epid and cbct on radiation therapy

TREATMENT VERIFICATION SYSTEMS IN RADIATION THERAPY

ANJU.K.VMSC.RADIATION PHYSICSCALICUT UNIVERSITY

IMPORTANCE OF TREATMENT VERIFICATION

All radiotherapy involves risk because even a small error in treatment planning , delivery or dosimetry can lead to negative consequences.

This is because human body is a complex organism and tumors are often located close proximity to the normal tissue and critical organs.

So ensuring that the right radiation dose has been given to the right place.

THE PATIENT PRESENTS CLINICAL CHALLENGES EVERY DAY…

set-up difficultiessoft tissue deformation tumor growth/shrinkage irregular respirationspontaneous motion

TWO IMPORTANT TYPE OF TREATMENT VERIFICATION SYSTEMS ARE

ELECRONIC PORTAL IMAGINGCONE BEAM COMPUTED TOMOGRAPHY

EPID EPID = Electronic portal imaging device Daily imaging for treatment localization and verification may become

feasible Acquisition, analysis, storage and distribution of portal images can be

performed with relative ease Mount on the linear accelerator

EPID

Electronic portal imaging devices-EPID can used for daily imaging for treatment localization and verification.

The portal image can be analyzed with computer software and before the treatment, no significant additional dose is required for patient.

The image can be saved in DICOM file format and it can be reviewed at any time.

EPID The first EPID system was video based. The beam transmitted through the patient excite a metal fluorescent screen.

A front-silvered mirror , placed diagonally, reflects the fluorescent light by 90 degree into the video camera.

The analog output of the video camera is converted into a digital array with a ADC known as “ frame grabber ”.

The special resolution depends on phosphor thickness.

Another kind of EPID has a matrix ionization chamber system that consist of 256*256 liquid ionization chamber(scanning liquid ionization chamber-SLIC).

EPIDFluoroscopy based system (Video based)

Matrix ionization chamber based system

Amorphous silicon based system

AMORPHOUS SILICON ELECTRONIC PORTAL IMAGING DEVICE

Detector : Amorphous silicon flat panel imagerActive imaging area : 40 x 30 cm2 Resolution : 1024 x 768 pixels

Spatial resolution : 0.391 mmMax frame acquisition rate : 9.574 frames/secondEnergy range : 4 - 25 MVDose rate range : 50 - 600 MU/min

SPECIFICATIONS:

AMORPHOUS SILICON BASED SYSTEM

Within this unit a scintillator converts the radiation into visible light The light is detected by an array of photodiodes implanted on an amorphous silicon panel

The photodiodes integrate the light into charge captures

Resolution and contrast greater than other system

EPID

CONE BEAM COMPUTED TOMOGRAPHY-CBCT

CBCT CBCT is an imaging modality that provide high imaging quality .

It has ability to provide submillimeter resolution in terms of images.

WHAT IS CONE BEAM AND HOW DOES IT WORKS…

Its called 3D cone beam imaging . It is fast , simple and completely painless.

3D cone beam imaging system is basically a digital x ray scanner mounted on a rotating arm . Like a digital camera. Its called “CONE BEAM”.

Because the scanner projects x rays in a carefully controlled ,cone shaped beam.

CBCT

In CBCT planar projection images are obtained from multiple directions as the source with the opposing detector panel rotates around the patient through 180 degree or more .

These multidirectional images provide sufficient information to reconstruct patient anatomy in three dimensions , including cross- sectional , sagittal and coronal planes.

CBCT Conventional CT scanner has circular ring of detectors , rotating opposite to X-ray tube . However , it is possible to perform CT scans with detectors embedded in a flat panel instead of circular ring . CT scanning that uses this type of geometry is known as CBCT.

CBCTCBCT system can be implemented either by using kilovoltage x ray source or The megavoltage therapeutic source

KILOVOLTAGE CBCT - KVCBCT

KILOVOLTAGE x rays for a KVCBCT system are generated by a conventional x ray tube that is mounted on a retractable arm at 90 degree to the therapy beam direction .

A flat panel of x ray detectors is mounted opposite to the x ray tube.

ADVANTAGE OF KVCBCT SYSTEMS ARE ..Its an ability to produce volumetric CT images with good contrast and submillimeter spatial resolution.

Acquire images in therapy room coordinatesUse 2-D radiographic and fluoroscopic modes to verify portal accuracy , management of patient motion and making positional and dosimetric adjustments before and during treatment.

MEGAVOLTAGE CBCT - MVCBCT

Megavoltage cone beam (MVCBCT) uses the megavoltage x ray beam of the linear accelerator and its EPID opposite to the source .

EPIDs with a- Si flat panel detectors are sensitive enough to allow rapid acquisition of multiple , low dose images as the gantry is rotated through 180 degrees or more . From these multidirectional 2D images , volumetric CT images are constructed.

MVCBCTMVCBCT system has a good image quality for the bony anatomy .

MVCBCT is a great tool for on-line or pretreatment CT, avoidance of critical structures such as spinal cord, and identification of implanted metal markers if used for patient setup.

ADVANTAGE OF MVCBCT OVER KVCBCT

Less susceptibility to artifacts due to high objects such as metallic markers in the target , metallic hip implants , and dental fillings.

No need for extrapolating attenuation coefficients from KV to megavoltage photon energies for dosimetric corrections.

CONCLUSION

Portal dosimetry using EPID for IMRT plans can be performed in less time compared to other existing tools.

High density data points and direct to digital data capture makes it easier to evaluate complex IMRT fields.

CBCT has high ability to provide submillimeter resolution in terms of images.

In radiation therapy treatment verification is an important process . And at that time make sure that right amount of radiation will given to the right place.