13 QUALITY ASSURANCE OF A LINEAR ACCELERATOR 13.1 COLLIMATOR ISOCENTER, JAWS, LIGHT FIELD VS INDICATORS, COLLIMATOR ANGLE INDICATORS.

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1 13 QUALITY ASSURANCE OF A LINEAR ACCELERATOR 13.1 COLLIMATOR ISOCENTER, JAWS, LIGHT FIELD VS INDICATORS, COLLIMATOR ANGLE INDICATORS TRAINING GOAL Among the responsabilities of a medical physicist, the implementation of as quality assurance program (a, b, c) is one of the most important. Many checks are required to assure the safety of the patients and of the personnel in radiation therapy. The purpose of this task is to give knowledge on how to perform the main checks on a teletherapy unit. This submodule deals with visual checks made to verify that the collimators are in good order Minimum time to be spent on the task: 1 day Competencies Addressed Ability to check the position of the collimator isocenter, the jaws symmetry, the field size and collimator angles indicated by the readout system Equipment, Materials Needed for the Task Mechanical front pointer (optional); sheets of paper; ruler Depending on local circumstances hands-on training may not be possible Important Note The use of equipment must be supervised by a qualified medical physicist Procedure Collimator Isocenter Place a sheet of paper horizontally on the table at the standard SSD. Project the image of a rectangular field on the paper and draw its diagonals, then marks their intersection. Rotate the collimator to 90, 180, 270 and repeat the procedure. Measure the maximum distance between the centers of the fields.

2 Jaws With the same setup used in procedure set the gantry to 0 and mark the corners of a rectangular light field. Rotate the collimators through 180 and check the correspondence of the new light field with the previous one Light Field vs Indicators With the same set-up used in mark the corners of a rectangular field and the image of the cross hairs on the paper. Measure the sizes of the light field and compare them with the readout values. Draw the diagonals of the light field and compare their intersection with the image of the cross hairs Collimator Angle Indicators With the same setup used in procedure rotate the collimator through different angles and check the correspondence between the effective angles and the readout values Measurements Integral part of the procedure Calculations No calculation is required Observations, Interpretations and Conclusions Observe that in procedures and the check is made on the light field. In order to check the position of the mechanical collimator isocenter a mechanical front pointer is needed (see optional) Compare the results to tolerance levels defined by the locally adopted quality assurance protocol Optional (Relative to procedure ) For a check of the mechanical collimator isocenter a mechanical front pointer held to the jaws is necessary. If this device is available, the procedure can be performed in the same way, by marking on the paper the point indicated by the pointer tip.

3 (Relative to procedure ) If a mechanical front pointer is available, check that the position of its tip is congruent with the center of the light field References W.R.Hendee, G.S.Ibbot, "Radiation Therapy Physics", 2nd ed., Mosby, 1996 J.R. Williams, D.I. Twaites, "Radiotherapy Physics", Oxford Medical Publications, 1993 UNICEN Routine Checks and Dosimetry of Therapeutic X-ray Units, AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No Verification Signature and date by the trainer: Name of the trainee: Comments: Date: Trainer's sign:

4 13.2 GANTRY ISOCENTER, GANTRY ANGLE INDICATOR, COUCH ISOCENTER,COUCH ANGLE, COUCH POSITION INDICATORS Training Goal This submodule deals with visual checks made to verify that the gantry and the couch are in good order Minimum time to be spent on the task: 1 day Competencies Addressed Ability to check the position of the gantry and the couch, to verify the readout relative to gantry and couch Equipment, Materials needed for the Task Mechanical front pointer; rod with fine point; ballast (weight of approximately 50 Kg); ruler Depending on local circumstances hands-on training may not be possible Important Note The use of equipment must be supervised by a qualified medical physicist Procedure Gantry Isocenter Mount the front pointer and fix the rod on the table so that its tip juts out and touches the tip of the front pointer. Then rotate the gantry from 0 to 360 with a 45 step. For each position measure the distance between the tips. Find the maximum value Gantry Angle Indicator Check that the gantry angle corresponds to the indicated value for at least four positions (0, 90, 180,270 ).

5 Couch Isocenter The general set-up is the same used for Measure the distance between the tips for different couch positions Couch Angle Check that the couch angle corresponds to the indicated value for at least four positions (0, 90, 180,270 ) Couch Position Indicators Put the ballast over the couch on the top. Move the couch along the three axes and check the correspondence between the real position and the redout indicators Measurements Integral part of the procedure Calculations No calculation is required Observations, Interpretations and Conclusions Observe that the position of the gantry and couch mechanical isocenter should be compared to the position of the radiation isocenter Compare the results to tolerance levels defined by the locally adopted quality assurance protocol Optional Nothing optional is suggested.

6 References W.R.Hendee, G.S.Ibbot, "Radiation Therapy Physics", 2nd ed., Mosby, 1996 J.R. Williams, D.I. Twaites, "Radiotherapy Physics", Oxford Medical Publications, 1993 UNICEN Routine Checks and Dosimetry of Therapeutic X-ray Units, AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No Verification Signature and date by the trainer: Name of the trainee: Comments: Date: Trainer's sign:

7 13.3 LOCALIZING LASER, LIGHT VS RADIATION FIELD CONGRUENCE, COLLIMATOR ROTATION, GANTRY ROTATION, COUCH ROTATION Training Goal The main purpose of this submodule is to give knowledge about how to find the radiation field and the radiation isocenter. This step in quality assurance is fundamental to check finally the coincidence between radiation, light and mechanical isocenter and field Minimum time to be spent on the task: 1 day Competencies Addressed Learning how to check the laser localization system. Understanding how to find the radiation isocenter for the collimators, the gantry and the couch Equipment, Materials Needed for the Task Ready pack films ; water equivalent plastic sheets; photodensitometer; pin Depending on local circumstances hands-on training may not be possible Important Note The use of equipment must be supervised by a qualified medical physicist Procedure Localizing Laser Place a paper sheet horizontally on the table and set the gantry to 0. Open the jaws to the maximum field size and project the light field on the paper. Then compare the field cross hairs to laser cross hairs and measure the maximum shifting between them. The check should be done also in vertical planes Light vs Radiation Field Congruence The check should be done for at least three fields: 5 cm x 5 cm, 10 cm x 10 cm and the maximum field size.

8 Place the film over at least 5 cm of plastic sheets and perpendicular to the beam axis, at the standard SSD. Then, make holes in the film in order to mark the corners and the center of the light field. Mark the film outside the field in a known position, in order to recognize its orientation with respect to the collimators after the development. Cover the film by other plastic sheets to ensure electronic buildup. Make an exposure in order to have a linear response by the film. Develop the film, then scan it with the photodensitometer along the two collimators axes to find the limits of the radiation field, defined where the net optical density is 50% of the central axis value. Compare the radiation field size to the light field size (corresponding to the holes) Collimator Rotation Arrange the film and the plastic sheets as in , without piercing it. Close one couple of jaws and open the other to obtain a strip-like field. Expose the film for different collimator angles. Develop the film, then draw the field midline for the different angles with a sharp pen. Sign the intersections of the fields on the film and measure the maximum shift between them. Repeat the procedure for a new film by inverting the collimators Gantry Rotation Place the film between two plastic sheets in the plane defined by the beam axis when the gantry rotates. Arrange the collimators to obtain a strip-like field parallel to the rotation axis of the gantry. Expose the film for different gantry positions, then develop it. Draw the field midline for the different angles with a sharp pen. Sign the intersections of the fields on the film and measure the maximum shift between them Couch Rotation Arrange the film on the table at the standard SSD between two plastic sheets as indicated in Close one couple of jaws and open the other to obtain a strip-like field. Expose the film for different couch angles. Develop the film, then draw the field midline for the different angles with a sharp pen. Sign the intersections of the fields on the film and measure the maximum shift between them. Repeat the procedure for a new film by inverting the collimators Measurements Integral part of the procedure.

9 Calculations No calculation is required Observations, Interpretations and Conclusions Compare the results to tolerance levels defined by the locally adopted quality assurance protocol Optional Nothing optional is suggested References W.R.Hendee, G.S.Ibbot, "Radiation Therapy Physics", 2nd ed., Mosby, 1996 J.R. Williams, D.I. Twaites, "Radiotherapy Physics", Oxford Medical Publications, 1993 UNICEN Routine Checks and Dosimetry of Therapeutic X-ray Units, AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No Verification Signature and date by the trainer: Name of the trainee: Comments: Date: Trainer's sign:

10 13.4 OUTPUT CONSTANCY AND LINEARITY Training goal This submodule deals with the verification that the timing and dosimetry properties of a teletherapy unit are in good order Related submodules: Calibration of a MVXRBeam Competencies Addressed Learning how to check the output constancy, the time constancy, the linearity Equipment, Materials Needed for the Task Water equivalent phantom; ionization chamber (e.g. 0.6 cc FARMER type ) with associated electrometer; termometer, barometer; stopwatch Depending on local circumstances hands-on training may not be possible Important Note The use of equipment must be supervised by a qualified medical physicist Procedure Output Constancy Check the output constancy by measuring the dose in reference condition. Repeat the measurement at least 10 times and calculate the standard deviation of the results. An index of output stability is the percent ratio of the standard deviation to the mean: check that it is smaller than 1% Linearity Measure the dose in reference condition for different MU pre-sets. Plot the resulted doses against the MU set-up and check the linearity of the curve Measurements Integral part of the procedure.

11 Calculations Integral part of procedure Observations, Interpretations and Conclusions Compare the results to tolerance levels defined by the locally adopted quality assurance protocol Optional Optionally, the output constancy should be checked also during the day, by repeating the measurement in different moments References W.R.Hendee, G.S.Ibbot, "Radiation Therapy Physics", 2nd ed., Mosby, 1996 J.R. Williams, D.I. Twaites, "Radiotherapy Physics", Oxford Medical Publications, 1993 UNICEN Routine Checks and Dosimetry of Therapeutic X-ray Units, AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No Verification Signature and date by the trainer: Name of the trainee: Comments: Date: Trainer's sign:

12 13.5 PDD (OR TPR), BEAM FLATNESS AND SYMMETRY Training Goal The purpose of this task is to check the constance of PDD (or TPR) for a linear accelerator Minimum time to be spent on the task: 1 day Related submodules: Measurement of PDD, Measurement of TPR Competencies Addressed Equipment, Materials Needed for the Task Water scanning system; couple of small ionization chamber (0.1 cc volume) Depending on local circumstances hands-on training may not be possible Important Note The use of equipment must be supervised by a qualified medical physicist Procedure For this measurement see the related submodules (Measurement of PDD, Measurement of TPR) Measurements Integral part of the procedure Calculations Integral part of the procedure.

13 Observations, Interpretations and Conclusions Find D2010 or TPR2010 and check that they are congruent with the expected values Optional Nothing optional is suggested References W.R.Hendee, G.S.Ibbot, "Radiation Therapy Physics", 2nd ed., Mosby, 1996 J.R. Williams, D.I. Twaites, "Radiotherapy Physics", Oxford Medical Publications, 1993 UNICEN Routine Checks and Dosimetry of Therapeutic X-ray Units, AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No Verification Signature and date by the trainer: Name of the trainee: Comments: Date: Trainer's sign:

14 13.6 BEAM FLATNESS AND SYMMETRY, TRANSMISSION FACTOR OF WEDGE FILTERS Training Goal The purpose of this task is to check the main charateristics of the beams: depth dose, flatness and symmetry (a, b). The beam profile of linacs can change because the flattening filter and the source are misaligned Minimum time to be spent on the task: 1 day Related submodules: Measuring MVXR Isodose Curves, Measurement of the Wedge Transmission Competencies Addressed Ability to check that the beam flatness and symmetry are within the tolerance limits. Ability to check the wedge transmission factors Equipment, Materials Needed for the Task Water scanning system; ionisation chambers; termometer, barometer; calculator. The materials are the same used for the measurement of isodose curves and wedge transmission factors (see the related submodules) Depending on local circumstances hands-on training may not be possible Important Note The use of equipment must be supervised by a qualified medical physicist Procedure Beam Flatness and Symmetry Measure the beam profiles at the buildup depth and at 10 cm depth along the X and Y directions as indicated in the related submodule. Calculate the beam flatness: R R max max R + R min min 100

15 where R max and R min are the maximum and minimum readings in the interval defined by the 80% of the central axis reading (reference region). Calculate the beam symmetry as the maximum percent difference between the doses at the left and right sides of the field. Optionally, repeat the assessment along the diagonals directions Transmission Factor of Wedge Filters Measure the transmission factors of all wedge filters for some fields as indicated in the related submodule. Compare the resulted values to those registered in the machine book Measurements Integral part of the procedure Calculations Integral part of the procedure Observations, Interpretations and Conclusions The manufacturers give tolerance limits both for the flatness and the symmetry: compare these limits to the the measured values Optional Nothing optional is suggested References W.R.Hendee, G.S.Ibbot, "Radiation Therapy Physics", 2nd ed., Mosby, 1996 J.R. Williams, D.I. Twaites, "Radiotherapy Physics", Oxford Medical Publications, 1993 UNICEN Routine Checks and Dosimetry of Therapeutic X-ray Units, AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No. 45.

16 Verification Signature and date by the trainer: Name of the trainee: Comments: Date: Trainer's sign: