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ARC THERAPY

ABHIJIT DAS2ND YEAR PG DEPT. OF RADIOTHERAPYAHRCCARC THERAPY

SHIFT OF MACHINES AND CONCEPTThe dawn of the 20th century, arc therapy Involving dynamic field shaping using a multileaf collimator was first described by Takahashi in 1965.1982,Brahme et al solved an integral equation for a hypothetical target wrapped around a critical structure and treated with arc therapy.In 1993, another form of IMRT using rotational fan beams, called Tomotherapy, was Proposed by Mackie et al.Intensity modulated arc therapy (IMAT) was introduced by Cedric X.Yu in 1995THE BASIC CONCEPT OF ARC THERAPY :

The delivery of radiation from a continuous rotation of the radiation source and allows the patient to be treated from a full 360 DEGREE beam angle.

Arc therapies have the ability to achieve highly conformal dose distributions and are essentially an alternative form of IMRT.

A major advantage over fixed gantry IMRT is the improvement in treatment delivery efficiency due to

The Reduction In Treatment Delivery Time .

The Reduction In MU Usage(the amount of radiation output per unit of time is referred as monitor unit).

Subsequent Reduction Of Integral Radiation Dose To The Rest Of The Body. The Availability Of Extra Time Within A Set To Employ IGRT.ARC-BASED THERAPIES:

Tomotherapy

IMAT

volumetric modulated arc therapy (VMAT)- single arc forms of IMAT

TOMOTHERAPYTomotherapy techniques can be subdivided

axial or serial tomotherapy (where the radiation is delivered slice by slice)

2. helical tomotherapy (HT) (where the radiation is delivered in a continuous spiral).

HT has been evaluated in a variety of tumor sites and it can generally achieve either similar or improved dose distributions compared with fixed field IMRT, with variable results on treatment time comparisons.

Machine Interior

The TomoHelical delivery mode provides IMRT and 3D CRT treatment delivery in a continuous (360) helical pattern, using thousands of narrow beamlets, which are individually optimized to target the tumor.The TomoDirect delivery mode is a discrete angle, non-rotational delivery mode. TomoDirect allows creation of treatment plans that include between 2 and 12 target-specific gantry angles. During treatment delivery, all beams for each target are delivered sequentially with the couch passing through the bore of the system at an appropriate speed for each gantry angle.HIART SYSTEMTomotherapy - a combination of a CT scanner and a linear accelerator that can deliver the radiation in a fan-shaped distribution, similar to CT imaging with a continuously rotating radiation source, while the patient is moved through the machine. PARTSDESCRIPTIONSLINAC6 MV S-band (3 GHz) linear acceleratorDIRECTION OF ROTATIONCLOCKWISE FROM FOOT END /speed varies according to plan.ENERGY FOR TREATMENT6 MV photon beamPOWRED BY MAGNETRONSAD85 CMMAXIMUM RADIATION FIELD LENGTH150 CM WITH COUCH AT HEIGHT OF ISOCENTER PLANETREATMENT VOLUME -Tomohelical 80 Cm (Transverse Diameter) X 135 Cm (Longitudinal) For Typical Patient Set-up.Tomodirect 40 Cm (Transverse Diameter) X 135 Cm (Longitudinal) For Typical Patient Set-up.PARTSDESCRIPTIONENERGY FOR IMAGING3.5 MV photon for imaging.DOSE FOR IMAGING0.5-3 CGY DETECTOR SYSTEM528 channels, single-row xenon ion chamber array used for image acquisitionIMAGE RESOLUTION512X512(0.78 PIXELS)SCAN TIMETYPICALLY 2 MINUTES PER 10 CM LENGTH AT 4 MM SLICE SPACING.( 2,4,6 mm slicing available)FIELD OF VIEW (FOV) 40 CM DIAMETERFIELD OF VIEW (FOV) 40 CM DIAMETERSOURCE TO DETECTOR DISTANCE145 CMIMAGING

The beam from the accelerator is collimated by a multileaf collimator consisting of 64 leaves each of which project a shadow of 6.25 mm at the patient generating a total fan beam width of 40 cm. (pneumatically driven)By using a separate collimation ("jaws") system above the multileaf collimators, the "slice thickness" can range between 0.5 to 5 cm. it is a specially designed machine for helical, fan beam delivery.multileaf collimation system is specifically designed to minimize leaf transmission and interleaf leakage - important considerations for narrow beam, multislice delivery procedures. Average MLC leakage - 0.25% (typical)

Axis of travel is in one direction.(IEC-y axis)

Radiation Characteristics

One of the key differences is the lack of flattening filter, which makes the dose more uniform at greater depths. As a result of this, the photon fluence profile is shaped differently when compared to a traditional radiotherapy system. the conical shape of the profile implies that there will be an increased average dose rate - thus reducing the imaging time & No scatter outside the field

Procedures descriptions3-D Imaging.standard diagnostic imaging equipment or CT-simulatorsDefinition of Target Volume and Organs at Riskcontour the target volume as well as the organs at risk. Thiscould be done at the CT-simulator or on a conventional 3-D treatment planning computer---> after the image data set has been transferred to the treatment planning system.Tomotherapy Co-registrationa rigid-body adjustment that will only provide translational, rotational, pitch and yaw calculations. The Registration process allows the user to define structures for co-registration including the Whole Image (Mutual information with no thresholding), Bone and Tissue Technique (pixel threshold > 0.3 g/cm3), or a Bone Technique (pixel threshold > 1.1 g/cm3) as the focus for registration. N.B- The simplest thresholding methods replace each pixel in an image with a black pixel if the image intensity is less than some fixed constant T (that is, ), or a white pixel if the image intensity is greater than that constantData Transfer to Planning Computer planning computer which will perform the delivery optimization calculations

PLANNING AND DOSE DELIVERY

the full gantry rotation is divided into 51 projections. Each projection is characterised by its own leaf opening pattern and covers an arc segment of approximately 7. The available rotation period may be between 15 and 60 s (typically around 20 s). As such, each projection takes between 0.2 and 1 s with all leaves shut for a short time between projections. The delivery assumes constant dose rate of the linac.Optimized Planning.The tomotherapy treatment planning system provides "inverse planning" capabilities.Three factors are predefined before starting the calculations:1.Pitch : 2.Modulation Factor: 3.Field width: The pitch factor is defined as couch movement per rotation in units of the FBT(fan beam thickness).The MF represents the ratio of maximum leaf opening time to the mean leaf opening time of all MLC leaves, which open in a projectionThe FBT is achieved a compromise between fast treatment times and dose modulation in the superior/inferior directionThe smaller the pitch factor, the longer the treatment time. a small pitch improves the capability of dose modulation and the ability to deliver high doses per fraction. A potential problem with large FBT and large pitch is the dose distribution away from the central axis. The beam divergence will cause variations in overlap between adjacent rotations, which increase with distance from the axis of rotation. -THREAD EFFECTMF is proportional to the overall treatment time, and with typical physical constraints for the tomotherapy delivery, can be selected between 1 and approximately 6. A small MF results in short treatment times and is adequate for relatively symmetrical targets close to the central axis of the patient.. A large FBT results in larger volumes covered in any projection and a higher central axis dose output while it reduces the scope for conformality and detailed dose modulation in cranio/caudal direction of the patient. Kissick et al showed a pitch factor of 0.86/integer number (e.g., 0.43, 0.287, 0.215, etc.) minimises the thread effect).Creation of Verification Data

consists of the expected beam intensity at the detector array for each gantry angle and couch position. This intensity pattern is referred to as a "sinogram" because each point irradiated in the patient maps a sine wave pattern at the CT detector as the gantry revolves. Sinograms can actually be obtained for various processes including a CT sinogram as described above, an MLC sinogram, a registration sinogram, a verification sinogram and a planned detector sinogram. each is implemented in a very specialized manner to address a specific task.

A sinogram is an array of pencil beam intensity values as a function of gantry angle (horizontal axis in this Figure). Each vertical row corresponds to one angular view. A point object that is straight and parallel to the z-axis will appear as one cycle of a sinusoidal curve when the gantry revolves by 360 degrees.

Procedures descriptionsPre-Treatment Megavoltage CTTomotherapy Delivery.Delivery Verification.While the patient is being treated, the detector array isactively measuring the radiation transmitted through the patient (for each pulse of the linac). This is used to determine actual radiation incident on the patient and can be used to verify dose delivery during or after treatment Dose Reconstruction. Using the incident radiation fluence delivered to the patient and the CT information that was obtained before the treatment, the dose actually deposited in the patient can be computed and compared to the planned dose. If necessary, corrections can be made to subsequent fractionsprosconsTotal Body Irradiation (TBI)greater control over the dose distribution and to spare organs that may be at risk.HT provided excellent conformal l