aapm task group 180chapter.aapm.org/nccaapm/z_meetings/2017-11-10/04_agenda-and... · aapm task...

AAPM Task Group 180Image 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, 2017

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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

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AAPM TG‐180

• 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)

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• Task Group proposal approved in August 2008• Final approved version submitted to Medical Physics in July 2017 ‐ Currently under review

• Members:

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)

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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

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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

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• Dose resulting from image guidance procedures• Dose calculation algorithms• Kilovoltage imaging beam dosimetry• Accounting for imaging dose• Recommendations

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Dose from Image Guidance Procedures

• MV EPID Imaging:~1‐4 cGy from pair of orthogonal images

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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)

Pelvis

OrganD50 Range (cGy)

6 MV 2.5 MVBladder 2.0‐3.5 1.0‐1.5Bowel 2.0‐4.0 1.0‐1.5

Femoral Heads 2.5‐3.5 0.8‐1.5Prostate 3.5‐3.5 0.9‐1.1Rectum 2.0‐4.0 0.8‐1.0


• MV CT Imaging: 1‐3 cGy, depends on pitch (and scan length)

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Tomo MVCT dose at the center of a 30 cm water phantom and its dependency on acquisition protocols. (Extracted from TG-180)

MVCT in Tomo

Acquisition mode Dose (cGy)Fine pitch (4mm couch

travel/rotation)2.5 cGy

Normal pitch (8mm couch travel/rotation)

1.2 cGy

Coarse pitch (12mm couch travel/rotation)

0.8 cGy


• MV CBCT Imaging: 2‐12 cGy, depends on the protocol

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LocationAverage Organ Dose (cGy/MU)

Total‐Brain 0.90 ± 0.01Left Lens 1.15 ± 0.03Right Lens 1.13 ± 0.03Left Eye 1.16 ± 0.01Right Eye 1.13 ± 0.01Left Lung 0.85 ± 0.06Right Lung 0.80 ± 0.06Total Lung 0.83 ± 0.06Spinal Canal 0.59 ± 0.10

Heart 0.86 ± 0.15Soft 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 (270-110 degrees). (Extracted from TG-180)


• kV Imaging:–kV DR: 0.1‐1 cGy from pair of orthogonal images

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• kV Imaging:–kV CBCT:

• Soft Tissue: 0.1 ‐ 3 cGy /acquisition • Bone: 0.3 ‐ 6 cGy /acquisition

–Depends on the technique/filter/arc length used

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kV CBCT Imaging Dose

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Standard Head, Head & NeckOrgan D50 Range (cGy) D10 Range (cGy)Brain 0.15‐0.22 0.16‐0.23Larynx 0.21‐00.29 0.25‐0.33

Oral Cavity 0.13‐0.26 0.20‐0.31Parotids 0.26‐0.42 0.31‐0.48

Spinal Cord 0.16‐0.25 0.19‐0.32Thyroid 0.07‐0.23 0.11‐0.32

Esophagus 0.07‐0.16 0.14‐0.26Skin 0.18‐0.27 0.34‐0.44Bones 0.25‐0.65 0.64‐1.07

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, 200o rotation). D50 and D10 are minimum dose delivered to 50% and 10% of the organ volume respectively. (Extracted from TG-180)


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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, 345-190 degree (IEC) rotation. (Extracted from TG-180)

Head and Neck

OrganD50 Range

(cGy)Brainstem 0.06‐0.08Rt Eye 0.08‐0.09Lt Eye 0.13‐0.13

Rt Parotid 0.05‐0.06Lt Parotid 0.16‐0.17Rt Cochlea 0.04‐0.05Lt Cochlea 0.09‐0.12Oral Cavity 0.09‐0.11


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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, 360o

rotation). (Extracted from TG-180)

Low‐dose Thorax

OrganD50 Range

(cGy)D10 Range

(cGy)Aorta 0.42‐0.58 0.44‐0.63Lungs 0.30‐0.61 0.43‐0.72Small Bowel

0.33‐0.54 0.39‐0.61

Esophagus 0.29‐0.60 0.35‐0.74Kidney 0.43‐0.54 0.49‐0.59Heart 0.31‐0.55 0.41‐0.63Liver 0.31‐0.51 0.38‐0.61

Spinal Cord 0.32‐0.57 0.35‐0.78Spleen 0.32‐0.52 0.36‐0.60Stomach 0.28‐0.57 0.31‐0.62Trachea 0.36‐0.71 0.47‐1.04Skin 0.46‐0.57 0.64‐0.89Bones 1.06‐1.74 1.47‐2.25


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Organ doses for the pelvis treatment site from Varian OBI v1.4 using Pelvis kV-CBCT scan (Half fan, 125 kVp, 700 mAs, 360o rotation). (Extracted from TG-180)

Pelvis Scan, Prostate Isocenter

Organ D50 Range (cGy) D10 Range (cGy)

Bladder 1.36‐2.20 1.72‐2.69

Bowel 1.54‐1.91 2.04‐2.65

Femoral Heads 2.40‐3.60 3.22‐4.88

Prostate 1.19‐1.79 1.33‐1.89

Rectum 1.51‐1.99 1.70‐2.22

Skin 1.80‐1.96 2.26‐2.92

Bone 2.93‐3.96 4.61‐5.72


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Organ doses for the pelvis treatment site from Elekta XVI kV-CBCT scan using M cassette (120 kVp, 650 mAs, 360o rotation). (Extracted from TG-180)

Pelvis

Organ D50 Range (cGy)

Bladder 1.1‐2.5

Rectum 1.3‐2.4

Small Bowel 1.1‐2.3

Pelvis

Organ D50 Range (cGy)

Bladder 0.9‐2.0

Rectum 1.1‐1.9

Small Bowel 1.0‐1.8

With Bowtie W/O Bowtie

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: 0.1‐1 cGy from pair of orthogonal images

• kV CBCT: 0.1 ‐ 3 cGy /acquisition (0.3‐6 cGy for bone)

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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: 1135-1144 (2008) Spezi et al. Med. Phys. 36: 127-136 (2009)


• 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)


• 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


• Model‐based methods:

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Alaei et al., Med. Phys. 37: 244-248 (2010)

Alaei and Spezi, J. Appl. Clin. Med. Phys. 13, 19-33 (2012)

Varian OBI Elekta XVI

*Bone dose not accurate



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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

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



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Accounting for Imaging Dose

• Patient specific calculations:–Straightforward for megavoltage imaging using TPS–Only possible using Monte Carlo for kilovoltage imaging

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



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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

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

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

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

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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

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

Question 1

• Dose from CBCT imaging:a) Is negligible and can be ignoredb) Must be accounted for all patientsc) Varies widely and depends on the imaging technique and modality usedd) Could be as high as 15 cGy for a single image acquisition

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Question 1

• Dose from CBCT imaging:a) Is negligible and can be ignoredb) Must be accounted for all patientsc) Varies widely and depends on the imaging technique and modality usedd) Could be as high as 15 cGy for a single image acquisition

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Reference: P. Alaei, E. Spezi, “Imaging dose from cone beam computed tomography in radiation therapy”, Phys. Med. 31 (2015) 647-658

Question 2

• AAPM TG‐180 recommendations include:a) Accounting for imaging dose regardless of the magnitudeb) Determining CTDI for all CBCT protocols usedc) Calculating patient‐specific doses for patientsd) Accounting for imaging dose above a threshold

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Question 2

• AAPM TG‐180 recommendations include:a) Accounting for imaging dose regardless of the magnitudeb) Determining CTDI for all CBCT protocols usedc) Calculating patient‐specific doses for patientsd) Accounting for imaging dose above a threshold

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Reference: An overview of TG-180 by George Ding. AAPM virtual library: http://www.aapm.org/education/vl/vl.asp?id=11482

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Questions?

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