Imaging for Proton Therapy · 2015-06-22 · Imaging for Proton Therapy Katja Langen(1), Reinhard...

Imaging for Proton Therapy Katja Langen(1), Reinhard Schulte (2)

1: University of Maryland

2: Loma Linda University

Introduction

Imaging is used for • Treatment planning • Image guidance during treatment • Develop methods for proton range

verification • Treatment monitoring/adaptation

Imaging for treatment planning

Prerequisite: In proton therapy the material along the beam’s path needs to be accurately known

Why ?

Tumor at 10 cm

sees 67 % of max dose

Patient role: bone

“appears” at 5 cm depth

Add 1 cm bone

Density is twice

that of water

(1cm water => 2 cm)

Pull back PDD by an

additional 1 cm

Tumor at 10 depth still sees 64% of dose

“new” material in photon beam’s path

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Tumor at 10 cm

sees 100 % of max dose

Patient role: bone

“appears” at 5 cm depth

Add 1 cm bone

Density is twice

that of water

(1cm water => 2 cm)

Pull back PDD by an

additional 1 cm

Tumor at 10 depth sees 0% of dose

“new” material in proton beam’s path

Imaging for treatment planning

Planning CT should be of the fully immobilized patient Need treatment table in planning CT CT field-of view must include all materials that are potentially in the beam’s path Setup reproducibility is paramount CT artifacts and contrast materials are of concern

Proton ranges and dosimetry

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ZS

proportional to electron density


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velocity

Stopping power increases as particle slows down (Bragg peak)


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ZS

Mean ionization energy of material ( uncertainty in Iwater: ~10% range in tabulated values, uncertainty in S is 1% for water)


Eliminate uncertainty in S for water by measuring dose and range in water at time of commissioning

Proton ranges and dosimetry For non-water materials: can use stopping power ratio (SPR) to water Possible since SPR is largely independent of proton energy SPR depends mainly on material

Energy (MeV) Energy (MeV)

Sto

pp

ing

Pow

er (

MeV

cm

2/g

)

Sto

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ing

Pow

er (

MeV

cm

2/g

)

CT HU to SPR calibration

𝑆𝑃𝑅𝑤𝑚=

𝜌𝑒,𝑚

𝜌𝑒,𝑤×ln(2𝑚𝑒𝑐

2𝛽2 𝐼𝑚 (1−𝛽2))−𝛽2

ln(2𝑚𝑒𝑐2𝛽2 𝐼𝑤 (1−𝛽2))−𝛽2

,


𝜌𝑒,𝑚

𝜌𝑒,𝑤×K,

K ranges from 1.025 to 0.975 for different materials • SPR is dominated by relative electron density • K is not unity • K is a function of Im which is uncertain, • K is uncertain by 1-1.5%

Allows HU to SPR calibration

Schneider et al., PMB 41, pp 111, 1996

HU calibration

HU Measure of x-ray attenuation (electron density and Z dependence)

Photon Relative electron density ρe

Proton SPR

Uncertainties in ρe and SPR ?



Proton SPR


HU depends on: Scanner Size of phantom Location of material within image Image beam (kVp, mAs) Single calibration curve? Ok in photon RT (2% in dose calc.) 3.5% range uncertainty for protons Guan et al., PMB 47:N223, 2002 Ainsley and Yeager, JACMP, 15:4721, 2014

1.



Proton SPR


2.

Uncertainty in calibration ?



Proton SPR

Typical calibration

Scan tissue-substitutes to approximate real tissue But …. differences in chemical composition Does it matter ???? CT attenuation depends on Compton, coherent scattering (~Z2), Photon electric (~Z3) attenuation coefficient

Typical calibration

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Subs real bone tissue

Real bone has 15-20 % Ca (Z=20) content

Mylar- no Ca PTFE – high F content (Z=9)

PVC- 50% Cl (Z=17)

B100/B110- well matched Z

Same ρe => different HU Same HU => different ρe

Typical calibration

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ρe SPR

How about soft tissues ?

ρe SPR

Schneider et al., PMB 41: 111

~ 3-5 %

Stoichiometric calibration


Issue: Need HU for real tissues but can only measure HU for tissue substitutes If we understand the exact dependence of HU on Z, then we can calculate HU for any material. Then we can calculate HU for any “real tissue”. We know density and SPR for “real tissue”. Virtual calibration

Need: exact relationship between HU and Z


Rutherford et al., Neuroradiology, 11:15, 1976 Schneider et al., PMB 41, pp 111, 1996

• Accurate parameterization: μ=ρ Ng (Z,A) [KphZ3.62+ Kcoh Z1.86+ KKN] • Kph is coefficient for photoelectric effect • Kcoh is coefficient for coherent scattering • KKN is coefficient for Klein-Nishina effect



1. Acquire CT scan of phantom with tissue equivalent materials with known density and elemental compositions. 2. Measure HUs for each tissue equivalent material. 3. Use measured HUs to determine Kph, Kcoh, KKN coefficients for stoichiometric equation. 4. Using Kph, Kcoh, KKN coefficients calculate HUs for a full range of “real” tissues using their published elemental compositions and physical densities. 5. Calculate the stopping power ratio (SPR)/ ρe for each “real” tissue based on elemental composition. 6. Use calculated SPR/ ρe vs. calculated HU values to establish respective calibration for TPS

Example of calibration workflow

1. Acquire CT scan of phantom with tissue equivalent materials with known density and elemental compositions. 2. Measure HUs for each tissue equivalent material.


3. Use measured HUs to determine Kph, Kcoh, KKN coefficients for stoichiometric equation.

For tissue substitute we need to know elemental composition to calculate and . . Use vendor data for elemental composition.

Use regression analysis to determine Kph, Kcoh, KKN .


4. Using Kph, Kcoh, KKN coefficients calculate HUs for a full range of “real” tissues using their published elemental compositions and physical densities.

ICRP publication 23: Task group on Reference man ICRU report 44 on Tissue substitutes in Radiation Dosimetry and Measurement. Woodward and White, BJR, 59 1209, 1986



𝜌𝑒,𝑚

𝜌𝑒,𝑤×ln(2𝑚𝑒𝑐

2𝛽2 𝐼𝑚 (1−𝛽2))−𝛽2

ln(2𝑚𝑒𝑐2𝛽2 𝐼𝑤 (1−𝛽2))−𝛽2

,

5. Calculate the stopping power ratio (SPR)/ ρe for each “real” tissue based on elemental composition

Mean ionization energy table are published by Janni (At. data Nucl. Data Tables, 27, 212, 1982)

Example of calibration curve (5-segments)

Schaffner and Pedroni et al., PMB 43, pp 1579, 1998

HU to SPR calibration uncertainty

For biological tissues (those that lie on calibrations curve): 1%

Schaffner and Pedroni et al., PMB 43, pp 1579, 1998

Higher uncertainty for non-biological materials. May need to assign RSP to these materials. Bolus/implants….

Practical considerations for stoichiometric calibration

Ainsley and Yeager, JACMP, 15:4721, 2014

• Tissues substitutes from different vendors?

• Use vendor supplied e-densities or calculate from compositions?

• CT slice thickness ?

• Position of plug in phantom ?

• Simultaneous vs sequential scan of test plugs ?

• Phantom size ?

Practical considerations for stoichiometric calibration


• Tissues substitutes from different vendors? Found consistency between vendors

• Use vendor supplied e-densities or calculate from compositions? With 1%

• CT slice thickness ? Calibration is independent of slice thickness

• Position of plug in phantom ? Independent of plug placement

• Simultaneous vs sequential scan of test plugs ? Within 1%

• Phantom size ? MATTERS!

Influence of phantom size


Rotate phantom 30 cm vs 40 cm phantom size

Conclusion: For larger differences, Child extremity vs Adult pelvis HU uncertainties in may exceed 3.5 %

What is total uncertainty with stoichiometric calibration ?

Yang et al., PMB, 57:4095, 2012

1. Uncertainties inherent to the CT imaging process

2. Uncertainties in the determination of the K parameters 3. Uncertainties that are introduced by using standard ICRU tissues that do

not include tissue variations with age or health of the patient 4. Uncertainties in the mean excitation energies 5. Uncertainties that are introduced in the dose calculation algorithm by

using a constant SPR assumption across all proton energies.


Yang et al., PMB, 57:4095, 2012

1. Uncertainties inherent to the CT imaging process


Yang et al., PMB, 57:4095, 2012

2. Uncertainties in the determination of the K parameters

After K parameters are calculated, calculate HU for tissue substitutes and compare with measured HU


Yang et al., PMB, 57:4095, 2012

3. Uncertainties that are introduced by using standard ICRU tissues that do not include tissue variations with age or health of the patient


Yang et al., PMB, 57:4095, 2012

Tissue variations: largest component in for soft tissue Uncertainty is highest for lung


Yang et al., PMB, 57:4095, 2012

In combination for clinical cases

Can dual energy CT help?

Dual energy CT images electron density ρ (rho) and effective atomic number Zeff


Electron density ρ (rho)

Excitation energy in water Iw

= 75 eV (ICRU)

Excitation energy in material Im

Dual energy CT images electron density ρ (rho) and effective atomic number Zeff


Excitation energy in material Im – is function of Zeff

Yang et al., PMB, 55, 1343 (2010)

SPR to rel. e-density


𝜌𝑒,𝑚

𝜌𝑒,𝑤× k,

and K ranges from 1.025 to 0.975 for different materials , but ….

0

0.5

1

1.5

2

2.5

0 1 2

RSP

rel. e-density

Substitutes

Real Tissues

Data from Schneider et al., PMB 41: 111

Hünemohr papers on DECT

Huenemohr et al., Z. f Med Physik, 23, 300: 2013 Huenemohr et al., Med Phys, 41, 61714-1: 2014

Use DECT for ρe determination: Establish WEPL to ρe lookup table

Reduce range uncertainties

from 1±1.8% to 0.1±0.7% (tissue)

From 7.8% to 1% (tissue-substitutes)

Improvement for non-standard tissues

(age and disease related variations)

Why has a stoichiometric calibration been widely adopted by the proton community? 1. It is a more precise because

it is based on real tissues

2. It is easier to perform

3. It is based on virtual HU and does not require any CT measurements

4. It is independent of the CT phantom size

5. Real tissues are easier to obtain than tissue substitutes

Answer: 1) It is a more precise because it is based on real tissues Ref: Schneider U, Pedroni E, Lomax A. The calibration of CT Hounsfield units for radiotherapy treatment planning. Phys Med Biol 1996;41:111-124.

Hounsfield numbers vary with imaging parameters such as phantom size. Regarding proton therapy which one of the flowing statements is true:

1. The use of multiple calibration curves does not improve the range calculation accuracy

2. The use of multiple calibration curves eliminates range uncertainties

3. .The use of a single calibration curve results in a 10 % range uncertainty

4. The use of a single calibration curve results in a 3.5 % range uncertainty

5. Proton therapy does not require a special HU calibration

Answer: 4. The use of a single calibration curve can result in a range uncertainty of more than 3.5 % for proton therapy Ref: Ainsley CG, Yeager CM. Practical considerations in the calibration of CT scanners for proton therapy. Journal of applied clinical medical physics / American College of Medical Physics 2014;15:4721.

Which of the following statements regarding tissue variations that occur with age and health status of patients is true:

1. They are accounted for by the stoichiometric calibration because it is based on standard ICRU tissues

2. They are accounted for by the stoichiometric calibration because it is based on real tissues

3. A dual energy CT scan allows a more accurate SPR calculation for tissue variations that are seen with age and health status

4. Tissue variations with age and health status are minimal and do not result in SPR variations

5. A dual energy CT scan offers no advantage in SPR accuracy for these variations

Answer: 3. A dual energy CT scan allows a more accurate SPR calculation for tissue variations that are seen with age and health status Ref: Yang M, Virshup G, Clayton J, et al. Theoretical variance analysis of single- and dual-energy computed tomography methods for calculating proton stopping power ratios of biological tissues. Phys Med Biol 2010;55:1343-1362.

Artifact Reduction

• CT contrast avoidance

• Beam Hardening

• Delineation and HU override

• Artifact reduction algorithms

CT numbers are modified after intravenous contrast injection

• Use of contrast agents (CA) is common in proton treatment planning CTs

• CA accumulate and their iodine content increases the HU of soft tissues significantly

• Treatment planning calculations must be done on a native planning CT scan to avoid significant range errors

Wertz H, Jäkel O. Med Phys. 2004,31:767-73

Beam Hardening Artifacts

• Besides uncertainty of the HU-SPR calibration curve, beam hardening contributes additional uncertainty in SPR values

• Lower-energy photons have a higher cross section for the photoelectric effect and are absorbed with a higher probability than higher-energy photons. This results in a hardening of the spectrum.

• In all diagnostic scanners, a correction for this effect is applied in the calculation of the HU by the scanner software

• This is only perfect for a standard situation (16 cm cylindrical water phantom) but is incorrect if high-Z materials (e.g. metals) are present

• Beam hardening makes the calibration curve dependent on patient size

Due to beam hardening, the calibration curve depends on body size (Schaffner & Pedroni, PBM, 1998)

Manual Segmentation & HU Override

• The standard practice to deal with metal CT artifacts is to delineate artifact regions and to reset the HU of these regions to standard soft tissue or bone values

• This is a time-consuming process but usually leads to adequate results (Dietlicher et al. PMB, 2014) Metal artifacts are outlined manually in red and CT

numbers in these regions are ret to standard soft-tissue values (Dietlicher et al PMB, 2014)

Artifact Reduction Algorithms – Projection Completion Method (1)

• The projection of the metal object in the sinogram (upper left) is treated as missing data

• In the reconstructed image (upper right), the metal objects are thresholded, isolated (lower left) and projected back into sinogram space, isolating the projections that are missing (lower right)

Yazdi et al, IJROB, 62, 2005

Artifact Reduction Algorithms – Projection Completion Method (2)

• The missing sinogram data are interpolated using, e.g., the sum of the weighted nearest unaffected projection values within a window centered on the missing projection.

• The modified sinogram is reconstructed using the CT scanner reconstruction software, resulting in reduced artifacts (bottom) compared to the uncorrected image (top)

Yazdi et al, IJROB, 62, 2005

Impact of PC-Based Artifact Reduction on Proton Dose Distribution

• Wei et al. demonstrated the impact of not applying an appropriate artifact reduction method on a hypothetical prostate proton treatment plan

• A projection completion method, designed to work in the presence of bone, was used to correct the proton planning CT of a prostate patient

• The original plan, calculated from the uncorrected CT scan (top), applied to the corrected planning scan showed severe underdosing of the target (bottom)

Wei et al., PMB, 51, 2006

Iterative Artifact Reduction Methods

• Iterative methods use a forward projection model applied to each projection line (index i) and maximize the likelihood L of observing the measured values 𝑦𝑖 given the modeled projection line values 𝑦 𝑖 by iteratively updating the object vector of linear attenuation coefficients

• While monochromatic models are sufficient for soft tissues, bone and metals require polychromatic models to take beam hardening into account

Iterative maximum likelihood reconstruction of a simple phantom (upper left) with two different forward models. The water-based polychromatic model (MLTR) handles beam hardening better (lower right) than the simpler monochromatic model (upper right)

Slambroek & Nuyts, Med Phys, 39, 2012

Iterative Artifact Reduction Methods – Illustrative Example

• PC-based metal artifact reduction algorithms are fast, but carry a risk of false reconstruction

• Iterative ML-based methods are more accurate, at the expense of being much slower

• Using a more complex model in high-density regions (patches) and a simpler model in soft-tissue regions results good and faster results

Iterative reconstruction in comparison with filtered back projection (FBP). FBP is highly impacted by metal artifacts (upper left). Creating patches of metal and metal –free regions in conjunction with the iterative ML polychromatic methods removes most artifacts (lower left and right)

Can Proton CT help?

• 1963, A. Cormack: alternative to x-ray CT

• 1979-1981 K. Hanson: lower dose, diagnostic radiology

• 2000, P Zygmanski: improved RSP definition for p-therapy planning

Proton CT Design Concept

• An energetic low intensity cone beam of protons traverses the patient

• The position and direction (entry & exit) and energy loss of each proton is measured

• Proton histories from multiple projection angles

• Minimal proton loss and high detection efficiency make this a very efficient low-dose imaging modality

Design concept of the proton CT scanner based on single-proton imaging (R Schulte et al, IEEE Trans Nucl Sci, 53, 2004)

Low intensity proton beam

The Most Likely Path (MLP) Concept

Proton CT Iterative Reconstruction

• With registration of single particle histories, the object solution can be found by solving a very large, sparse linear system

• Iterative reconstruction algorithms use MLP concept and exploit massive computational parallelism

𝐴𝑥 = 𝑏

Status of Proton CT

• Several groups have attempted to build pCT systems based on the concept shown above

• The most advanced is the Phase II head scanner built by the U.S. pCT collaboration, which can handle proton rates of up to 1.3 Mps without pile-up

The Phase II pCT Head Scanner

• The Phase II head scanner prototype is a compact scanner system, mountable on any horizontal proton beam line

• The scanner area is 36 cm x 9 cm allowing to scan head-sized objects

• Fast data acquisition (goal > 1 Mp per sec) allows 360 deg acquisition in ~10 minutes Components of the Phase II pCT scanner

pCT Reconstruction of a Custom Edge Phantom with an Iterative Reconstruction Technique

• The r=2 median filtered FBP as starting point for the iterative reconstruction

• 7 iterations of DROP-TVS

• 1 mm x 1 mm pixels, 2.5 mm slices

• ~200M p histories (~0.5 nSv per p, ~1 mSv)

Proton CT – Future Development

• Proton CT may provide a good solution providing – A mostly artifact free definition of SPR images for

proton treatment planning with better range definition

– A low-dose method for daily IGRT and adaptive proton therapy

• With current accelerator systems (230 MeV-250 MeV), proton CT would be used for head scanning, but needs to be integrated into existing gantry rooms and work flows

You are reviewing a proton planning head CT and notice that iodine contrast was used to enhance tumor and blood vessels. This is the only CT scan that was provided for planning purposes. The correct next step would be:

1. Reset contrast-filled vessels to HU of water

2. Reset contrast enhanced tumor to HU of water

3. Reset all enhancing structures to HU of water

4. Use original scan for planning

5. Repeat planning study providing a native scan

Answer: 5. Repeat planning study providing a native scan Ref: Wertz H, Jäkel O. Influence of iodine contrast agent on the range of ion beams for radiotherapy. Med Phys. 2004;31:767-73.

Proton CT is not yet used in clinical practice. When replacing x-ray planning with proton CT planning the main advantage would be:

1. it is less sensitive to motion artifacts

2. it provides larger image contrast

3. it provides more accurate RSP

4. it provides better-quality DRRs

5. it is low-cost technology

Answer: 3. it provides more accurate RSP Ref: Zygmanski P, Gall KP, Rabin MS, Rosenthal SJ. The measurement of proton stopping power using proton-cone-beam computed tomography. Phys Med Biol. 2000;45:511-28.

Use of 4DCT

• Used to access motion amplitude

• Proton Centers may have motion threshold above which IMPT or Proton therapy in general is not used

• Common threshold for moving targets is 5 mm

Use of 4DCT for planning

If 4DCT is available: • Average CT • Any single phase • MIP image • End-expiration phase can be used for planning

Use of 4DCT for planning

If 4DCT is available: • Average CT • Any single phase • MIP image • End-expiration phase can be used for planning

?

Use of 4DCT

MIP (maximum intensity projection) Conservative since maximum density is provided Should guarantee dose coverage of distal target But …….

Use of 4DCT

MIP (maximum intensity projection) Conservative since maximum density is provided Should guarantee dose coverage of distal target But ……. Proton that are aimed at proximal target also have high energy In true anatomy proton may stop distal to proximal target Lose coverage in proximal target region

Kang et al. Int J Radiat Oncol Biol, 67, 906, 2007

Use of 4DCT

Average CT Better approximation of breathing patient But ……. Protons may encounter denser tissue along path for some phases May under dose distal target


Planning strategy for mobile tumors

• Use average CT for planning • Use 4DCT to determine ITV • Density override ITV to tumor density with HU of 100 • Compare nominal (apparent) plan with dose accumulated over all phases (4D) plan


Access robustness of plan against breathing- Breast

Depauw et al. Int J Radiat Oncol Biol, 91, 427, 2015

Use 4DCT to simulate the interplay effects

Grassberger et al, IJROBP (86) 2, 380, 2013

Use 4DCT to simulate the interplay effect


Open symbols: n=1

Solid symbols: n=4

Open symbols: n=1

Solid symbols: n=4


Use 4DCT to simulate the interplay effect

Use 4DCT to select optimal gantry angles

Breathing can cause variations in tumor motion And Range changes due to anatomical variations Both effect can compromise plan quality

Use 4DCT to select optimal gantry angles

Use T0 and T50 phase project water equivalent thickness (WET) of distal tumor surface Plot WET difference as a function of gantry angle

Chang et al. Int J Radiat Oncol Biol, 90, 809, 2014

Gantry angles near 30 and 160 degrees are optimal for robustness

According to Kang et al the treatment plan of a moving tumor should be calculated on what CT image ?

1. MIP image with ITV density override

2. MIP image without ITV density override

3. Average CT with ITV density override

4. Average CT without ITV density override

5. Exhale phase

Answer: 3) Average CT with ITV density override Ref: Kang Y, Zhang X, Chang JY, et al. 4D Proton treatment planning strategy for mobile lung tumors. Int J Radiat Oncol Biol Phys 2007;67:906-914.

If a tumor in the thorax is treated which the following statements is true:

1. AP and PA beam are always the most robust beam angles

2. Breathing does not affect the delivered plan if the average CT is used for treatment planning

3. All beam angels are equally affected by breathing motion

4. Some beam angles are more robust against breathing motion than others

5. The beam angle selection is unimportant as long as the MIP image is used for treatment planning

Answer: 4) Some beam angles are more robust against breathing motion than others

Ref: Chang JY, Li H, Zhu XR, et al. Clinical implementation of intensity modulated proton therapy for thoracic malignancies. Int J Radiat Oncol Biol Phys 2014; 90:809-818.

Image Guidance for Proton Therapy

• Purpose

• Digital radiography

• 3D and 4D CBCT

• CT on rails

• Other methods: ultrasound, optical tracking, EM transponders

• Current practice and emerging trends

Purpose of image guidance

• IGRT generally refers to frequent, serial imaging of some kind performed in the treatment room prior to delivery of RT

• The main purpose of IGRT is better localization of target and normal tissue volumes and, thereby reducing the uncertainty PTV margins and avoiding missing the tumor or overexposing OARs

How can IGRT be done?

• Two-dimensional (planar) radiographic imaging (usually digital now, MV or kV)

• 3D (volumetric) CT imaging (3D-CBCT or CT-on-rails)

• 4D (time-sensitive) CT imaging (4D-CBCT or CT- on-rails)

• Non-ionizing radiation imaging: ultrasound

• Surface optical markers

• EM trackers (transponders)

Planar versus Volumetric IGRT

• Planar – Pros: inexpensive, fast (< 1min), very low dose (<1

mSv) – Cons: no soft tissue detail, insensitive to deformation,

susceptible to axial rotation

• Volumetric – Pros: image soft tissues and bone, sensitive to

deformation and other interfraction changes, improved alignment accuracy

– Cons: more expensive, time-intensive (> 1min), higher dose (CBCT, >10 mSv)

Should we use Implanted Markers for Proton IGRT?

• Gold markers are commonly used to track interfraction and interfraction motion in photon IMRT and radiosurgery (Cyberknife)

• Proton therapy is sensitive to presence of high-density markers, which my influence the dose distribution and cause additional artifacts in the planning CT

• Gold markers should be used with caution, and smallest possible markers should be preferred if used

Should we align to Bony or Soft Tissue Landmarks?

• Proton treatment plans are sensitive to misalignment between bones and tumor

• Common practice has been to align to bony landmarks and ignoring soft tissues

• With in-room CBCT and CT-on-rails, new protocols are likely to evolve that check whether a plan is adequate or needs to be adapted

Digital Radiography

• Digital radiography with 2D-to-2D alignment based on DRRs and comparison of bony anatomy is still the most commonly used IGRT technique in proton therapy

• Alignment algorithms can be automatic or interactive

A radiation therapist at Loma Linda Medical Center performing a digital radiography based alignment procedure on a prostate cancer patient

CBCT

• It is expected that 2D radiography will continue to be used for proton IGRT in the foreseeable future

• CBCT is now starting to be employed at some centers, and is expected to be installed in most new installations in the next few years mostly for more precise patient positioning

• CBCT has the added advantage of visualizing soft-tissue changes, which is important for adaptive proton RT, especially for head & neck tumors

• CBCT is however not accurate enough for proton plan assessment before treatment

User interface for a proton CBCT system in clinical use.

CT-on-Rails

• CT-on-rails is a diagnostic quality MSCT scanner that has recently been installed in two European proton centers (Trento, Dresden)

• It will have use in adaptive replanning of patients, particularly useful for head & neck tumors

CT-on-rails as in-room IGRT tool installed at the Trento facility in Italy

Auxiliary Methods for Proton IGRT

• Ultrasound: – In-room ultrasound pretreatment alignment is used for prostate, lung,

abdominal, and breast tumor sites, most experience with interfraction tracking of prostate

– At least 5 commercial systems are available – See AAPM TG-154 report for guidelines on commissioning and QA

• Surface Tracking: – Surface tracking with ceiling-mounted camera systems at the time of planning

CT and in-room – Detects intrafraction motion – uses rigid body transformations in combination with a least-square fit to

minimize the difference between the actual expected surface – Currently two commercial systems were available for patient surface tracking

(AlignRT and C-Rad Sentinel) – Commissioning and routine QA described in AAPM TG-147 report

• Real-time tracking (Electromagnetic Transponders): – Only one commercial system (Calypso) – Currently not used in proton therapy due potential interference of

transponder beacons with proton dose distribution

Example Ultrasound Guided RT

Current and Emerging IGRT Practice

• IGRT has been common practice in proton therapy, and is currently done with kV planar imaging*

• IGRT of the future will shift to 3D imaging with soft-tissue registration

• CBCT and CT-on-rails will become more commonly used in the future and new protocols will emerge

*Alcorn SR, Chen MJ, Claude L, et al Pract Radiat Oncol. 2014, 4:336-41

Emerging Imaging Techniques for Range Verification

• Prompt gamma imaging

• PET-imaging

• Proton CT and radiography

Prompt -Ray Imaging

• Elemental prompt gamma (PG) rays arise during proton irradiation of tissue.

• PG ray lines are specific for the excited nucleus

• The intensity and profile of the PG-ray emission is strongly correlated to delivered dose and Bragg peak position

• Compton cameras for PG detection and intra-treatment beam range verification are under development

PET Imaging

• PET imaging of proton-induced isotopes has been shown to be valuable for range verification during and after treatment.

• In soft tissues, the most important radionuclide species are 11C, 13N and 15O, of which 15O is dominant but decays fastest

• Three operational modalities are in use – in-beam PET during tx (GSI, NCC

Kashiwa, Japan) – in-room PET post-tx (MGH) – off-line PET (MGH, Heidelberg)

• Each technique has pros & cons

Proton Radiography and CT • Proton radiography and

proton CT are emerging technologies with promising properties

• Proton radiography could track lung tumors in real-time providing accurate validations of tumor motion models

• Proton CT would provide accurate 3D maps of the patient just before treatment, opening the possibility of low-dose imaging daily imaging for adaptive proton RT

• First clinical systems should become available in a few years from now

Proton radiography results in a 2D distribution of water equivalent path length (WEP) shown as a 3D bar graph (top). Three dimensional reconstruction of the relative stopping power (RSP) of a pediatric head phantom with an experimental proton CT scanner (top)

Testa et al. PMB, 2013

Future use of proton CT: daily IGRT for advanced head & neck cancer

• The head & neck region is anatomically complex (many critical structures can abut the GTV)

• Bulky disease (T3-4, N2-3) often responds dramatically during tx

• Use of IGRT & IMPT in the non-surgical setting is awaiting technology development, e.g., pCT

• Largest benefit of daily IGRT will be in the sparing of OARs Ahn et al. Cancer J 2014;20: 421–426

The currently most commonly used method for IGRT in children treated with proton therapy is:

1. ultrasound

2. planar MV imaging

3. CBCT

4. proton CT

5. planar kV imaging

Answer: 5. planar kV imaging Ref: Alcorn SR, Chen MJ, Claude L, et al. Practice patterns of photon and proton pediatric image guided radiation treatment: results from an International Pediatric Research consortium. Pract Radiat Oncol. 2014;4:336-41.

Prompt gamma (PG) imaging as a novel method for proton range verification is best done:

1. during treatment

2. before treatment

3. after treatment

4. before or after treatment

5. after injection of a gamma-emitting isotope

Answer: 1. during treatment Ref: Polf JC, Peterson S, Ciangaru G, Gillin M, Beddar S. Prompt gamma-ray emission from biological tissues during proton irradiation: a preliminary study. Phys Med Biol. 2009;54:731-43.

PET imaging as a novel method for proton range verification is best done:

1. during treatment

2. before treatment

3. during or after treatment

4. before or after treatment

5. after injection of a positron-emitting isotope

Answer: 3. during or after treatment Ref: Zhu X, España S, Daartz J, Liebsch N, Ouyang J, Paganetti H, Bortfeld TR, El Fakhri G. Monitoring proton radiation therapy with in-room PET imaging. Phys Med Biol. 2011;56:4041-57.

One main advantage of daily IGRT in proton therapy of head and neck cancer would be:

1. additional prognostic information

2. better RBE estimation

3. prevention of overdosing OARs

4. shortening of the daily treatment time

5. reduction of operational costs

Answer: 3. prevention of overdosing OARs Ref: Ahn PH, Lukens JN, Teo BK, Kirk M, Lin A. The use of proton therapy in the treatment of head and neck cancers. Cancer J. 2014;20:421-6.

Adaptive radiation therapy for protons

Protons plans are sensitive to • Tumor growth- can cause under dosing of tumor • Tumor shrinkage- can cause protons to range out into OAR

• Weight gain- can cause under dosing of distal target

More sensitive than photon plans since dose distribution can change significantly Use repeat imaging more frequently

Adaptive radiation therapy for protons

MD Anderson study for lung and mediastinal tumors Repeat 4DCT weekly or after 2-3 weeks of treatment • Was target coverage maintained ? • Were OAR over dosed?

• 9 of 34 patient were re-planned • 7 due to under coverage of target • 2 due to over dose of OAR

• Median time for re-plan was after 20 fractions (5-25 range)

Chang et al. Int J Radiat Oncol Biol, 90, 809, 2014

Tumors with cystic components

• Pediatric craniopharyngiomas:

• Weekly MRI imaging

• Effect of PTV change on PTV coverage for IMRT, Passive scattered, and IMPT plans

Beltran et al. Int J Radiat Oncol Biol, 82, e281, 2012

Tumors with cystic components

• 5 mm CTV margin, 3 mm PTV margins

Beltran et al. Int J Radiat Oncol Biol, 82, e281, 2012

IMPT plan most sensitive to PTV changes IMPT: 5% change in PTV- investigate re-plan IMRT/DSB: 10% change in PTV- investigate re-plan

Considering adaptive proton therapy for thoracic tumors, treatment related anatomical changes may result in …..

1. Increased target coverage due to patient weight loss

2. Increase target coverage due to tumor growth

3. Decreased target coverage due to patient weight loss

4. Decreased target coverage due to patient weight gain

5. Overdosing of organs at risk due to tumor growth

Answer: 4) Decreased target coverage due to patient weight gain

Ref: Chang JY, Li H, Zhu XR, et al. Clinical implementation of intensity modulated proton therapy for thoracic malignancies. Int J Radiat Oncol Biol Phys 2014; 90:809-818.

Imaging for Proton Therapy Katja Langen(1), Reinhard Schulte (2)

1: University of Maryland

2: Loma Linda University

Imaging for Proton Therapy · 2015-06-22 · Imaging for Proton Therapy Katja Langen(1), Reinhard...

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