Intensity Modulated Radiation Therapy

Vipin Singh Pal Presenter :-

What is IMRT Principle and goal of IMRT Why IMRT Used Intensity-Modulated Radiadiation Therapy

Planning IMRT Proceses Intensity –Modulated Radiation Therapy

Delivery conclusion

Objectives

The terms Intensity modulated radiation therapy-(IMRT) refers to a radiation therapy technique in which a nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution.

IMRT is an advanced from 3DCRT .

What is IMRT

The principle of IMRT is to treat a patient from a number of different directions with beams of nonuniform fluence.

The IMRT technique is currently the most advanced from of conformal radiotherapy and holds great promise for improving radiotherapy both through increased tumour control probability and decreased treatment morbidity .i .e. decreased normal tissue complication probability (NTCP).

Principle and goal of IMRT

To improve target dose uniformity. To selectively avoided critical structure and

normal tissue. To create concave isodose surface of dose

area surrounded by high dose. Focal dose escalation to specific sub volume

in the target volume. Better sparing of critical structure specially

during reirradiation.

Why used IMRT

Based on “Inverse planning”1. Initially, a CT scan is performed on the

affected region.2. A radiation oncologist defines the PTV3. Enters the plan criteria: max dose, mini

dose, desired -limiting dose(for critical structures) and a dose-volume histogram

4. Then, an optimisation program is run to find the treatment plan which best matches all the input criteria.

Intensity-Modulated Radiadiation Therapy Planning

WORKFLOW OF IMRT TREATMENT

Intensity modulated radiation therapy are delivered by three technique.

1-IMRT with fixed gantry angle 2- IMRT with Tomotherapy based 3 - IMRT With rotating cone beams

Intensity –Modulated Radiation Therapy Delivery

For a given gantry angle ,a computer controlled mulltileaf collimator is not only use full in shaping beam apertures for conventional radiotherapy.

But ,it can also be programmed to deliver . IMRT.

This has been done in two different ways. A-segmental MLC Delivery B- Dynamic MLC Delivery

1-IMRT with fixed gantry angles

In segmental MLC (SMLC) delivery the patients is treated by multiple field and each field is subdivided into a set of subfield with uniform beam intensity level.

The subfield are created by the MLC and delivered in a stack arrangement one at a time in sequence without operator intervention.

The accelerator is turned off while the MLC moves to create next subfield.

This method of IMRT delivery is also called “step and shoot OR stop and shoot

A-segmental MLC Delivery

Step & shoot IMRT technique available in seimens linear accelerator like .primus, Artiste etc.

Cont.

Advantage –- Easy to plan ,delivery and to verify- An interputted treatment is easy to resume - Fewer MUs in comparison to DMLC- Radiation leckage less as compare to DMLC Disadvantage- - Slow dose delivery- Treatment time is more (2-3 min/field)

Advantage and disadvantage of “step and shoot” IMRT

Unlike SMLC. In the dynamic OR sliding window

mode ,the leaves of MLC are moving during irradiation.

Each pair of composing leaf sweeps across target volumes under computer control.

B- Dynamic MLC Delivery(DMLC)

Advantage –- Better dose homogeneity for target volume- Shorter treatment time Disadvantage -- Beams remains on through out –leakage

radiation increased- Total MU required is more than that for

SMLC

Advantage and disadvantage of “DMLC” IMRT

Tomotherapy is an IMRT technique in which the patients is treated by slice by slice by intensity modulated beams in a manner analogous to computer tomography (CT) imaging.

Tomotherapy is delivered in two ways-slice based tomotherapy-Helical tomotherapy

2- IMRT with tomotherapy based

The radiation is collimated to a narrow fan beam defining a transaxial slice of the patient.

The multileaf intensity modulating collimator (MIMiC) is retrofit to any accelerator

It consists of 2x20 finger attenuators that can be driven into & out of the field by electro pneumatic action for variable dwell time to create spatial modulation.

Slice based tomotherapy

This is a method of IMRT delivery in which the linac head and gantry rotate with the patients is translated through the doughnut – shaped aperature in a manner analogous to helical ct scanner.

The tomotherapy unit also provides megavoltage CT- scanning for image guided radiation therapy.

Helical based tomotherapy

In this technique combines the dynamic motion of the MLC with Arc rotation of the accelerator gantry.

IMRT delivered with rotating gantry by two ways.

-intensity –modulated Arc Therapy (IMAT)

- Volumetric –modulated Arc therapy(VMAT)

3 - IMRT with rotating gantry beams

IMAT Intensity-modulated arc therapy (IMAT) technique uses

MLC dynamically to shape fields as rotate gantry rotate in an arc.

IMAT, which uses five to seven overlapping concentric arcs to deliver a conformal dose distribution

Beam is ON all the time.

VMAT is delivery of a rotational cone beam with variable shape and intensity.

In VMAT treatment gantry rotates continuously with the MLC leaves and dose rate varying throughout the arc.

Volumetric-modulated arc therapy(VMAT)

IMRT: Treatment Delivery on Linear accelerator

Check beam data transfer Patients positioning &immobilization

Match isocenter with help of laser

Take a portable image 2D &3D

Match with DRRs in EPID Software

Beam delivery

Dose distribution more homogenous within PTV.

Reduction of normal tissue dose. Radiobiological advantage. Selectively avoided critical structure and

tissue.

Conclusion