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THREE-DIMENSIONAL CONFORMAL

RADIATION THERAPY

P Sathish Kumar

RNT MEDICAL COLLEGE

UDAIPUR

INTRODUCTION

The aims of 3-D CRT are to achieve conformity of the

high dose region to the target volume and consequently to

reduce the dose to the surrounding normal tissues.

3-D conformal radiotherapy (3-D CRT) is the term used to

describe the design and delivery of radiotherapy treatment

plans based on 3-D image data with treatment fields

individually shaped to treat only the target tissue.

A typical 3-D CRT Process

CLINICAL IMPLEMENTATION OF 3-D

CONFORMAL RADIOTHERAPY

PATIENT ASSESSMENT AND DECISION TO TREAT

WITH RADIATION.

IMMOBILIZATION AND PATIENT POSITIONING.

IMAGE ACQUISITION AND TARGET

LOCALIZATION.

CT imaging

MRI and other imaging modalities

• SEGMENTATION OF STRUCTURES.

• TREATMENT PLANNING FOR 3-D CONFORMAL

RADIOTHERAPY.

PATIENT ASSESSMENT AND DECISION TO

TREAT WITH RADIATION

The first step in the process is patient assessment and

deciding how the patient should be treated. During

assessment various diagnostic and investigative

procedures are undertaken to define the state of the

disease.

This involves imaging, biochemical testing and review

of pathologic information to identify the type, stage and

grade of the cancer. The decision to treat the patient with

radiation should be made by a team of clinicians

IMMOBILIZATION AND PATIENT

POSITIONING .Radiation treatment accuracy can be divided into two

separate but highly interrelated issues:

Dosimetric accuracy- deal with issues such as radiation

beam calibration and dose calculations.

Geometric accuracy- It covers issues related to patient

positioning and immobilization that have a strong effect on

how well we can accurately cover a specified anatomic

volume with a desired radiation dose

An accurately set up laser alignment system

is an essential requirement for accurate

radiotherapy.

This should consist of at least three lasers

to provide two lateral crosses and a sagittal

line which can be used in conjunction with

appropriately placed tattoos to ensure the

patient is not rotated.

Special immobilization systems are

available for immobilizing different parts of

the body

For example, knee supports and ankle

stocks are used for pelvic and abdominal

immobilization, adjustable breast boards

are used for breast and vacuum

immobilization bags or alpha cradles are

used for chest, thermoplastic masks are

used for head and neck treatment, and

relocatable stereotactic frames are used for

brain tumours.

transparent headresthead rest-

Polyurethane

Head Rest

H &N Molds

FLATTENED AT 70° C IN WATER

HARDENS IN FEW MINUTES

USED FOR HEAD & NECK AND PELVIS CASES

EASILY FIXED TO THE COUCH

REUSABLE

CAN BE CUT AT STRATEGIC PLACES FOR

BEAM ENTRANCE

LASTS LONG WITHOUT DETERIORATION

Patient is placed on the couch in the same position as

was during simulation using the same immobilization

devices

Image acquisition and target Localization

CT imaging

For many tumour sites CT scanning provides

the optimal method of tumour localization. All

CT planning must be carried out under

conditions as nearly identical as possible to

those in the treatment room, including the

patient support system (couch top), laser

positioning lights and any patient positioning

aids.

LASER CROSS HAIR SYSTEM

For conformal therapy a slice separation andthickness of between 3 mm and 5 mm isrecommended for CT scanning.

For head and neck and Central NervousSystem (CNS) planning this may be reducedto between 2 mm and 3 mm.

The CT scanner couch top must be flat,securely fitted and compatible with thetherapy machine couch. Transverse andlongitudinal lasers with additional laserpositioning lights are needed in the CT roomidentical to those in the treatment room toensure exact positioning of the patient.

Digitally reconstructed radiographs

One of the important features of 3-D treatment planning is

the ability to reconstruct images in planes other than that of

the original transverse image. These are called the digitally

reconstructed radiographs (DRRs).

DRRs are produced by tracing ray lines from a virtual

source position through the CT data of the patient to a

virtual film plane. The sum of the attenuation coefficients

along any one ray line gives a quantity analogous to optical

density (OD) on a radiographic film.

Digitally reconstructed

radiographs

In treatment planning, MRI images may be usedalone or in conjunction with CT images. In general,MRI is considered superior to CT in soft-tissuediscrimination such as central nervous systemtumors and abnormalities in the brain.

Also, MRI is well suited to imaging head and neckcancers, sarcomas, the prostate gland, and lymphnodes. On the other hand, it is insensitive tocalcification and bony structures, which are bestimaged with CT.

Although important differences exist between CT andMRI image characteristics, the two are consideredcomplementary in their roles in treatment planning.

The most basic difference between CT and MRI is

that the former is related to electron density and

atomic number (actually representing x-ray linear

attenuation coefficients), while the latter shows

proton density distribution.

One of the most important requirements in

treatment planning is the geometric accuracy. Of

all the imaging modalities, CT provides the best

geometric accuracy and, therefore, CT images are

considered a reference for anatomic landmarks,

when compared with the other modality images.

SEGMENTATION OF STRUCTURES

The term image segmentation in treatment planning

refers to slice-by-slice delineation of anatomic

regions of interest, for example, external contours,

targets, critical normal structures, anatomic

landmarks, etc.

The segmented regions can be rendered in

different colors and can be viewed in BEV

configuration or in other planes using DRRs.

Segmentation is also essential for calculating dose

volume histograms (DVHs) for the selected regions

of interest.

Image segmentation is one of the most laborious but

important processes in treatment planning.

Although the process can be aided for automatic

delineation based on image contrast near the

boundaries of structures, target delineation requires

clinical judgment, which cannot be automated or

completely image based.

CT: attenuation coefficients (m), can be converted to

electron density,

used for treatment planning/dose calculation.

Spatial resolution ~1mm in X/Y directions,

variable (1-10 mm) in Z-direction, which affects the

quality of DRR

MRI: proton density, better soft tissue delineation

(brain, head/neck, prostate), but insensitive to

calcification and bony structures.

Spatial resolution ~1mm in all directions.

PET: functional image

TREATMENT PLANNING FOR 3-D CONFORMAL RADIOTHERAPY

DEFINITION OF VOLUMES

Volume definition is a prerequisite for meaningful 3-D treatment planning and for accurate dose reporting. ICRU Reports 62 and 83 define and describe several target and critical structure volumes that aid in the treatment planning process and provide a basis for comparison of treatment outcomes

Gross tumor volume or GTV

Clinical target volume or CTV

Planning target volume or PTV

Organ at risk or OAR

Planning organ-at-risk volume or PRV

Internal target volume or ITV

Treated volume or TV

Remaining volume at risk or RVR

The Gross Tumor Volume (GTV) is the

gross palpable or visible/ demonstrable

extent and location of malignant growth.

The GTV is usually based on information

obtained from a combination of imaging

modalities (computed tomography (CT),

magnetic resonance imaging (MRI),

ultrasound, etc.), diagnostic modalities

(pathology and histological reports, etc.)

and clinical examination.

Clinical target volume.

The clinical target volume (CTV) is the tissue volume that

contains a demonstrable GTV and/or sub-clinical

microscopic malignant disease, which has to be

eliminated. This volume thus has to be treated

adequately in order to achieve the aim of therapy, cure or

palliation.

The CTV often includes the area directly surrounding the

GTV, which may contain microscopic disease and other

areas considered to be at risk and requiring treatment

(e.g. positive lymph nodes).

The CTV is usually stated as a fixed or variable margin

around the GTV (e.g. CTV = GTV + 1 cm margin), but in

some cases it is the same as the GTV (e.g. prostate

boost to the gland only)

Internal target volume

The ITV consists of the CTV plus an internal margin.

The internal margin is designed to take into account

the variations in the size and position of the CTV

relative to the patient’s reference frame (usually

defined by the bony anatomy); that is, variations due

to organ motions such as breathing and bladder or

rectal contents.

Planning target volume

The planning target volume (PTV) is a

geometrical concept, and it is defined to select

appropriate beam arrangements, taking into

consideration the net effect of all possible

geometrical variations, in order to ensure that the

prescribed dose is actually absorbed in the CTV.

The PTV includes the internal target margin

(ICRU Report No. 62) and an additional margin

for set-up uncertainties, machine tolerances and

intratreatment variations. The PTV is linked to the

reference frame of the treatment machine and is

often described as the CTV plus a fixed or

variable margin (e.g. PTV = CTV + 1 cm).

The PTV depends on the precision of such tools

as immobilization devices and lasers, but does

not include a margin for the dosimetric

characteristics of the radiation beam (i.e.

penumbral areas and buildup region), as these

will require an additional margin during treatment

planning and shielding design.

Organ at risk

The organ at risk is an organ whose sensitivity to

radiation is such that the dose received from a

treatment plan may be significant compared with its

tolerance, possibly requiring a change in the beam

arrangement or a change in the dose.

Specific attention should be paid to organs that,

although not immediately adjacent to the CTV, have a

very low tolerance dose (e.g. the eye lens during

nasopharyngeal or brain tumor treatments).

Organs with a radiation tolerance that depends on the

fractionation scheme should be outlined completely to

prevent biasing during treatment plan evaluation.

Planning Organ at Risk Volume

As is the case with the PTV, uncertainties and

variations in the position of the OAR during

treatment must be considered to avoid serious

complications. For this reason, margins have to be

added to the OARs to compensate for these

uncertainties and variations, using similar principles

as for the PTV.

A margin around an OAR with a serial-like structure

(e.g., spinal cord) is more clinically relevant than that

around an OAR with a parallel-like structure (e.g.,

liver, lung, parotid).

Treated Volume

Because of the limitations of irradiation

techniques, the volume receiving the prescribed

absorbed dose might be different than the PTV; it

might be larger (sometimes much larger) or

smaller, and in general more simply shaped (less

so with IMRT than with conventional or three

dimensional radiation therapy).

The treatment planning process

Once the target volume, organs at risk, and

the required doses have been defined, the

treatment plan will be produced by a person

trained in 3-D planning.

The aim of the treatment planning process is

to achieve the dose objectives to the target

and critical structures and to produce a dose

distribution that is “optimal”.

The beam angles can be chosen using standard

templates such as a six field prostate plan or by

using a beam’s-eye-view display to maximize PTV

coverage and to minimize irradiation of critical

structures.

When a beam aperture is defined, an additional

margin of about 7 to 8 mm needs to be added

beyond the PTV in all directions in the transverse

plane to obtain the desired dose coverage to the

PTV.

In the superior inferior directions one needs to add

about 12 to 15 mm margin because of beam

divergence effects.

In its most basic implementation conformal

radiotherapy may consist of coplanar static beams in

a standard geometric configuration with MLCs or

conformal blocks used to achieve the required

conformal shape.

Dose-volume Histograms

Display of dose distribution in the form of isodosecurves or surfaces is useful because it shows not onlyregions of uniform dose, high dose, or low dose, butalso their anatomic location and extent.

In 3-D treatment planning, this information is essentialbut should be supplemented by DVHs for thesegmented structures, for example, targets, criticalstructures, etc.

A DVH not only provides quantitative information withregard to how much dose is absorbed in how muchvolume, but also summarizes the entire dosedistribution into a single curve for each anatomicstructure of interest. It is, therefore, a great tool forevaluating a given plan or comparing competing plans

The DVH may be represented in two forms:

the cumulative integral DVH and the

differential DVH.

The cumulative DVH is a plot of the volume

of a given structure receiving a certain dose

or higher as a function of dose .

Any point on the cumulative DVH curve

shows the volume that receives the

indicated dose or higher. The differential

DVH is a plot of volume receiving a dose

within a specified dose interval (or dose

bin) as a function of dose

0

25

50

75

100

Volu

me

(%)

0 25 50 75 100

Dose (Gy)

femurs

bladder

target

rectum

D=77GyV=90%

D=75GyV=30%

D=72Gy

DVH for a prostate plan

Dose Computation Algorithms

Dose calculation algorithms for computerized

treatment planning have been evolving since the

middle of the 1950s. In broad terms the algorithms

fall into three categories.

a) correction based,

(b) model based,

(c) direct Monte Carlo

Either one of the methods can be used for 3-D

treatment planning, although with a varying degree of

accuracy and speed. However, the model-based

algorithms and the direct Monte Carlo are becoming

more and more the algorithms of the future.

Correction-based Algorithms

These algorithms are semiempirical. They are based primarily on measured data (e.g., percent depth doses and cross-beam profiles, etc.) obtained in a cubic water phantom.

The corrections typically consist of

(a) attenuation corrections for contour irregularity;

(b) scatter corrections as a function of scattering volume, field size, shape, and radial distance;

(c) geometric corrections for source to point of calculation distance based on inverse square law;

(d) attenuation corrections for beam intensity modifiers such as wedge filters, compensators, blocks, etc.;

(e) attenuation corrections for tissue heterogeneities based on radiologic path length

Model-based Algorithms Convolution-superposition is currently the most

accurate model-based algorithm.

Direct Monte Carlo is the most accurate method for

treatment planning, but currently it is not feasible

because it requires prohibitively long computational

times. However, with the continuing advancement of

computer technology, it is possible that direct Monte

Carlo will be used routinely for treatment planning in

the not too distant future.

Thank

you