Treatment Planning Target and Structure Definition Jack Yang, Ph.D., DABR Institute for Advanced Radiation Oncology Barnabas Health Long Branch, NJ 07740 New Technologies in Modern Radiotherapy, Chulabhorn Research Institute, Bangkok, Thailand, August 22-25, 2012

Outlines Identify the patient data acquisition process and target (GTV) definitions Image modalities with discussion for target delineation Atlas (RTOG and more) Published papers and Netter/Gray Anatomy books for common disease sites

ICRU50/62 Target Volume Definition Gross Target Volume (GTV) visible or palpable disease Clinical Target Volume (CTV) GTV + subclinical extension site dependent typically 5 to 10 mm beyond GTV Planning Target Volume (PTV) CTV + margin for internal organ motion and variation in daily setup site dependent, usually 5 to 10 mm Beam aperture PTV + margin for beam penumbra (LIANC and energy dependent) 7 to 8 mm from 50% to 95% 10 -12 mm from 50% to 100%

Light Field

margin for penumbra

Target and Normal Tissue Delineation Tumor  CTV (Clinical Target Volume)  ITV (Internal Target Volume) =CTV + IM (internal margins)  PTV (Planning Target Volume) =ITV + SM (Set up margins) =CTV+ IM+ SM Normal Tissue  OAR (Organ at Risk)  PRV (Planning organ at risk volume)

ITV IM CTV SM

PTV

OAR PRV

Target and Normal Tissue Delineation •

Internal Margin (IM) Margin accounts for variations in size shape and position of CTV in relation to anatomy Setup Margin (SM) Accounts for uncertainties in patient-beam positioning Planning Organ at Risk Volume (PRV) Similar to PTV Volume accounting for margin about critical structures near or inside irradiated volume –

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Class C Global safety margins Smaller margins Based on clinical experiences of toxicity, or proximity of organs at risk Class B Reduce PTV Non-linear addition of IM + SM Class A Linear addition of IM + SM Internal margin accounts for variations in position or shape of CTV Setup margin accounts for variations in positioning

PTV/PRV Margins (ICRU 62) PTV (Planning Target volume) PRV (Planning Organs at Risk (OAR) Volume) PTV (PRV)= CTV (OAR) + Margins Uncertainty sources Internal Margins (IM): size, shape and position of the CTV in relation to anatomical reference points

Setup Margins (SM): To account specifically for uncertainties (inaccuracies and lack of reproducibility) in patient positioning and alignment of the therapeutic beams during treatment planning and through all treatment sessions.

Target and Normal Tissue Delineation

SBRT SBRT: –

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Gating Body fixation IGRT 4D scan estimation of tumor trajectories

Can thus reduce SM, IM, and even some CTV by selecting a portion of phase of tumor motion.

2D/3D

Target and Normal Tissue Delineation Target and normal structure delineation requirements for conformal radiotherapy have been addressed by the ICRU. Often most time consuming portion of the virtual simulation process and care should be taken to simplify this task as much as possible.

Well designed contouring software package is a prerequisite and should be one of main concerns when selecting virtual simulation software.

GTV/CTV Unfortunately, in many sites anatomic imaging techniques (i.e., CT or MRI) do not always distinguish malignant from normal tissues.

PET or PET/CT has been utilized to routinely delineate GTV/CTV w/ proper margin tools

PTV – Defining Margin Reviewing article by Langen and Jones, IJROBP 50, 265278, 2001

Defining PTV When CTV/GTV changes shape dramatically from one CT slice to next, larger PTV margin must be used in slice containing smaller CTV/GTV to obtain PTV coverage in the inferior-superior direction.

Proper PTV margin

PTV – Positional Uncertainties Physician should account for the asymmetrical nature of positional uncertainties.

No. of Patients

Comments

Displacement (mm)

Max. (mm)

10

Radio-opaque marker full bladder weekly portals, relative to a reference portal image

AP: 4.5

AP: 7.5

Lat: 1.7

Lat: 2.0

SI: 3.7

SI: 5.0

Balter, JM, Lam, K, Sandler, HM, Littles, JF, Bree, RL, Ten Haken, RK: Measurement of prostate movement over the course of routine radiotherapy using implanted markers. Int. J. Radiat. Oncol. Biol. Phys. 31:113-118, 1995

4D CT – Application in Simulation An imaging technique for providing a set of CT images for a specific breathing phase. A multi-slice CT scanner is used and the couch speed is reduced to accommodate breathing cycle During the scanning the patient’s breathing phase is monitored using a device such as Philips Bellows, Varian’s Real-time Position Management System (RPM) or Elekta’s Active Breathing Coordinator (ABC).

The data acquired data is then sorted by breathing phase (binning), and phase specific images can be reconstructed.

4D CT – Image Cycle • • • •

Tool Principle Advantages Limitations

4D CT – a Lung Case mid exhale

mid inhale

end exhale

end inhale

4D CT - Process for Sorting/Binning

GTV – 4D CT

Std light breathing scan

0% Phase of 4D scan

Courtesy G. Chen, Ph.D./MGH

SBRT Contouring Tips Because of the tight field margins used, errors in GTV delineation is much less forgiving than conventional treatment. Some variables to consider     

CT slice thickness (both diagnostic and planning) and field of view Axial vs. helical imaging Patient breathing/breath hold, 4DCT Diagnostic modality that best represents the tumor CT vs.MR vs. PET For each of the modalities windowing/leveling plays a key role

Contouring of Lung Lesions

Use the appropriate windowing/leveling for the anatomy being contoured. (Mediastinal window/level is not appropriate for identifying this lung lesion.) Contour the entire cross-section of the structure, e.g. esophagus includes the mucosa, submucosa & all muscular layers out to the fatty adventitia. (RTOG 0813) Subtract the GTV from the organ being treated.

Optical Chiasm

Brain Stem

Landmarks – Sella, CSF

Landmarks – Clivus, C1

Spinal Cord

Landmarks – C1, Vertebrae body, end at L1-L2 5 mm cord margin

Parotid

Landmarks – mandible, styoid, and masseter, can visualize well by changing window/level

Breast Breast Heart Lumpectomy

Tips for Contouring Male OARs This is the slice which defines the last slice of rectum, the following slice will connect anteriorly with the rectosigmoid.

Still counted as the femoral head

RTOG 1106 - Thoracic

Lung Spinal cord should also start at this level just below the cricord or from the base of skull C1 if scan is available, particularly when the tumors involve neck or apex.

RTOG 1106 - Thoracic

RTOG 1106 – Locating Brachial Plexus Timmerman’s Trick-1

clavicle

Vein, artery, and nerve (VAN, anterior to posterior) will go over the 1st rib and under the clavicle Using coronal images, find the plane where vascular/nerve structures (tubes and wires) pass between the 1st rib and clavicle

1st rib

Roughly contour these neurovascular tissues in this coronal plane (as shown in yellow) You will use these rough contours in the next step

Courtesy of RTOG 1106 protocol

RTOG Hippocampus Contouring Red: Hippocampus

Green: Hippocampal Avoidance Zone

Hippocampal 1) Hippocampal 1) Tail Tail 2) Body Body 3)2) Head 3) Head

The hippocampus has three anatomic subdivisions: the head, body, and tail; note that the head is inferior or caudad, the body is superoposterior and the tail is most cephalad (superior) and posterior, and an overall “banana” shape emerges on sagittal images, located in the plane of the lateral ventricle.

MR Images courtesy of: Holmes CJ, Hoge R, Collins L, et al. "Enhancement of MR Images Using Registration for Signal Averaging" Journal of Computer Assisted Tomography 22, 324-333 (1998)

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RTOG Sarcoma Working Group

RTOG Sarcoma Radiation Oncologists Reach Consensus on Gross Tumor Volume (GTV) and Clinical Target Volume (CTV) on Computed Tomographic Images for Preoperative Radiotherapy of Primary Soft Tissue Sarcoma of Extremity in RTOG Studies. IJROBP June 2011

Conclusion Imaging modalities (CT/MR/PET/NM) are basics for target and OAR delineation. CT (4D CT) Simulation has gradually replaced the current simulation process and can improve the quality for target definition if properly operated. Clinical volume margins vary from clinic to clinic, standards such as RTOG can be the starting references. Target and critical structures are the most important ingredients for quality treatments, anatomy knowledge from physicians is critical.