4D CT in Radiation Therapy

Parminder S. Basran, PhDDept. Medical Physics

BCCA-Vancouver Island Centre

Outline

• The PTV problem– ITV, MTV

• Various solutions• Our solution• Protocol

ICRU standard definitions for margins in radiation therapy

GTV

CTV

ITV

PTV

PTV margin assessment

• Requires compartmentalizing “independent” sources of displacement errors

– Combine errors in quadrature– ‘Recipe’ based (ex: VanHerk formulas)

PTV margin components- Inter-fraction motion: i.e., motion between

fractions- Primarily a function of the immobilization and

localization methodology- Other considerations: patient preparation (ex:

bowel status), other confounding factors during treatment (ex: degrading pulmonary status)

- Intra-fraction motion- Primarily a function of the extent of the target

motion during the course of treatment

PTV: Intra-fraction motion

- Intra-fraction motion- motion during the course of treatment- Major factors determine the extent of

target motion during treatment- Voluntary motion- Involuntary motion (done without intention)

- Breathing motion- Gastro-intestinal motion- Cardiac motion and pulsatile motion- Coughing, sneezing, swallowing, twitches, etc.,

PTV: Intra-fraction motion due to Breathing

Large magnitude of displacement errors for lung, liver, upper GI disease : MTV

Somewhat periodic/regular

Confounds the treatment two key ways- localization of the target- Irradiation of the surrounding normal tissue

(i.e., lung)

Breathing Motion: Target Localization (current approach)

GTV

CTV

PTV = CTV + standard margin

Breathing Motion: normal tissue toxicity

• Growing evidence that significant portions of lung can be spared if the margin associated with respiratory motion not included in the PTV definition

GTV

CTV

PTV = CTV + standard margin

Breathing Motion: Target Localization

• Ample evidence that the PTV margin allotted for respiratory motion may be severely under- or over- estimated if one simply allots a standard ‘1 cm’ margin for the PTV

GTV

CTV

PTV = CTV + standard margin

Breathing Motion: Target Localization

• Methods for assessing the PTV– Fluoroscopy and CT– CT scans at exhale, inhale and normal

respiration• CT1 RPM Gating System• DIBH LSB

– 4D CT• New CT scanner

Breathing Motion: What is 4-D CT?

• CT simulation provides a volumetric dataset for dose calculation

• 4D CT provides volumetric datasets for assessing target motion due to breathing

Assume a simple moving target

What is 4-D CT?

GTV25%

50%

75%100%

Snap-shots of motion at

0, 25, 50, 75 and 100%

What is 4-D CT?

What is 4-D CT?• A longer than conventional CT scan (about 5X

longer, ~ 40 seconds)

• Slightly more dose , ~ extra 6 cGy (versus ~2 cGy for the first scan)

• Requires a signal to sort out the phases of the breathing cycle (use the RPM gating block)

• Provides up to 10 different phases, or sets of CT images (divided in roughly equal parts of the breathing cycle)

What is 4-D CT?

• Assumptions– Block motion correlates with lung motion

• Highly dependent on the patients pulmonary status

– The (reconstructed) images provide an accurate measure of the lung motion

– The respiratory motion exhibited during the scan is representative of that during treatment• Not quantified, but seems reasonable for those

patients whose pulmonary status has not changed significantly

Result

• 4D CT will result in a number of CT image sets that can be used for PTV definition

Question: Do you want to contour on all 10 datasets to define you PTVs?

4D CT at VIC

• 4D CT will result in a number of CT image sets that can be used for MTV definition

• One can then fuse all 10 data sets with the original planning CT for MTV definition

Question:

• Do you want to contour on all 10 datasets to define you PTVs?

Answer

• No because I can contour on a MIP instead of all of the different phase-related images

Question

• What is a MIP?

• Maximum Intensity Projection

MIPs

• Use the fact that in lung, higher density tumors will appear to move against a lower density lung

0% 25% 50% 75% = phases of motion

GTV25%

50%

75%100%

5 different phases

MIP =

MIPs

• Will provide a ‘blurred’ image of the target

• Ignores surrounding tissues, and will therefore– Make lung volumes smaller than reality

(due to chest wall and heart motion)– Make external contours larger than

reality– ‘blur’ vessels in the lung

Some examples: Case 1Conventional MIP

Case 1 Conventional MIP

Case 1 Conventional MIP

Case 2 Conventional MIP

Case 2 Conventional MIP

Case 3 Conventional MIP

Case 4 Conventional MIP

Case 5 Conventional MIP

MIPs

Pros Cons- 1 dataset to contour the GTV (ITV), not 10

-Difficult to define ITV for tumors near chestwall, mediastinum structures and diaphragm

- Captures 3D range of motion, as opposed to just exhale/inhale

- Artificially increases all CT values (hence standard window/level will result in brighter images)

- Great for lung tumors against background lung, and liver nodes against normal liver

- Provides data for only 1 of many structures that need contouring

Proposal When should I use 4D CT?

• Non-palliative cases• When lung toxicity is a concern• When the target is close to the

diaphragm• When the target is not in the apex ?

Suggested Workflow

• CT Simulation– Set-up for RPM scan– Perform a standard CT– Perform a retrospective 4DCT w/ RPM– Mark-up patient– Start 4DCT analysis …

Suggested Workflow

• Advantage Workstation– Reconstruct CT images using 10 phases– Create the MIP AND Average CT– For SBRT cases, also export 0 and 50%

phases– Export 2-4 datasets into Eclipse– Start contouring …

Suggested Workflow• Eclipse

– There will be at least 2 CT datasets• MIP

– Contour GTV_MIP• Planning CT

– Contour GTV– All other normal tissues– May be used for planning planning purposes

• Average CT– Contour GTV– All other normal tissue– May be used for planning purposes

Once we are comfortable with the AVERAGE CT datasets for normal tissue contouring, drop the conventional CT and just perform a 4DCT scan

Conclusions• MIPs are a simple means of collapsing the 4D data

into a single data set

• There are some shortcomings

• MIPs along with the 0% (rest exhale) and 50% (inhale) phase images will be available for planning purposes

• Audio coaching… some data to suggest better quality images…