Radiotherapy intervention has a important and critical axial rotation in the intervention of cancerous diseases. Different types of techniques were used for this intent. In past, conventional radiation therapy technique was used. But it did non efficaciously used for intervention of tumours located in sensitive countries of homo organic structure which have limited tolerance for radiation doses [ 1 ] .Modern twenty-four hours radiation therapy consists of 3D conformal radiation therapy and strength modulated radiation therapy ( IMRT ) . After the development of oculus position show of beam, 3D conformal radiation therapy was used for clinical intent in 1978. [ 2 ] ( Reinstein et Al. ( 28 ) and McShan et Al )
In 3D conformal radiation therapy, present the high doses to aim volume while determining the beam to minimise the dosage to the environing normal tissues. To avoid the inauspicious effects of radiations during intervention, protect the normal tissues by determining the intervention Fieldss with lead blocks [ 12 ] . In this procedure, 3D image computing machine imaging and magnetic resonance imagination ( CT, MRI ) are used to turn up the tumours and other septic variety meats accurately [ 13 ] . In this technique we could non used more than four intervention field programs before the development of MLCs in CLINAC [ 14 ] . After the development of MLCs, it is possible to protect the normal tissues by determining the intervention Fieldss to present the high doses to the mark volume accurately.
In some instances like prostate, caput and cervix, it is impossible to salvage normal tissues during the bringing of high energy dosage to the tumour even by utilizing the 3D conformal technique [ 15 ] . For this intent a new technique Intensity modulated radiation therapy was introduced. In this technique, a patient is treated from a figure of different waies by utilizing a non unvarying intense beam. In IMRT technique, it is possible to present the high doses to aim volume while determining the beam to minimise the dosage to the environing normal tissues. It is achieved by utilizing compensators, cuneuss or MLCs. [ 16 ] IMRT techniques are dearly-won and significantly complex than 3D CRT.
In radiation therapy intervention, radiations are used for malignant neoplastic disease intervention by avoiding the healthy tissues. These radiations destroy malignant neoplastic disease infected cells wholly. Generally radiation beams of X raies, gamma beams, and negatrons are used during intervention of malignant neoplastic disease diseases. In radiation therapy, assorted types of machines are used for the intervention of malignant neoplastic disease tumours by utilizing high energy photons and negatron beams [ 1 ] .
In early 1900 ‘s, ace electromotive force and low energy X raies machines were used for malignant neoplastic disease intervention. But these machines could non handle the deep sitting tumours due to their hapless deepness dose distribution [ 2 ] . In 1950s, with the development of first practical mega-voltage machine Co-60, it was possible to handle deep seated tumours with photons. As the beginning of radiation in Co-60 unit has no point beginning, so its dimensions produce the geometric and transmittal penumbra. [ 3 ] Due to this penumbra, the dose distribution is non unvarying at the field border. Therefore Co-60 unit does non supply the ideal deepness dose distribution at the field edges.
After the development of magnetron or klystrons, microwave tubings, charge atoms were accelerated by high frequence electromagnetic moving ridges through these tubings. For this intent foremost additive gas pedal were developed in 1950s. [ 4 ] In 1953 first medical linear gas pedal was used in Hammersmith Hospital in London [ 5 ] . At the beginning, accurate bringing of radiations to septic cells of a patient was major job. To decide this job, British Institute of Radiology ( BIR ) compiled a cardinal axis dose distribution informations, published in British diary of Radiology ( BJR ) supplement 5 in 1953. [ 1 ]
Clinical additive gas pedal ( CLINAC ) is a megavoltage machine with less geometric and transmittal penumbra, used for malignant neoplastic disease intervention [ 6 ] . Clinical additive gas pedal has quickly improved its design and engineering with the transition of clip. A modern twenty-four hours computing machine controlled clinical additive gas pedals with double energy and multileaf collimators ( MLC ) technique provides fast, more accurate and precise dose bringing to the malignant neoplastic disease infected cells by avoiding the normal environing tissues [ 7 ] . It provides both negatrons and X-ray photons of assorted mega electromotive force energy scopes. In modern twenty-four hours additive gas pedal, high energy negatrons are produced which are used for superficial tumours. These negatrons are besides used to bring forth high energy x beams which are used for deep sitting tumours [ 8 ] .
The Role of Medical Physicists
The medical physicists must carry through following duties before choice of CLINAC
To develop demands, specifications and recommendations for the choice of the CLINAC
To program and supervise the building installations including screening design to house the CLINAC.
To supervise all the installing processs
To execute credence testing of the machine
To execute commissioning of the machine for clinical intent
To come in all get informations during commissioning into intervention planning system
To set up new processs for the effectual and safety usage of gas pedal
To set up processs for quality confidence and quality control on the day-to-day footing
After the installing of radiation therapy machine, for credence trial a series of undertakings and measurings must execute to verify the specifications and safety criterion. In credence trial, physicists must verify three chief processs
In safety cheques, the medical physicists must verify that all interlocks and warning visible radiations are proper operation. In mechanical cheques, it must guarantee that all the parts of intervention unit including intervention tabular array are accurately working. In dosimetry measurings, specifications and uniformity of photon and negatron beams verified.
After complete and satisfactory credence trial, it is non possible to utilize clinical additive gas pedal ( CLINAC ) straight for intervention intent. All informations acquired during the credence proving are non sufficient to committee a radiation therapy machine in the intervention planning system.
Commission is the procedure of fixing the processs, protocols, instructions and dosimetry informations for clinical prospectus. AAMP codification of pattern for gas pedals ( TG 45 ) provides elaborate information about all these commissioning processs and besides discuses the demands for specific beam informations. All measurings of commissioning should be made by to the full trained and qualified medical physicist.
During commissioning, following undertakings must be achieved.
Acquire all informations for external beam radiation therapy for intervention intent.
Form all informations into a dosimetry informations book
Enter all information into the intervention planning system ( TPS )
Develop all intervention planning processs
Verifying the truth of all processs
Establish quality control processs
Training all individuals
Machine-specific beam informations for commissioning is extremely dependent on the dose calculation- algorithms used in the intervention planning system. The model-base dose calculation-algorithms ( convolution/superposition ) require much less measured informations than correction-based algorithms ( tantamount TAR, etc. ) . Irrespective of the dose calculation-algorithm, it is necessary to hold a minimal dataset for several energy scopes of photon and negatron beams that includes
Percentage deepness dosage ( unfastened, cuneus ) for several field sizes
Off axis unfastened and wedge beam profiles ( cross plane, In plane ) for all field sizes
End product factors
Wedge factors ( difficult and practical )
Calibrated dose rate
Effective beginning to come up distance ( merely for negatron )
It is ensured that all beam and dosimetry informations acquired during commissioning of CLINAC must compare with all the specifications and published informations of the same available theoretical account.
It is strongly recommended that all the processs of commissioning must be carefully executed and planned because all the get beam informations is used for patient ‘s intervention. Therefore, for completion of all undertakings and measurings in commissioning, an appropriate clip tabular array must be scheduled. All the undertakings achieved during commissioning procedure of CLINAC can be estimated to necessitate 1 to 2 hebdomad per energy.
Percentage Depth Dose
Percentage deepness dosage is defined as the ratio of absorbed dosage at any depth D on the cardinal axis of beam to absorbed dosage at Dmax multiplied by 100. ( F. Khan )
Percentage deepness dosage
It depends upon a figure of parametric quantities beam energy, beam collimation, deepness D, field size degree Fahrenheit, and beginning to come up distance ( SSD ) .
The uniformity of off axis deepness dosage normally called beam profile is measured by scanning the H2O apparition for assorted deepnesss both beam axes. Following parametric quantities are measured to look into the uniformity of the beam profiles.
Two-dimensionality can be specified as a maximal allowable per centum fluctuation from the norm dose across the cardinal 80 % of the full breadth at half maximal ( FWHM ) of the profile in a plane transverse to the beam axis. That is, the two-dimensionality F is given by
Two-dimensionality must be less than 3 % for mention field size 10 ten 10 centimeter measurings in H2O apparition at deepness of 10 centimeter and beginning to surface ( SSD ) .
Symmetry is frequently defined as a maximal allowable per centum divergence of the “ left-side ” dosage from the “ right-side ” dosage of a beam profile frequently at 80 % of the FWHM points.
Symmetry must be less than 2 % for mention field size 10 ten 10 centimeter measurings at deepness of Dmax and 10 centimeter in H2O apparition at SSD 100 centimeter.
The penumbra, by and large defined as the sidelong distance between the 80 % and 20 % of upper limit dose points on one side of a beam profile, must be within specification. It depends upon deepness, SSD, beginning size, beginning to collimator distance and beam energy. ( Ervin page196 ) Since commissioning beam informations are treated as a mention and finally used by intervention be aftering systems, it is vitally of import that the collected informations are of the highest quality to avoid dosi metric and patient intervention mistakes that may later take to a hapless radiation result.
End product is defined as the ratio of the end product in air for a given field to that for a mention field. It is the merchandise of collimator spread factor and apparition spread factor. It depends upon field size. It increases with field size. It is measured for each field size at a fixed deepness Dmax.
The cuneus factor is defined as the ratio of dosage at a specified deepness normally Dmax on the cardinal axis with the cuneus in the beam to the dosage under the same status without the cuneus. Wedge factor alterations at deepness as a map of cuneus angle fading coefficient and field size. It does non depend on deepness. Wedge factor for practical cuneus is one because the figure of monitor units entered at control console, are delivered at the cardinal axis.
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