2C - Tumor Margins and Field Design

Treatment planning considerations

Diagnosis

• Tumor pathobiology / aggressiveness

• Staging / lymph nodes

Therapeutic decisions

• Curative or palliation

• Treatment modalities

Target volume localization

• Tumor/normal tissue definition

• Tumor doses

• Organs at risk

• Field shaping

Treatment planning

• Technique selected

• Dose distribution

• Field definition

• Confirmation of measurements

Fabrication of treatment aids

• MLCs/ blocks

• Compensators / bolus

Consequences of inaccurate planning

• Dose difference of even 5% may lead to impairment or enhancement of tumor response, may risk morbidity

• Inadequate documentation of planning parameters may lead to false interpretation for daily treatments

Gross Tumor Volume (GTV)

The true palpable or visible extent and location of malignant growth

Actual tumor

May also apply to positive lymph nodes (GTV-T, GTV-N)

Why is it important to describe and report the GTV accurately

Required for staging

Adequate dose must be delivered to whole GTV

Regression of the GTV during treatment may used with care as predictive of tumor response

If the tumor shrinks, will they usually also shrink the GTV

No

Ways to determine the GTV

Clinical examination (inspection, palpation, endoscopy)

Imaging (x-ray, ct, mri, us, etc.)

Clinical Target Volume (CTV)

The GTV and margin of tissue around it that may have microscopic disease in it

How is the CTV determined

Clinical-anatomical concept, based on available data regarding probability of malignant cells around GTV

Subclinical volume (more of a guesstimate)

Radiation oncologist’s judgement and decision

Internal margin (IM)

Planning for movement during treatment (like breathing)

Internal target volume (ITV) accounts for this

Set-up margin

Couple mm for buffer during set-up

• Patient positioning

• Mechanical uncertainty of equipment

• Dosimetric uncertanitures

• Transfer of set-up errors from CT simulator to treatment unit

• Human factors

The actual size of the margin depends on the selection of beam geometries

Setup margin consideration

Inter-fraction motion = change in target position from one fraction to another

• Changes caused by uncertainties in PTV at isocenter each time patient is on table

Intra-fraction motion = change in target position during treatment delivery (like breathing, bladder, rectum)

• Changes while on treatment table

Planning Target Volume (PTV)

Contains tissues that are to be irradiated to a prescribed dose

PTV surrounds CTV with additional margin to compensate for variation and certainties of the beam

PTV is used to help select beam size and arrangement

Name of the volume for inside to outside

GTV

CTV

ITV

PTV

Treated volume

Irradiated volume

Organs at risk (OAR)

Normal tissues whose radiation sensitivity may influence treatment planning and/or prescribed dose

Planning Organ at Risk Volume (PRV)

Any movements of the OARs, as well as set-up uncertainties

Margin must be added to OAR

Types of OARs

Serial = loss of function in one part of organ causes entire organ to stop working

• No threshold volume = high dose to small volume can cause injury

• Ex: brain stem, spinal cord, optic nerve

Parallel = loss of function in one part of organ only affects that part of the organ

• There is a threshold volume = you can take out entire kidney without causing renal failure if other kidney is healthy

• Ex: kidney, liver, lung

Treated Volume (TV)

Receives the prescribed therapeutic dose of radiation necessary to achieve the purpose of treatment

• Very similar to PTV, but a little more tissue

• Selected by radiation oncologist

• Around 95-100 isodose curve

Irradiated volume

Volume of tissue that receives a dose considered significant in relation to normal tissue tolerance

Ex: 50% isodose line

Main reasons for adding margins

Dose fall off at the edge of the beam

• Geometric edge of beam is defined by the 50% isodose line

Inaccuracies in defining target volume

• Physicians may not have defined the complete CTV or PTV

Inaccuracies in beam alignment and probability of organ and patient motion

Advantages of smaller margins

Less normal tissue treated, less radiation induced complications

Increase dose to tumor since decreasing amount of normal tissue treated