax) is 400cGy per fraction. Two Pairs of Parallel Opposed Fields! Using two pairs of parallel opposed fields at 90 degrees to each other results in maximum peripheral doses on the order of only percent of the isocenter dose.! The Four-Field Box 46

47 Four Field Box 10 MV Cobalt 10 MV vs Cobalt 60 Four Field Box 25 MV 10 MV 47

48 Two Pairs of Parallel Opposed Fields! Using two pairs of parallel opposed fields at gantry angles other than 0/180 degrees and 90/270degrees also results in maximum peripheral doses less than the isocenter dose.! Opposed obliques Three Field Beam Arrangements! Avoiding critical structures is often achieved using a three field technique! Three fields at equal angular separations can introduce some challenges into daily setup! table rail obstruction for beam entry! center spine obstruction for port filming 48

49 Three Field Beam Arrangements Rotation Therapy! Best suited for small, deep seated target volumes! Not good if:! target volume is too large! external surface contour differs greatly from a cylinder! target volume to far from center of patient 49

50 Rotation Plan Calculation 50

51 Field matching 51

52 Field matching Field abutment 52

53 Heterogeneity corrections Heterogeneity corrections 53