The Physics of Oesophageal Cancer Radiotherapy

Transcription

1 The Physics of Oesophageal Cancer Radiotherapy Dr. Philip Wai Radiotherapy Physics Royal Marsden Hospital 1

2 Contents Brief clinical introduction Imaging and Target definition Dose prescription & patient positioning Conventional planning method Beam shaping issues Effect of lungs and beam energy selection Advance treatments Stereotactic Body Radiotherapy (SBRT) Treatment 2

3 Clinical introduction 3

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5 Oesophageal Cancer Location of tumour in oesophagus <10% upper third 40-50% middle third 40-50% lower third Surgical, Chemo, Radiotherapy or combination Radiotherapy Treatment of choice for upper-third tumours Also used in lower two-thirds if surgery not possible 5

6 Brachytherapy High dose rate brachytherapy (HDR) Similar to the treatment for bronchial carcinoma Generally for palliation 6

7 Imaging and Target definition 7

8 Target Definition (1) Possible investigations Barium swallow Endoscopy CT MRI PET 8

9 PET image: Transverse 9

10 PET image-sagittal 10

11 CT scan at same time as PET scan to assist fusion of images 11

12 PTV Volume from PET image fused onto Planning CT 12

13 CT Localisation of Target Volume Gross Tumour Volume (GTV) Clinical Target Volume (CTV) Planning Target Volume (PTV) 13

14 Target Definition (2) Radical: Clinical target volume (CTV) = Gross tumour volume (GTV) + 5 cm sup. and inf. margin + 1 to 2.5 cm margin in AP and lateral directions Sup.-inf. margin may be reduced to 2 cm for a second phase of treatment Palliative: Smaller margin may be used to reduce morbidity 14

15 CT Localisation of Target Volume Sup. Inf. 15

16 CT Localisation of Target Volume PTV CTV GTV 16

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18 CT The Localisation Royal Marsden of Target Volume: Variation between Clinicians (Canada, Tai et al, IJROBP, 1998) Sup. Inf. 18

19 Dose Limiting Structures 19

20 Liver Lungs Heart PTV Spinal Cord Kidney 20

21 OAR dose constraints (1.8 Gy fractions.) Cord Max point dose 48 Gy Lung V20 < 20%, Mean lung dose <15Gy. If pre-operative radiotherapy is used calculate the volume of lung spared of 5Gy (receiving 0-5Gy). If >1900cc then risk of pulmonary post-operative complications is <10%.[8]. (Volume spared VSdose = total lung volume absolute Vdose). Heart V40<30-40%, D50 <25Gy the whole heart must not have more than 30-40% exposed to a total of dose of 40Gy, and no more than 50% exposed to a dose of 25Gy. Liver (no more than 30% of its volume exposed to 30Gy) D30<30Gy, mean liver dose<25gy. Kidney At least 70% of one physiologically functioning kidney should receive a total dose less than 20Gy. Overall no more that 50% of the each functional kidney volume should receive more that 18Gy(D50<18Gy) and mean combined kidney volume dose<18gy. If one functioning kidney D15%<18Gy 21

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23 Dose Prescription and Patient position 23

24 Dose Prescriptions Palliative 20 Gy in 5# in 1 week, or 36 Gy in 12# Radical (RT alone) 60 Gy in 30# or 55 Gy in 20# Radical (with chemo-radiotherapy) 54 Gy in 30# in 6 weeks (one or two phases) 24

25 Dose Prescriptions Dose Per Fraction Number of Fraction 25

26 Patient position and immobilisation Patient is treated supine Upper 1/3 Cervical immobilization in posicast, arms by side. Mid 1/3 Lung board and arms up Lower 1/3 Lung board and arms up 26

27 Conventional Planning Method 27

28 Conventional (Old) 2D Planning Technique Two phases, both simulator based Phase 1: Parallel-opposed pair Phase 2: Three fields 28

29 Conventional Localisation of Target Volume 29

30 Simulator Film Magnification FFD patient SSD depth FAD The magnification at the anterior skin surface:- Magnificat ion = FFD SSD film Image intensifier F F= 4 cm for RMH Sutton simulator IIRAD (Distance between isocentre and image intensifier Magnification at a depth of interest in the patient:- Magnification = FFD SSD + depth 30

31 Conventional Planning: Organs at Risk Spinal cord Lungs 31

32 2D + 3D Planning Technique (current in some radiotherapy centres) Two phases Phase 1 simulator based (2D), to avoid delaying the start of treatment for CT scan appointment Phase 2 CT based (3D) Phase 1: Parallel-opposed pair Phase 2: Three fields 32

33 3D CT Planning in Two Phases Phase 1: AP/PA Phase 2: 3 fields Same PTV for both phases Used for an interim period at the Royal Marsden Hospital 33

34 Beam shaping techniques 34

35 Beam shaping devices: 1 cm wide MLC vs conformal blocking Ant. Oesophagus field ( green circle is normalisation point) 35

36 green colourwash = ROI 1 ROI 1 = 2 cm volume of normal tissue surrounding the PTV (excluding the PTV) 36

37 DVH to show difference between blocking methods 1 cm MLC: solid line Block: dashed line ROI 1 = 2cm normal tissue margin around PTV 37

38 Effect of Lungs and Beam energy selection 38

39 DRR settings in chest region kv bone enhancement MV airway enhancement 39

40 DOSE CT-Density tables BONE DRR LUNG DRR Convert the CT numbers in a CT volume into physical densities for dose calculation or image enhancement 40

41 Phase 1: 6 MV versus 10 MV 10 MV gives lower hot spots but worse PTV coverage 41

42 Lung Correction Approximate Physical Densities of Different Tissues Soft tissue: 1.0 g/cm 3 Fat: 0.9 g/cm 3 Skeletal muscle: 1.0 g/cm 3 Brain: 1.0 g/cm 3 Skin: 1.1 g/cm 3 Hard (cortical) bone: g/cm 3 Average bone (e.g., ribs): g/cm 3 Inflated lung: 0.3 g/cm 3 Air cavities: 0.0 g/cm 3 42

43 Lung Correction 6 MV Same MUs set for both cases 10# Dose to isocentre (prescription point) = Gy with lung density included = Gy ignoring lung density (9.4% difference) False indication of PTV coverage when ignoring lung 43

44 Phase 2: 6 MV versus 10 MV Better PTV coverage near lung with 6 MV 44

45 Effect of Lungs: Summary 1. Attenuation of primary beam For same prescription dose, ignoring the true lung density leads to calculation of a higher number of MUs to be delivered. Scale the prescription dose to account for this if changing over from an old to a modern dosimetry technique that include lung density. 2. Scattered radiation Lack of dose coverage to PTV at the periphery of a field near lungs Reduced scatter contribution from lungs (compared to other tissues) to a point of interest, e.g., prescription point Scattered electrons and photons are attenuated less readily in lungs and travel further in them than other tissues. 45

46 Cardiac toxicity 46

47 Cardiac Toxicity Modern diagnostic techniques & chemo-radiotherapy schedules Increased long-term patient survival Late effects including cardiac problems observed with the two-phase treatment 47

48 Comparison of Plans with Different MLC Fitting Orientations % Heart Volume Receiving 30 Gy MLC fitting along LEFT-RIGHT % Heart Volume Receiving 55 Gy MLC fitting along SUP-INF 48

49 Four-field, Single-phase Plan 49

50 Heart Cord PTV Volume (%) Four Fields vs. Best Two-Phase Plan Thick Lines: 4 Fields R. Lung L. Lung Dose (Gy) Thin Lines: 2 Phases 50

51 Dose calculation must cover entire lung volume 51

52 600 Receiving more than 30 Gy 500 Volume of heart [cm3] phase 3 field 4 field Receiving more than 45 Gy 0 Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Volume of heart [cm3] phase 3 field 4 field 600 Receiving more than 40 Gy 0 50 Volume of heart [cm3] phase 3 field 4 field Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 0 Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 52

53 Study Conclusions (1) Standard two-phase oesophageal RT Delivers high cardiac doses Sup-inf MLC fitting worst cardiac DVHs 53

54 Study Conclusions (2) Should consider lung, cord, and cardiac toxicity If quick access to a CT scanner is available, then use a single phase plan e.g., 4 fields (now standard treatment at Royal Marsden) 54

55 Example plans 55

56 Clinical case 1: Oesophagus + pharynx 56

57 Clinical case 1: Oesophagus + pharynx Not Normalised to Isocentre 100% 57

58 Not Normalised to Isocentre 100% >107% 58

59 Clinical case 1: Oesophagus + pharynx 59

60 Scanned with arms down- and a shell made for head immobilisation 60

61 Field arrangement 61

62 Case 3 Which direction to wedge? But over-wedged at this level 62

63 Case 3: Which direction to wedge? Looks good on this slice 63

64 Longitudinal wedge (May depend on MLC direction) 64

65 Case 4: Coplanar vs non coplanar 3 field coplanar 3 field non-coplanar (AIO field) 65

66 Case 4: Coplanar vs non coplanar 66

67 DVH comparison of coplanar and non-coplanar plans Coplanar Non Coplanar 67

68 Irradiated Volume (ICRU 62) The Irradiated Volume is the tissue volume that receives a dose that is considered significant in relation to normal tissue tolerance 68

69 Clinical Case 5 Volumetric Arc Therapy: VMAT 69

70 Clinical Case 5:Volumetric Arc Therapy: VMAT 70

71 Clinical Case 5: Volumetric Arc Therapy: VMAT 71

72 Clinical Case 5: Volumetric Arc Therapy: VMAT 72

73 Clinical Case 5 : Volumetric Arc Therapy: VMAT 73

74 Summary 74

75 Summary History Two-phase simulator-based technique (2D) Phase 1 simulator based, Phase 2 CT-based (3D) Phase 1 and Phase 2 CT-based Single-phase CT-planned (organs at risk: cord, lungs and heart) Volumetric Arc Therapy: VMAT New dose calculation algorithms are better at predicting dose distributions near inhomogeneities. Using higher-energy X-rays can reduce dose to organs-atrisk but can also reduce PTV dose coverage near lungs. 75

76 Stereotactic Body Radiotherapy (SBRT) for peripheral lung tumours In-operable Outside of 2cm No-Fly-Zone No previous treatment within PTV 76

77 Stereotactic Body Radiotherapy (SBRT) for peripheral lung tumours 77

78 Biologically Effective Dose (BED) D BED = nd (1+ ) α / β n: number of fraction, D: dose per fraction, α/ β =10 BED 100Gy improves survival, less toxicity for peripheral tumor 60Gy 30# = 72 BED, 50Gy 5# = 100 BED 78

79 Acknowledgements and References J Dobbs, A Barrett, D Ash, Practical radiotherapy planning -the 4th edition 2009 is much more up to date) P Tai et al, Variability of target volume definition in cervical esophageal cancer, Int J Radiation Oncology Biol Phys, Vol 42, (1998) AJ Neal and PJ Hoskin, Clinical Oncology: Basic principles and practice, 3rd edition (2003) Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic radiation. IntJRadiatOncolBiolPhys 1991;21: Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) International Journal of Radiation Oncology * Biology * Physics - 1 March 2010 (Vol. 76, Issue 3, Supplement, Pages S1-S2, DOI: /j.ijrobp ) 79

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