Importance of 4D CT For Target

Volume Definition in Stereotactic Lung

Radiotherapy

Derya Çöne, RTT

Acıbadem Kozyatağı Hospital

Istanbul, Turkey

Introduction

SBRT

• High local control rates for early

stage NSCLC

• High doses in1-5 fractions

• Limited margins

Introduction

Target delineation accuracy is very

important for success of the treatment

Intrafractional tumor movement

caused by breathing must be taken

into account when treating lung

tumors.

Introduction

Various techniques are being used for lung tumor target delineation including

4DCT, 3D MIP, 3D insp-exp or 3D free+margin.

However, the accuracy of some of these techniques to delineate the target is

controversial.

Introduction

Recommendations about delineation lung tumor for SBRT treatment

intend.

Introduction Another recommendation is coming from AAPM TG 101.

And it is directing us to the TG 76

Considerations of Breathing

Motion in CT Sim

Breath Hold CT Scan

• Voluntery breath hold

• Active breathing control

• Combine inhale and exhale GTVs to get ITV

Considerations of Breathing

Motion in CT Sim

Breath Hold CT Scan

• Voluntery breath hold

• Active breathing control

• Combine inhale and exhale GTVs to get ITV

Slow CT scan

• 4 seconds per slice for axial scan

Considerations of Breathing

Motion in CT Sim

Breath Hold CT Scan

• Voluntery breath hold

• Active breathing control

• Combine inhale and exhale GTVs to get ITV

Slow CT scan

• 4 seconds per slice for axial scan

Gated CT scan

• Images at only one phase, 4-5 times longer acquisition time

Considerations of Breathing

Motion in CT Sim

Breath Hold CT Scan

• Voluntery breath hold

• Active breathing control

• Combine inhale and exhale GTVs to get ITV

Slow CT scan

• 4 seconds per slice for axial scan

Gated CT scan

• Images at only one phase, 4-5 times longer acquisition time

4D CT scan

• 3D CT scans at multiple breating phases

Considerations of Breathing

Motion in CT Sim

Breath Hold CT Scan

• Voluntery breath hold

• Active breathing control

• Combine inhale and exhale GTVs to get ITV

Slow CT scan

• 4 seconds per slice for axial scan

Gated CT scan

• Images at only one phase, 4-5 times longer acquisition time

4D CT scan

• 3D CT scans at multiple breating phases

Processed image data sets

• Maximum intensity projections

• Avarage intensity projections

• Minimum intensity projections

Aim

In this study, we investigated if the ITV volume defined on 4D CT

images, received the prescription dose, when the target volume was

determined on Free Breath CT and MIP images for SBRT for lung

tumors.

Patients and Setup

Twelve patients treated with fractionated stereotactic body

radiotherapy for early-stage lung cancer or lung metastasis in our

clinic between June 2013 and December 2014 were included in this

study

Median age 69.5 (range 42-86y)

6 primary lung cancer, 6 metastases

Patients were immobilized with a vacuum bag in supine position,

arms extended above the head.

Methods CT scans were performed with 3mm slice thickness for 4D and free

breathing CT using Varian RPM system (Siemens Somatom

Sensetion 64).

Free Breathing CT GTV-FB contoured on Free Breathing CT

4D CT ITV was created by merging GTV contours defined on each phase of

the respiratory cycle (8 phases on 4D CT)

MIP 4D CT images were digitally processed to obtain maximum intensity

projection (MIP). GTV-MIP contoured on MIP images.

Methods: Contouring

PTV-FB and PTV-MIP volumes were created on free breathing (FB)

and MIP planning CT scans respectively (GTV+5mm).

CTV contour was not defined.

PTV-4D was generated by adding 5 mm margin to ITV

FB, MIP and 4D CT data sets were automatically matched

according to DICOM coordinates.

Methods

PTV-MIP and PTV-4D contours were transferred on the free breathing

CT scan

Plans were constructed on FB datasets for PTV4D (95% of PTV-4D

volume to receive 100% of the prescription dose).

This plan was defined as a reference plan.

Two treatment plans were generated for PTV-FB and PTV-MIP

volumes (95 % of PTV–FB and PTV-FB volumes to receive

100% the prescribed dose) on free breathing CT dataset.

Methods

Red GTV-FB

Magenta: GTV-MIP

Cyan: ITV

Methods: Planning

Prescription dose defined as 7x850 cGy=6000 cGy.

For treatment plans, 'RapidArc' technique was performed using 2

noncoplanar partial arcs (Eclipse Version 11).

AAA algorithm was used for dose calculation.

Grid size: 0.125 cm

Heterogeneity correction: ON

The ITV volume that received the prescription dose and dose received

by 95% of ITV volume (D95) were recorded for each plan.

Methods

Dose distribution for PTV-FB plan

Methods

Dose distribution for PTV-MIP plan

Results

Mean PTV-4D, PTV-MIP and PTV-FB volumes were 23.2 cc, 15.4cc

and 11cc, respectively

Plans for PTV-MIP volume: P PTV-MIP,

Plans for PTV-FB volume : P PTV FB

Conclusion

We found the ITV volume was not covered by the prescription dose if

the target volume was not defined on each phase of 4D CT .

Even, the MIP datasets constructed via 4DCT images were not

sufficient to reflect the respiratory effect.

4D CT scans should be considered as standard imaging modality for

stereotactic lung radiotherapy.

Thank you...