Gaganpreet Singh

Dr. Oinam Arun Singh

DEPARTMENT OF RADIOTHERAPY

RCC , PGIMER , CHANDIGARH

STEREOTACTIC RADIOSURGERY, RADIOTHERAPY AND QUALTIY

ASSURANCE PROCEDURES

INTRODUCTION

Stereotaxis : It is a Greek word. Stereo (3-D approach) + Taxis (Touch) i.e. ordered in 3-dimensions

Concept of Stereotaxis:Use of well-defined 3-dimensional co-ordinate system to locate accurately any desired region in the target in space.It involves delivery of high dose of radiation to stereotactically defined targets (benign / malignant tumors and tissue abnormalities) within the body either in single fraction (SRS) or in multiple fractions (SRT) in such a way that the dose fall-off outside the target volume is very sharp.

TYPES OF STEREOTACTIC IRRADIATION

With regard to sessions of dose delivery stereotactic irradiation is divided into two categories Stereo tactic radio surgery (SRS) Stereo tactic radio therapy (SRT)STEREOTACTIC RADIO SURGERY (SRS)

High dose is delivered in a single session.To treat stereo tactically localized intracranial lesions

using rigid frame attachment while avoiding nearby normal tissue and critical structures.

STEREOTACTIC RADIO THERAPY (SRT)Delivering dose in multiple fractions to sterotactically

localized target.Using relocatable frames.

PATIENT SELECTION CRITERIA

Suitably sized, radiologically distinct lesion ≤ 3cm (SRS) or for Larger lesions (SRT).

Surgical hazards or inaccessibility. Conventional EBRT entails increased morbidity and decreased

effectiveness. Can be eradicated by single or multiple doses of radiation.

LIMITATIONS OF SRS

Lesions involved with or intrinsic to critical structures such as optic apparatus or brain stem cannot be treated well.

Lesions > 3cm cannot be treated well.

SRT INDICATIONS

Particularly important when the tumor is adjacent to or involving vital neurological structures such as the optic nerve or optic chiasm.

Can be used in situations where single dose radiosurgery cannot be applied safely.

Larger lesions (> 3cm) that are not appropriate for radio surgery i.e. SRS.

PRINCIPLE OF SRS AND SRT

Precisely locate the target stereotactically using special frames which Hold the target during treatment Accurately aim the radiation beam Shape the radiation beam to the target Deliver a radiation dose that damages abnormal cells yet sparing

normal cells

GOALS OF SRS AND SRT

Rapid dose fall off outside the target volume. Conformity of prescribed dose to the target volume. Repositioning accuracy.

TYPES OF RADIATIONS USED

GAMMA RADIATION from Co-60 sources. HIGH ENERGY X RAYS from LINAC. PROTON Particle beam accelerated from cyclotron.

MACHINES USED

Gamma Knife. LINAC-based systems (X-Knife). Cyber Knife.

GAMMA KNIFE

GAMMA KNIFE

Developed by Leksell and Larson in 1967 at Stockholm for treatment of benign tumors of intracranial lesions.

Models of GAMMA KNIFE

There are five versions of the Leksell Gamma Knife (Models U or A, B, C, Perfexion model).

Each system consists of six components: - the radiation unit, - the beam-focusing technology, - the patient couch,- an electric bed system,- the control console, and- the planning computer system.

SPECIFICATIONS OF GAMMA KNIFE

To meet the requirements of radiosurgery It should be possible to align the target point with unit

center point within 0.5mm.The axes of all beams should intersect at the unit center

point within 0.3mm.The dose rate any where within the target should not be

lower than 0.5Gy/min.Stray radiation from unit should not be hazardous to patient

or staff.

MODEL A OR U

179 Co-60 sources are used Arranged in hemi-spherical array and sources are at the pole of

hemisphere. for radiation protection point of view, it has challenging

loading and reloading issue.

MODEL B

It is developed by ELEKTA Sources are in annular section of hemisphere Due to development of imaging system and computers,

make use of 3D and complex treatment planning by using modern CT.

MODEL C (1999)

201 Co-60 sources arranged spherically Total activity of GAMMA KNIFE is 6000 Ci. Most important feature is APS(Automatic Positioning

System). This system carries target to focus automatically according to treatment plan with very high precision

Reduces treatment time and more safe treatment. The beam is focused to unit center point through conical

beam channels. Subsequent collimation is achieved by interchangeable

‘helmet’ with 201 collimators to which patient and frame are attached.

Helmets have removable tungsten collimators. It is docked with primary collimator at time of treatment.

The helmet defines approximately spherical dose distribution at isocenter with nominal diameter of 4,8,14,18mm.

Upto 100 holes in helmet can be plugged with 6cm thick tungsten alloy blocks to optimize dose distribution or to save critical structures.

The point of interest in the patient is positioned at isocenter.

MODEL 4-C (2005) Having advance programs to improve the workflow Increase accuracy Provide integrating imaging capability

PERFEXION MODEL(INSTALLED IN PGIMER) Latest model with 192 Co-60 sources arranged in 8 sectors

and encapsulated in double stainless steel capsules with welded closures.

24 sources in each sector. Motor control the source position mechanism. Sector off position is position between collimators of 1 and

8 mm size where source is shielded. In built helmets. Co-60 sources house in cast iron for radiation protection.

STEPS IN SRS/SRT

Frame Placement Image Acquisition Image fusion Delineation of Target and Critical Structures Treatment Planning Plan Evaluation Quality Assurance Treatment Execution.

FRAME FIXATION 1st step of stereotactic procedure. Remains rigidly attached through out the procedure. Unique feature of frame is that a rigid relationship between

patient’s intracranial anatomy and frame’s coordinate systems is established.

DEPTH HELMET The Depth Helmet provides quantitative quality assurance for

frame. Checks that frame has not moved between scanning &

treatment delivery. Confirms that frame have been accurately repositioned on

patient prior to treatment. Depth Probe with millimeter scale passed into the guide tubes

on the helmet to measure distance from the hole to the patient's head with millimeter accuracy.

This measurement data provides a permanent and precise record of positional accuracy for every treatment.

DEPTH HELMET

IMAGE ACQUISITION Imaging modalities selected for optimum target localization and

determined by nature of target and surrounding structures. More than one modalities required to link the target volume to

stereotactic frame coordinate system. Each modality use special designed localization box which fits

over head and attached to frame having fiducials to link the image to coordinate system. e.g. CT,MRI,ANGIOGRAPHY

IMAGE FUSION Image fusion automatically correlates thousands of points from two

image set ,provided true volumetric fusion of anatomical data sets. e.g. CT and MRI fusion improve accuracy

Delineation of target volume and organ at risk is done

TREATMENT PLANNING OF GAMMA KNIFE Acquired images transferred to TPS Coordinates of target are recognized by computer with help of

fiducials. Target volume defined by drawing margins to lesion. Organ At Risk outlined. Dose level that would ideally cover entire target volume selected. Maximum dose to tumor is set. Target volume is covered with shots. Each application of radiation

is referred to as shot.

CONTINUED

No. of shots required to deliver dose to entire Target Volume is determined by size, geometry and location of Target Volume.

Usually, More than one shot / isocentre is required to treat a tumor. Because of Smaller Field Size.Fixed beam geometry.

Optimization parameters are:No. of shots.Shot sizes.Shot location.Shot weight.Collimator helmetPlugging pattern.

The intensity of each shot and its weight depends on how long the involved collimators remain open

Shot positions defined relative to stereotactic frame by x, y and z coordinates and used to position the patient.

Stereotactic frame is locked into shot coordinate position so that target is precisely at the center of collimator helmet

Once the treatment plan is completed, plan is exported via network to control console computer.

CONTINUED

DOSE PRESCRIPTION In Gamma-knife, Entire Target Volume encompassed by

50% isodose curve. Correction for sloping or curved surfaces neglected due to

small field size. Inhomogeneity not taken into account as brain tissue has

water equivalent density, influence of skull on dose distribution < 3%.

PLAN EVALUATION

Techniques of plan evaluation Display of isodose lines over

target volume. Display of 3-D dose shells. Display of DVHs.

EXECUTION (GAMMA KNIFE)

Patient lies down on sliding couch of gamma unit Head frame is attached to frame adapter. Couch moves either electrical mechanism or hydraulic

system and helmet docks in to beam channels. After treatment, frame removed and patient discharged the

same day

GAMMA KNIFEAdvantages– Large clinical experience.– Appropriate for any shaped target.– High degree of targeting accuracy.– Better normal tissue sparing. – Simplicity for planning & treatment.– Fixed beam geometry, requirement for QA is less.

Disadvantages– Dedicated unit for intracranial radio surgical

treatments.– High cost of installation and Cobalt sources needs

periodically replacement.– Invasive frame fixation, hence procedure completed in

a single session.– Peripherally located tumors difficult to treat.

LINACS FOR RADIOSURGERY

For Linac based SRS/SRT : tertiary collimation is used so that collimator diaphragm closer to the surfaceTo reduce geometric penumbra (inversely proportional to

SDD) Tertiary collimation is either :

Cone basedMicro MLC based

CONE BASED

Highly collimated narrow beams defined by cone.

15cm long circular cones made of Cerro-bend Lead, encased in stainless steel.

The cones are mounted below X-ray jaws.

A range of cones with diameter from 5mm to 30mm in steps of 2.5mm are used for SRS lesions

A few cones of larger diameter Are also available for treating larger lesions with SRT

MICRO MLC BASED Used for small irregular field SRS/SRT, for greater

conformation accuracy and efficiency than circular collimators Simple mounting and dismounting. Computer controlled motorized leaves. Number of Leaves : 52 (26 pair). Field Size (max):10 cm x 10 cm. Leaf Width at Isocentre: 3 mm at isocentre. Leaf Height: 7 cm Leakage (through Leaf): < 1% Leakage (Interleaf): Avg.: 1.2%, Max.: 2.0% Leaf Speed:2.5 cm/sec.

Micro MLC

TOOLS FOR SRS/SRT X- KNIFE

CT localizer. MR localizer. Angiolocalizer. Fixation Frame.

BRW frame (SRS).GTC frame (SRT).

Depth Helmet.

LCMA (Linear couch mounted adaptor) for supporting localizing frame.

RLPP (Rectilinear LINAC Phantom Pointer)

LTLF (LASER Target Localizer Frame): for precise positioning of target at beam isocentre

CT IMAGING

CT localizer frame has nine

fiducial rods that appear as dots

in trans axial slice.

Double contrast axial images

with slice thickness 1.5-3mm for

clear delineation of tumor.

Gives detailed information of

bony structures.

Better delineation of target

volume and surrounding

structures in relation to external

contour. 33

MRI IMAGING Requires MRI compatible localization box. Can be used to scan directly in axial, saggital,

coronal or oblique planes Better imaging of soft tissue tumors. Having fiducial rods whose location are known

with respect to BRW frames.

34

ANGIOGRAPHY

Important for Arteriovenous malformations Angiographic localizer is attached to BRW

head ring. Consists of four plates embedded with four lead markers which

act as fiducial markers for angio-graphic images.

STEREOTACTIC FRAME Light weight aluminum box shaped head frame defines fixed

coordinate system for accurate localization & irradiation of PTV. The head frame serves following purposes: 

It prevents movement of patient’s head during the treatment It enables accurate reproducibility of the treatment position

for each planning and treatment session.

BRW(BROWN ROBERT WELLS) Head ring used for SRS Frame fixation is done Fixed for whole procedure i.e.

localization & treatment delivery. Procedure is completed in a single

session

GTC (GILL THOMAS COSMAN)

Relocatable Individualized by bite block

system, head rest bracket & Velcro straps attached to GTC frame

Used for fractionated SRT

X-KNIFEAdvantages Permits fractionation Flexibility in selecting field size. Max available Field Size

using cones is 4cm. Potential for treatment of extra-cranial lesion.

Disadvantages More QA requirements Allowed isocentricity error is +/- 1mm or less

QUALITY ASSURANCE QA involves both clinical & physical aspects PHYSICAL ASPECT. Treatment QA and Routine QA. Treatment QA: Basic QA for all equipment used for target localization ,3D

treatment planning and radio surgical dose delivery including phantom base, Localizer system, Couch Mount.

Treatment QA protocol dealing with calibration and preparation of equipment just before radio surgical treatment

Treatment QA during surgical procedure on patient Routine QA: To check hardware and software performance

SRS equipment on a scheduled frequency basis

ACCEPTENCE TEST AAPM - 54

Accurate localization. Mechanical precision. Accurate and optimal dose distribution. Patient safety.

ACCURATE LOCALISATION

This technique shall be able to determine the coordinates of a well defined object ( pointer or ball bearing a phantom) in frame coordinate system.

1mm for angiography. 2mm for CT and MRI.

MECHANICAL PRECISION

Alignment of patient frame coordinate system with LINAC coordinate system..

COUCH AND GANTRY STABILITY TEST

Before doing any SRS treatment, overall stability of isocentre under rotation of all axes - couch and collimator , Gantry to be established

Limit - 1mm radius sphere ( all angles)

LASERS

Three lasers ( 2 in opposite and 1 on ceiling) must coincide at isocentre.

Must coincide with in 1mm of axes locus.

PATIENT DOCKING SYSTEM Allow fine adjustment system to align the patient at

desired isocentre. Limit of 1mm

TARGET VERIFICATION DEVICES These devices ensure that patient is treated at target

correct coordinates, that target coordinates is aligned with isocentre these devices are calibrated with respect to frame based coordinate system.

FRAME SYSTEM Performance of component relating to frame

coordinate system must be verified as to compliance with manufacture’s specification. e.g. BRW and phantom axis should be accurate within 0.6 mm for each axis.

By these geometric error margin is calculated.

SRS SYSTEM VERIFICATION TEST

Before patient treatment, testing the entire system for geometric accuracy

Lead balls(hidden targets) placed in head phantom

These hidden targets are treated with number of fixed beams representing entrance points spread over upper hemisphere of skull.

A port film exposure is made for each beam and displacement of image of lead ball from center of field is measured for each beam .

DOSE DELIVERY Accuracy of absorbed dose be uncertain by less than 5%

PATIENT SAFETY AND INTERLOCKS

In LINAC limit gantry rotation, to prevent injury to patient by means of software and hardware.

Interlock system for disable power and immobilize the couch during the treatment.

ADDITIONAL TEST FOR GAMMA KNIFE

Radiation survey. Radiation leakage test. Timer constancy and

linearity test. Timer accuracy test. Timer on – off test error. Safety and QA test.

Door interlock. Emergency off switches. Beam on – off light. Audio – visual system. Couch movement. Micro switches to verify

helmet alignment with Co-60 source to 0.1mm.

HYDRAULIC SYSTEM

At the time of power failure to withdraw the couch/ patient from the gamma knife.

PROFILE MEASUREMENTS Beam profile is taken by film dosimetry. Beam profile is taken by

plugging 200 collimators out of 201. Background radiation measured with all 201 collimators. Relative helmet factor – films , diodes and TLDs are used. Ion

chamber is used in small phantom to calibrate the helmet.

TEST FOR GAMMA KNIFE

Physics Timer constancy – monthly Timer linearity and accuracy – monthly Radiation output – monthly (limit +-2%) Relative helmet factor – annually (limit +-3%)

Dosimetry Computer output vs measured – monthly (+_ 3%) Isocenter coincidence -- annually (+_4mm) Dose profile – after source change or acceptance test

SAFETY Timer termination of exposure – daily Door interlock – daily Emergency buttons – daily Beam status light – monthly Audio visual communication – daily Couch movement time deviation – monthly (<0.5 min from

initial couch movement)

HEAVY CHARGED PARTICLE BEAM Well defined range so beam stops at distal edge of target resulting

in little or no exit dose. (travel relatively straight path). Bragg peak can be adjusted to conform the location and size of

target. Very sharp lateral edges that can be shaped to conform with

target, less dose to normal tissues. Fixed beam and one isocentre are adequate in generating a plan

of high dose conformity and homogeneity. Decreased the integral dose to normal tissues. MLCs for beam shaping. Used large and irregular shaped lesion and lesions in the vicinity

of critical structure. Pediatric cases which require maximum normal tissue sparing. Energy require for SRS 200 MeV.

DISADVANTAGES

High cost of installation. Charge particle not widely available. Proton produced by ionizing hydrogen gas are accelerated to

therapeutic energy using particle accelerator i.e synchrotron and synchrocyclotron which is very complex process.

ADVANTAGE

Very precise dose delivery at the target volume. Sharp dose fall off due to bragg’s peaks. More normal tissue sparing due to the finite range of particles

in tissue.

CYBER KNIFE

EXTRACRANIAL RADIO SURGERY

SRS also applied to treat small localized tumors outside the cranium. These techniques are frameless (don’t need stereotactic frame to immobilize the body )

Robotic adjustment are made through gantry and table top motion to compensate for target displacement.

Extra cranial radio surgery and stereotactic body irradiation therapy applied to tumors in spine, lung, liver.

Treatment deliver in single or multiple fraction according to minimize risk of complication and optimize the off effectiveness.

COMPONENTS

High energy 6MV light weight linac due to X band. X-band means – microwave of frequency range from 8-12GHz Utility of X band is using a higher microwave frequency for

accelerating electrons reduces size and weight of accelerator(120Kg)

Robotic arm pointing linac beam from wide variety of angles. X ray cameras used to track patient position. 12 interchangeable collimator , at SSD of 80cm , collimator

provide a beam diameter of 5 to 60mm SSD can be varied from 65 to 100 cm

WORKING

Cyber knife radio surgery system continuously checks and compensate patient position and movement through an image guidance system during treatment

Image Guidance System consist of diagnostic X ray tube and cameras . The cameras use detectors (Si) linked to image registration computer.

Data is then transferred to computer control ,robot update robot gantry position to assure that beam from linac points at intended location within patient with millimeter accuracy.

It responds in real time to any patient movement during treatment. Patient lie on couch while robotic arm move around patient to deliver the dose.

Operator controls the progress treatment from out side the room via video monitors

SYNCHRONY RESPIRATORY TRACKING SYSTEM

Tumor is moving continuously, to continuously image its location using X-rays, a large amount of radiation delivered to the patient skin .

Uses a combination of surgically placed internal fiducial (electromagnetic transponder oscillating circuit 1.8* 8.6mm ) and light emitting optical markers (external markers) usually placed on patient abdomen .

When excited by electromagnetic field they emit a unique resonant frequency signal that can be detected by magnetic array placed on patient surface.

In this system, take images of internal fiducials and predict their location using motion of marker .

The light form markers can be tracked continuously using a CCD camera, and placed so that motion is corrected with motion of tumor.

A computer algorithms creates a correlation model that represent how internal markers moving compared to external markers.

CONTINUED

ADVANTAGES

Allow frameless radiosurgery. Monitor and track patient continuously and uses online images

for exact position of target. Dose delivery accuracy of 1mm through image guided dose

delivery method.

DISADVANTAGES

No posterior treatment possible( below couch). Prolonged treatment time. Significant QualityAssurance required to treatment to ensure

that robotic arm performs as expected.

CONCLUSION

• Recent advance modalities like SRS and SRT provides a perspective approach for the treatment of intracranial lesions with more accuracy and conformity.

• In near future perhaps Heavy Charged particles therapy will be the best treatment modality for the different types of intracranial tumors and other types of tumors.

REFERENCES

• Faiz M. Khan, The Physics Of Radiation Therapy.• AAPM Report No.54, Stereotactic Radiosurgery

THANK YOU