AAPM Task Group 180 Image Guidance Doses Delivered During Radiotherapy: Quantification, Management, and Reduction

Transcription

1 AAPM Task Group 180 Image Guidance Doses Delivered During Radiotherapy: Quantification, Management, and Reduction Parham Alaei, Ph.D. Department of Radiation Oncology University of Minnesota NCCAAPM Fall Meeting La Crosse, WI, Nov. 10,

2 Learning Objectives Understand the: Magnitude of imaging dose in radiation therapy Available dose calculation algorithms for imaging beams kilovoltage imaging beam dosimetry methods Methods of accounting for imaging dose Recommendations of TG 180 2

3 AAPM TG 180 Task Group proposal approved in August 2008 Final approved version submitted to Medical Physics in July 2017 Currently under review Members: 3 George Ding, Chair Parham Alaei Bruce Curran Ryan Flynn Michael Gossman Walter Bosch (consultant) Choonsik Lee (consultant) Peter Munro (consultant) Rock Mackie Moyed Miften Richard Morin X. George Xu Timothy Zhu Ying Xiao (consultant) Jun Deng (consultant)

4 AAPM TG 180 Charge (1) To identify the important issues such as the large variations between dose to bone (or bone marrow) and dose to soft tissues for x rays at kilovoltage energy range To provide an overview on the general approach to clinical implementation of accounting for the imaging guidance dose from x ray imaging procedures in radiotherapy including megavoltage electronic portal imaging (MV EPI), kilovoltage digital radiography (kv DR), tomotherapy MV CT, megavoltage cone beam CT (MV CBCT) and kilovoltage cone beam CT (kv CBCT) 4

5 AAPM TG 180 Charge (2) To provide general guidelines for: Commissioning an imaging beam in a treatment planning system Various verification techniques and experimental methods to assure an accurate imaging beam model commission process Specific recommendations on the dose calculation accuracy from an imaging procedure in a treatment planning system 5

6 AAPM TG 180 Outline Overview of dose resulting from image guidance procedures Review of dose calculation algorithms Kilovoltage imaging beam dosimetry Review of methods of accounting for imaging dose Recommendations 6

7 AAPM TG 180 Outline Dose resulting from image guidance procedures Dose calculation algorithms Kilovoltage imaging beam dosimetry Accounting for imaging dose Recommendations 7

8 Dose from Image Guidance Procedures MV EPID Imaging:~1 4 cgy from pair of orthogonal images Pelvis Organ D50 Range (cgy) 6 MV 2.5 MV Bladder Bowel Femoral Heads Prostate Rectum Typical organ doses for the pelvis treatment site with MV EPID portal imaging for a typical pair of orthogonal setup fields (2 MU/field for 6 MV and 1 MU/field for 2.5 MV). (Extracted from TG-180) 8

9 Dose from Image Guidance Procedures MV CT Imaging: 1 3 cgy, depends on pitch (and scan length) MVCT in Tomo Acquisition mode Fine pitch (4mm couch travel/rotation) Normal pitch (8mm couch travel/rotation) Coarse pitch (12mm couch travel/rotation) Dose (cgy) 2.5 cgy 1.2 cgy 0.8 cgy Tomo MVCT dose at the center of a 30 cm water phantom and its dependency on acquisition protocols. (Extracted from TG-180) 9

10 Dose from Image Guidance Procedures MV CBCT Imaging: 2 12 cgy, depends on the protocol 10 Location Average Organ Dose (cgy/mu) Total Brain 0.90 ± 0.01 Left Lens 1.15 ± 0.03 Right Lens 1.13 ± 0.03 Left Eye 1.16 ± 0.01 Right Eye 1.13 ± 0.01 Left Lung 0.85 ± 0.06 Right Lung 0.80 ± 0.06 Total Lung 0.83 ± 0.06 Spinal Canal 0.59 ± 0.10 Heart 0.86 ± 0.15 Soft Tissue 0.61 ± 0.09 Femoral Heads 0.80 ± 0.14 MV-CBCT doses per monitor unit using a 6 MV treatment beam with an acquisition arc of 200 degrees ( degrees). (Extracted from TG-180)

11 Dose from Image Guidance Procedures kv Imaging: kv DR: cgy from pair of orthogonal images 11

12 Dose from Image Guidance Procedures kv Imaging: kv CBCT: Soft Tissue: cgy /acquisition Bone: cgy /acquisition Depends on the technique/filter/arc length used 12

13 kv CBCT Imaging Dose Standard Head, Head & Neck Organ D50 Range (cgy) D10 Range (cgy) Brain Larynx Oral Cavity Parotids Spinal Cord Thyroid Esophagus Skin Bones Organ doses for the head & neck and brain treatment sites from Varian OBI v1.4 using Standard Head kv-cbct scan ( Full fan,100 kvp, 145 mas, 200 o rotation). D50 and D10 are minimum dose delivered to 50% and 10% of the organ volume respectively. (Extracted from TG-180) 13

14 kv CBCT Imaging Dose Head and Neck Organ D50 Range (cgy) Brainstem Rt Eye Lt Eye Rt Parotid Lt Parotid Rt Cochlea Lt Cochlea Oral Cavity Organ doses for the head & neck treatment site from Elekta XVI kv- CBCT scan using S cassettes, 100 kvp, 0.1 mas/acquisition, 360 acquisitions, degree (IEC) rotation. (Extracted from TG-180) 14

15 kv CBCT Imaging Dose 15 Low dose Thorax Organ D50 Range D10 Range (cgy) (cgy) Aorta Lungs Small Bowel Esophagus Kidney Heart Liver Spinal Cord Spleen Stomach Trachea Skin Bones Organ doses for the chest treatment site from Varian OBI v1.4 using Low-dose Thorax kv-cbct scan (Half fan,110 kvp, 262 mas, 360 o rotation). (Extracted from TG-180)

16 kv CBCT Imaging Dose Pelvis Scan, Prostate Isocenter Organ D50 Range (cgy) D10 Range (cgy) Bladder Bowel Femoral Heads Prostate Rectum Skin Bone Organ doses for the pelvis treatment site from Varian OBI v1.4 using Pelvis kv-cbct scan (Half fan, 125 kvp, 700 mas, 360 o rotation). (Extracted from TG-180) 16

17 kv CBCT Imaging Dose Pelvis Pelvis Organ D50 Range (cgy) Organ D50 Range (cgy) Bladder Rectum Small Bowel With Bowtie Bladder Rectum Small Bowel W/O Bowtie Organ doses for the pelvis treatment site from Elekta XVI kv-cbct scan using M cassette (120 kvp, 650 mas, 360 o rotation). (Extracted from TG-180) 17

18 Imaging Dose Summary MV EPID Imaging:~1 4 cgy from pair of orthogonal images (~1 2 for 2.5 MV beam) MV CT Imaging: 1 3 cgy MV CBCT Imaging: 2 12 cgy kv DR: cgy from pair of orthogonal images kv CBCT: cgy /acquisition (0.3 6 cgy for bone) 18

19 AAPM TG 180 Outline Dose resulting from image guidance procedures Dose calculation algorithms Kilovoltage imaging beam dosimetry Accounting for imaging dose Recommendations 19

20 Dose Calculation Algorithms Monte Carlo methods: Widely used to characterize imaging beams, as well as calculating dose to phantom and patients Ding et al. Med. Phys. 35: (2008) Spezi et al. Med. Phys. 36: (2009)

21 Dose Calculation Algorithms Model based methods: Model based dose calculation algorithms provide accurate dose calculations at MV, but not kv, range They have successfully been used for MV CBCT imaging dose calculations Dose distributions resulting from an MV-CBCT localization procedure of a prostate cancer patient using a 15 MU imaging protocol with a 6 MV beam. (Miften et al. 2007)

22 Dose Calculation Algorithms Model based methods: It is possible to use model based dose calculation algorithms to perform dose calculations for kv range beams with inherent inaccuracies in bony structures Commercially available model based algorithms underestimate dose to bone by up to 300% when used for kv beams due to photoelectric effect which is not accounted for by them A proposed algorithm, Medium Dependent Correction, has the potential to calculate dose at kv range but is not available commercially

23 Dose Calculation Algorithms Model based methods: Varian OBI Elekta XVI Alaei et al., Med. Phys. 37: (2010) Alaei and Spezi, J. Appl. Clin. Med. Phys. 13, (2012) *Bone dose not accurate 23

24 AAPM TG 180 Outline Dose resulting from image guidance procedures Dose calculation algorithms Kilovoltage imaging beam dosimetry Accounting for imaging dose Recommendations 24

25 Kilovoltage Imaging Beam Dosimetry kv imaging beam cannot be calculated using TPS at the moment If that becomes possible, it requires: Beam data collection (PDDs, profiles, output factors) Not straightforward Difficulties include low dose rate and dependence on the phantom media Possible to use MC data validated by measurements Beam modeling/commissioning

26 Kilovoltage Imaging Beam Dosimetry In vivo dosimetry If patient specific imaging dose verification is needed, commonly used dosimeters can be used Dosimeters should be calibrated for the beam energy used Diodes with buildup are not suitable

27 AAPM TG 180 Outline Dose resulting from image guidance procedures Dose calculation algorithms Kilovoltage imaging beam dosimetry Accounting for imaging dose Recommendations 27

28 Accounting for Imaging Dose Patient specific calculations: Straightforward for megavoltage imaging using TPS Only possible using Monte Carlo for kilovoltage imaging

29 Accounting for Imaging Dose Non patient specific estimations: Dependence of imaging dose on patient anatomy is small in most cases, hence dose estimates could be provided in the form of organ dose look up tables Several tables provided in TG 180 They can be scaled with mas used for imaging

30 AAPM TG 180 Outline Dose resulting from image guidance procedures Dose calculation algorithms Kilovoltage imaging beam dosimetry Accounting for imaging dose Recommendations 30

31 Recommendations (1) General Recommendations: Create local imaging protocols with image modality, techniques, and frequency that are suitable for the clinical imaging intent Develop protocols that are specific for pediatric patients Communicate typical imaging doses associated with the imaging procedures used to the radiation oncologists

32 Recommendations (2) Imaging dose output and consistency checks: The anticipated imaging dose for each image acquisition procedure with specified protocol parameters should be measured in air or in phantom according to the AAPM dosimetry protocols Consistency checks should be performed yearly and after each system upgrade If patient specific image dose verification is desired for a particular patient, in vivo patient dose measurements should be performed with suitable detectors

33 Recommendations (3) Accounting for imaging dose to RT patients: It is recommended that imaging dose be considered as part of the total dose at the treatment planning stage if the dose from repeated imaging procedures is expected to exceed 5% of prescribed therapeutic target dose Why 5%? Clinical relevance, accuracy of dose calculation and delivery, organ dose tolerances, and feasibility in clinical practice

34 Dose from different imaging devices/techniques 2D imaging MV portal imaging kv digital radiography Room mounted kv imaging systems 3D imaging Cone Beam CT MV CBCT kv CBCT MV CT Imaging dose < 5% threshold, unless there are a large number of images no need to account for Imaging dose may be > 5% threshold, depending on protocol may need to account for 34

35 Recommendations (4) Available techniques to reduce imaging dose to patients: For MV portal images, minimize the imaging field size without removing reference structures needed for patient alignment For Tomotherapy units, select MVCT scan pitch parameters that balance imaging dose with clinical need For MV CBCT, select a patient specific MV imaging protocol and restrict the imaging field of view

36 Recommendations (5) Available techniques to reduce imaging dose to patients (continued): Consider the type of imaging needs (2D vs. 3D). Choose 2D if two planar orthogonal kv images are sufficient for the task The kv CBCT scan protocols that use a partial rotation provide the opportunity of selectively avoiding irradiation of superficial organs Optimize the kv beam entry and exit direction for orthogonal planar images to reduce organ doses Consider the use of beam filters when acquiring planar kv images

37 Question 1 Dose from CBCT imaging: a) Is negligible and can be ignored b) Must be accounted for all patients c) Varies widely and depends on the imaging technique and modality used d) Could be as high as 15 cgy for a single image acquisition 37

38 Question 1 Dose from CBCT imaging: a) Is negligible and can be ignored b) Must be accounted for all patients c) Varies widely and depends on the imaging technique and modality used d) Could be as high as 15 cgy for a single image acquisition Reference: P. Alaei, E. Spezi, Imaging dose from cone beam computed tomography in radiation therapy, Phys. Med. 31 (2015)

39 Question 2 AAPM TG 180 recommendations include: a) Accounting for imaging dose regardless of the magnitude b) Determining CTDI for all CBCT protocols used c) Calculating patient specific doses for patients d) Accounting for imaging dose above a threshold 39

40 Question 2 AAPM TG 180 recommendations include: a) Accounting for imaging dose regardless of the magnitude b) Determining CTDI for all CBCT protocols used c) Calculating patient specific doses for patients d) Accounting for imaging dose above a threshold Reference: An overview of TG-180 by George Ding. AAPM virtual library: 40

41 41 Questions?