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A Report on Cancer
Background
Cancer is a class of diseases characterized by out-of-control cell growth. There are over 100 different types of cancer, and each is classified by the type of cell that is initially affected.
Cancer harms the body when damaged cells divide uncontrollably to form lumps or masses of tissue called tumors. Early detection of cancer can greatly improve the odds of successful treatment and survival. [1]
There are three ways to treat cancer such as radiotherapy, chemotherapy and surgery. Radiation treatment, also known as radiotherapy, destroys cancer by focusing high-energy rays on the cancer cells. This causes damage to the molecules that make up the cancer cells and leads them to commit suicide. Radiotherapy utilizes high-energy gamma-rays that are emitted from metals such as radium or high-energy x-rays that are created in special machines like Cobalt-60 or (LINAC) Linear Accelerators. [1] Radiotherapy is a multidisciplinary specialty which uses complex equipment and radiation source for delivery of treatment for cancer patients.
In Bangladesh about 200,000 people diagnose cancer annually among which 150,000 people die. Among the three major treatment modalities for cancer, radiotherapy is the cheapest modality in comparison to chemotherapy and surgery.
Cobalt 60 and Linear Accelerator are used in radiotherapy now-a-days all over the world. Although proton therapy has already started its journey in the state of the art technologies for the treatment of cancer, it still remains applied in the developed countries only. For developing country like our Bangladesh we still depend on the Cobalt 60 machines and the Linacs. We are now able to serve the patients with IMRT and in the near future with IGRT.
After the Linac is installed Acceptance tests and Commissioning is a compulsory job. Acceptance tests and Commissioning are the steps that need to be performed in order to prepare or optimize the radiotherapy machine for the treatment. Acceptance tests and Commissioning posse a major part of quality assurance for radiotherapy.
In Delta Hospital Ltd. the machine I worked with is the Varian Clinac 2100C DMX Linear accelerator. The Clinac DMX is a streamlined, high-performance and reliable platform that incorporates a broad range of imaging and treatment options, including dynamic motion management. The linear accelerator is built on the new, high-performance iX platform, and the system can be custom configured. That means every facility always starts with the best and builds forward. Some of its standard features are as follows:[2]
It has two photon energies: 6MV and 10MV and four electron energies: 6 MeV, 9 MeV, 12 MeV, 15 MeV. It has tight isocenter alignment of the gantry, couch, collimator, and imagers; exact couch, remote control of patient position, improved positional accuracy and compact stand. It has 40 pair MLC. The machine is provided with wedges of 15°, 30°, 45° and 60°.
Acceptance tests assure that the specifications contained in the purchase order are fulfilled and that the environment is free of radiation and electrical hazards to staff and patients. The tests are performed in the presence of a manufacturer’s representative. Upon satisfactory completion of the acceptance tests, the physicist signs a document certifying that these conditions are met. [1]
Commissioning is the process to optimize and calibrate the machine to deliver the treatment to the patients. Radiation treatment outcome is directly related to the accuracy in the delivered dose to the patient that is dependent on the accuracy of beam data used in the treatment planning process. These data are obtained during the initial commissioning of the linear accelerator and are treated as the standard data for clinical use and should be verified periodically. [1]
Main Objectives of the study
The main objective of the study is to observe the acceptance testing and determine the commissioning for photon beams of a linear accelerator.
Acceptance tests of the Clinical Linear Accelerator (CLINAC) 2100C DMX needs to be done to check if the vendor provided all the equipments and facilities as required by the hospital.
· For the mechanical checks the followings have to be observed and checked:
The CLINAC is provided with modulator, couch, collimator, proper wedges and applicators, chilling system, collimator rotation, gantry rotation, couch rotation, crosshair alignment, independent jaw position readouts, gantry rotation readout calibration, couch mechanical motions, collimator rotation readout calibration and all other necessary equipments to carry out proper patient treatment.
· For the radiative checks:
The machine has the proper field size alignment; optical distance indicator (ODI), dose linearity with MU settings, coincidence of light field and X-ray field, static MLC, photon and electron depth of ionization, photon and electron field flatness and symmetry, short term dose reproducibility, gantry rotation spoke shot.
For commissioning the objectives are:
· To determine the central axis percentage depth dose curves,
· To determine beam profiles,
· To determine output factors,
· To determine tissue maximum ratio (TMR),
· To carry out absolute dosimetry,
· To calculate monitor unit.
Present Condition and Scope of Acceptance Tests and Commissioning
Currently there are six hospitals which are equipped with modern Linear Accelerators (LINACs).
· National Institute of Cancer Research and Hospital – 3 Linacs
· Dhaka Medical College and Hospital – 1 Linac
· Bogra Ziaur Rahman Medical College and Hospital – 1 Linac
· Delta Hospital Limited – 1 Linac
· Khaja Younus Ali Medical College and Hospital – 1 Linac
· Square Hospital – 1 Linac
We have done my research work at the Delta Hospital Limited. This hospital is well equipped with two Cobalt-60 teletherapy machines and a new Varian Linear Accelerator for radiotherapy and a Varian Acuity Simulator. The hospital provides 3D CRT (Conformal Radiotherapy) for the cancer patients. The hospital has well trained medical physicist and staff. They have been trained in foreign countries like India, Germany, USA and several other countries.
Materials and Methods
Instruments Required
To perform the acceptance tests and commissioning of a linear accelerator some instruments and procedures are required:
1.1.1 Radiation Survey Equipments:
Radiation survey equipments such as a Geiger counter and a large volume ionization chamber are required to carry out radiation survey for all treatment rooms. For facilities with a treatment unit operated above 10 MeV, neutron survey equipment such as Bonner spheres, long counters and BF3 counters are necessary.[1]
Table: 01. Survey Meter Unit – 1
| Gamma Beta Survey meter |
Unit 01 |
| Model | 290 |
| Serial | 91532 |
| Manufacturer | VICTOREEN, USA |
| Calibration date | 10.20.2000(previously checked by checked source) |
| Dose scale mSv/h, mR/h | 2.10 mSv/h |
Table: 02. Survey Meter Unit – 2
| Gamma Beta Survey meter |
Unit 02 |
| Model | 451B-RYR |
| Serial | 0000001612 |
| Manufacturer | Fluke Biomedical |
| Calibration date | 29-06-2009 |
| Dose scale mSv/h, mR/h | mR/h |
Table: 03. Survey Meter Unit – 3
| Gamma Beta Survey meter |
Unit 03 |
| Model | 451B-DE-SIRYR |
| Serial | 0000001150 |
| Manufacturer | Fluke Biomedical |
| Calibration date | 01-07-2010 |
| Dose scale mSv/h, mR/h | mSv/h |
Ionometric dosimetry equipments refer to equipments such as several ionization chambers (thimble or plane-parallel type), a versatile electrometer cable and connectors fitting to the electrometer and all chambers, thermometer, barometer (for absolute dose measurements). Ionization chambers are required to compile the radiation beam properties measured during the acceptance testing and commissioning of a radiation treatment unit. There are two types of ionization chambers such as Thimble type and Plane Parallel type. A thimble ionization chamber is mainly used for photon beam while the plane parallel ionization chamber is mainly used for electron beam. They measure a number of relative quantities and factors, which include central axis percentage depth doses (PDDs), output factors and penumbra.[1]
Phantoms:
There are two types of phantom such as Radiation Field Analyzer (RFA) or Water Phantom and Plastic Phantom.
Radiation Field Analyzer or Water Phantoms:
A water phantom that field is required for acceptance testing and commissioning. This type of water phantom is frequently referred to as a radiation field analyzer (RFA) or an isodose plotter. Although a 2-D RFA is adequate, a 3-D RFA is preferable, as it allows the scanning of the radiation field in orthogonal directions without changing the phantom set-up. The traversing mechanism have an accuracy of movement of 1 mm and a precision of 0.5 mm. A 3-D scanner of an RFA can able to scan 50 cm in both horizontal dimensions and 40 cm in the vertical dimension. The water tank can at least 10 cm larger than the scan in each dimension. The RFA can be filled with water and then positioned with the radiation detector centered on the central axis of the radiation beam. The traversing mechanism can move the radiation detector along the principal axes of the radiation beam. After the gantry has been leveled with the beam directed vertically downwards, leveling of the traversing mechanism is accomplished by scanning the radiation detector along the central axis of the radiation beam, indicated by the image of the cross-hair. Any deviation of the radiation detector from the central axis, as the detector is moved away from the water surface, indicates that the traversing mechanism is not leveled.[1]
Plastic Phantoms:
Fig: 05. Plastic Phantom
For ionometric measurements in the buildup region a polystyrene or water equivalent plastic phantom is convenient. A useful configuration for this phantom consists of ten blocks of 25 × 25 × 5 cm3. One block was drilled to accommodate a Farmer type ionization chamber with the centre of the hole 1 cm from one surface. A second block was machined to place the entrance window of a parallel-plate chamber at the level of one surface of the block. This arrangement allows measurements with the parallel-plate chamber with no material between the window and the radiation beam. An additional seven blocks of the same material as the rest of the phantom should be 25 × 25 cm2. These blocks should be 0.5, 1, 2, 4, 8, 16 and 32 mm thick. These seven blocks combined with the 5 cm thick blocks allow measurement of depth ionization curves in 0.5 mm increments to any depth from the surface to 40 cm with the parallel-plate chamber and from 1 to 40 cm with the Farmer chamber. The depth of 40 cm is the limit, because 10 cm of backscatter should be maintained downstream from the measurement point. A plastic phantom for film dosimetry is also required. It is convenient to design one section of the phantom to serve as a film cassette. Other phantom sections can be placed adjacent to the cassette holder to provide full scattering conditions.
Use of ready pack film irradiated parallel to the central axis of the beam requires that the edge of the film be placed at the surface of the phantom and that the excess paper be folded down and secured to the entrance surface of the phantom. Pinholes should be placed in a corner of the downstream edge of the paper package so that air can be squeezed out before placing the ready pack in the phantom, otherwise air bubbles will be trapped between the film and the paper. Radiation will be transmitted no attenuated through these air bubbles, producing incorrect data. Plastic phantoms are also commonly used for routine quality control measurements. The design of these phantoms will depend on the requirements of the quality control program. [1]
Acceptance Tests for Photon Beams
The acceptance tests have been performed by the Varian supplied Bio-medical Engineer Mr. Armin Von Desuhwanden along with other local engineers supplied by Tradevision Ltd. The duration of the acceptance tests was about 3 weeks. Medical Physicists of Delta Medical Center were present there. I was there as an observer while the acceptance tests took place.
Safety Checks
Interlocks
The initial safety check was verified that all interlocks were working properly and reliably. There are four types of interlocks such as:
Door Interlocks
Door interlocks prevented the irradiation when the door of the treatment room was open. Keeping the treatment door open it was tried to switch on the beam, but the beam was not on.
Radiation Beam-off Interlocks
The radiation beam-off interlocks stopped irradiation but they did not stop the motion of the treatment unit or patient treatment couch.
Motion Disable Interlocks
The motion-disable interlocks stopped motion of the treatment unit and patient treatment couch but they did not stop machine irradiation.
Emergency Off Interlocks
Emergency-off interlocks disabled power to the motors that drive treatment unit and treatment couch motions and power to some of the radiation producing elements of the treatment unit. The idea was to prevent both collisions between the treatment unit and personnel, patients and other equipments and to halt undesirable irradiation.
Warning Lights
After verifying that all interlocks and emergency off switches were operational, all warning lights were checked.
Patient Monitoring Equipments
To monitor and communicate with the patient inside the Linac room the proper functioning of the patient monitoring audio-video equipment has been verified. The audio-video equipment is often useful for monitoring equipments or gauges during the acceptance testing and commissioning involving radiation measurements.
Radiation Survey
A radiation survey was performed in all areas outside the treatment room. A survey for neutrons in addition to photons has been done since the linear accelerator can operate above 10MeV. The survey has been conducted using the highest energy photon beam because it has the highest penetration power.
Collimator and Head Leakage
The target on a linear accelerator is surrounded by a shielding. Most regulations require this shielding to limit the leakage radiation to a 0.1% of the useful beam at one meter from the source. The adequacy of this shielding was verified during acceptance testing. The leakage test was accomplished by closing the collimator jaws and covering the head of the treatment unit with films. The films were marked to permit the determination of their position on the machine after they are exposed and processed.
Mechanical Checks
Collimator Rotation
At first the gantry was leveled at 0° then the level was rotated end-over-end to check the accuracy. A calibrated 100 cm front pointer was installed. The tip of another front pointer was extended over the front edge of the couch and the Gantry was rotated between 90° to 270° to accurately set the distance of the Collimator front pointer to precisely 100 cm TSD. Then the couch top front pointer was removed. A piece 1 mm ruled graph paper was taped on the top of the couch. Then the couch top was positioned until the graph paper was just below the front pointer but not touching. Then with the graph paper aligned under the front pointer tip the collimator was rotated from 90° to 270° while observing the pointer run-out.
Gantry Rotation
The front pointer, gantry and couch were set-up like the previous step with the collimator at 0°. The short front pointer was attached to the end of the couch so that the 2 mm tip was extended over the end of the couch top. With the couch at 0°, the couch vertical axis was positioned so that the tip of the collimator front pointer was aligned to the center of the short front pointer tip. Then the couch longitudinal axis was positioned so that the tip of the short front pointer was approximately 1mm away from the collimator front pointer. Then the couch lateral axis was positioned so the tip of the short front pointer was centered on but not touching the tip of the Collimator front pointer. The gantry was rotated through the full 360° while visually checking the front pointer run-out. It was verified that the front pointer tip was ? 1.0 mm radius throughout the entire 360° of rotation of counterweight systems.
Couch Rotation
The front pointer, gantry and couch were setup as the previous section with the collimator at 0°. The couch was slowly rotated from 90° to 270° while observing the front pointer run-out every 45°.
Crosshair Alignment
The couch top was set to 100 cm TSD and a mm ruled graph paper was stuck on to the couch top. With the crosshairs aligned on the graph paper the collimator was rotated from 90° to 270° and it was verified that the crosshair run-out was ?1.0 mm at isocenter. It was verified that each crosshair was parallel to the upper and lower jaws. The collimator was positioned to the center position and the crosshairs were aligned to the graph paper. The upper jaws were set to 35 cm and each of the lower jaws was driven independently until both jaws were 1cm away from one end of the projected crosshair line. The distance from the crosshair to each jaw at the other end of the crosshair line was measured and verified that the worst-case error for the radial crosshair line was within specification. This line should be as accurate as possible for MLC leaf calibration.
Independent Jaw Position Readouts
The gantry was leveled at 0°. Then by using a calibration front pointer the Couch top was set to 100cm TSD. A piece of accurately ruled mm graph paper was attached to the couch top. The field light was turned on and the graph paper was aligned to the Linac crosshairs. Each jaw was independently driven so that 50% isodensity point of the projected jaw shadow corresponds to the jaw positions.
Gantry Rotation Readout Calibration
Using a precision level placed on a true surface of the interface mount, the gantry was leveled at each position.
Collimator Rotation Readout Calibration
The gantry was positioned to 90° or 270° and the collimator to approximately 0°. The top of the couch was placed near the isocenter and both sets of jaws were opened. A level on the couch top was placed so that the light field projected a shadow of the level on the treatment room wall. Shims were used to level the level. Then the light field was turned on. While observing the shadows cast by the lower jaw edge and upper edge of the level, the lower jaws were closed and the Collimator position was adjusted until both shadows were parallel. This was the reference for the 0°. Rotate and level the gantry to 0°.
Couch Mechanical Motions
Couch Rotation
The gantry was rotated to 90° or 270°. The collimator was positioned to 0°. Then the couch was rotated to 0° and raised close to the isocenter, and moved laterally to the furthest position from the Gantry. The light field was turned on and projected the crosshair onto the edge of the Couch. A mark was placed on the edge of the couch showing the location of the vertical cross-hair. The pendant was used to move the couch laterally until it was as close as possible to the Gantry. The couch was rotated until the crosshair lines up with it again. The couch was run laterally to the furthest position again. The crosshair and couch mark were aligned. A piece of graph paper was aligned to the couch.
Couch Longitudinal Readouts
The wooden service panel was replaced with the carbon fiber panel and the Varian provided tape measure was installed into the alignment tool. Then the longitudinal alignment tool was installed onto the couch carbon fiber top at the 0 position index. The end of the tape measure was extended toward the gantry display. The couch was positioned to 0° and 100 cm TSD. A measuring tape was gently supported to keep its level and float the couch top until the crosshair aligned with the 20 cm mark. It was verified that the digital display met the specification. The couch top was floated until it aligned with the 150 cm mark on the tape.
Couch Lateral Readouts
Using the same alignment tool setup from the previous test the couch top was centered laterally by aligning the cross-hair to the scribe mark on the alignment tool. The table top was moved 23 cm to the right by measuring the distance between the crosshair and the tool scribe mark. The test was repeated with the tabletop moved 23 cm to the left of center.
Couch Vertical Readouts
The gantry was positioned to 0° using a calibration front pointer. With the couch in the test setup position, it was verified that the digital display met the specification for the 0 cm position. The gantry was rotated 50° in either direction to allow full couch extension. Sequentially the couch was driven 35 cm above the reference and it was verified that the digital display met the specification at both positions.
Optical Distance Indicator (ODI)
The field light was turned on and the crosshair and ODI rangefinder display was projected on a piece of white paper. It was verified that the ODI meets specification for the 100 cm position in the following table. Using the same tape measuring technique in the previous test, sequentially the couch was driven 20 cm above then 30 cm below the 100 cm reference position, it was verified that the ODI display met the specification at both positions.
Gantry Rotation Spoke Shot
The collimator was set to 0° angle and the upper jaw was full opened. Using the independent jaw mode both the lower jaws were closed to 0.5 cm so that a symmetric 1.0 cm field was projected relative to the crosshair. An X-ray film was positioned using support blocks in such a way that it was standing vertically on the couch top and perpendicular to the lower jaws. Then the couch was positioned at a height so that the center of the film was near 100 cm TSD. The couch lateral was positioned so that the approximate center of the film was aligned to the Linac crosshair. The gantry was rotated successively from 90°, 0°, 275° and 185° while exposing the film at each position, then the film was developed. The 1.0 cm exposures in the center were bisected with a sharp line. At the intersection of lines the longest line of the trapezoid was measured. This represents the diameter error.
Coincidence of Light Field and X-ray Field
The gantry was positioned to 0°, the collimator was positioned to 0°, the field size was set to 30×30 cm2 and the couch vertical was 100 cm TSD. Then an X-ray film was taped on the couch top and field light was turned on. The edges of the field on the film package at the 50% density region were marked with a small pin. Then the film was exposed in low X-ray and another film was exposed in high X-ray. Then the films were developed and the 50% isodensity lines of the X-ray field edges were compared to the field light edges.
Static MLC
Leaf Positioning Accuracy
Using a calibrated front pointer the couch top was set to 100 cm SSD. A piece of graph paper was attached to the couch top and the gantry was set to 0°. The jaws were opened to 40×40 cm2. The collimator was rotated to accurately align the crosshairs to the graph paper keeping the MLC in the park mode. One at a time all four leaf position patterns were set. Then the leaves were verified that they were at the plan position within ±1 mm.
Leaf Position Repeatability
The repeatability file pattern was set and the actual leaf positions were marked on the graph paper by drawing a line across each row of the leaves. Then the autocycle application was opened and the MLC was gone through 10 different patterns. The repeatability pattern test was again set and the leaf positions were compared to the previous measurements.
Collimator Spoke Shot
At first the film developer was turned on to warm up. Entering the Linac Service Mode the MLC interlock was override. The MLC leaves were retracted so it was possible to view the crosshair. The jaws were reopened to 25x25cm2 and the couch was set to 100 cm SSD. A piece of X-ray film was taped on the couch top at the center to the crosshairs. The MLC was then shaped to the spoke shot pattern. The collimator was then rotated to 90°, 135°, 180° and 225°. On the developed film, each spoke shot was bisected with very thin lines, then it was verified that the lines intersect within a circle of ?1.0 mm radius.
Coincidence of Light Field and X-ray Field
Both the gantry and the collimator were set to 180° and the couch was set to height 100cm SSD. For each exposure, the jaws were set to the MLC field size +0.5cm. Then 10x10cm2 MLC pattern was selected. A piece of film was set up on the couch with the light field visible on one half of the film. By using a pin the edges of the light field were marked. The film was then exposed to low X-ray. The film was then shifted to expose the other half of the film with high X-ray using the same setup. The film was then developed and compared to the 50% intensity regions of X-ray edges to the marks representing the light field edges. The difference between the light field marks and the X-ray edges was verified that they are within ±2.0 mm.
Dosimetry Measurements
Photon Depth of Ionization
All photon depth of ionization tests were specified using a water phantom at 100 cm TSD and 10×10 cm2 field size. Then the Water phantom was set for depth dose scanning and the jaws were set to 10×10 cm2. At first the central axis depth of ionization was scanned with low X-ray then it was repeated with high X-ray.
Photon Field Flatness and Symmetry
The gantry was leveled at 0°. The Water phantom scanning system was set for crossplane scanning then the tank was leveled in both planes, using the front pointer the top surface was set at 100 cm TSD and the jaws were opened to 40×40 cm2. The probe was set to zero position and a 1 cm depth was allowed to visual probe alignment to the crosshairs. The reference probe was positioned as far as possible away from the scanning path so that it did not cause scatter and affect the scan result.
Short Term Dose Reproducibility
Short-term dose reproducibility for each energy was found ±1% or 1 Monitor Unit at a fixed dose rate. The test results were recorded and calculated using the following equation:
Equation 1 = {[Reading #1- Reading #2] ÷ Reading #2} x 100
Dose Linearity with MU settings
The test results were recorded and calculated using the following equations:
50 MU ERROR = {[2×50 MUAVG – 100 MUAVG] ÷ 100 MUAVG}x 100
300 MU ERROR = {[(300 MUAVG ÷3) – 100 MUAVG] ÷ 100 MUAVG}x 100
Dose Linearity with Dose Rate
The test results were recorded and calculated using the following equations:
RR1 ERROR = {[RR1AVG – RRmid (AVG)] ÷ RRmid (AVG)} x 100
RRmax ERROR = {[(RRmax)(AVG) – RRmid (AVG)] ÷ RRmid (AVG)} x 100
Dose Reproducibility with Gantry Angle
The test results were recorded and calculated using the following equations:
90° ERROR = {[90° AVG – 180° AVG] ÷ 180° AVG} x 100
270° ERROR = {[270° AVG – 180° AVG] ÷ 180° AVG} x 100
Commissioning for Photon Beams
1.1.2 Anisotropic Analytical Algorithm (AAA)
The Anisotropic Analytical Algorithm (AAA) is a three dimensional pencil beam convolution or superposition algorithm that uses separate Monte Carlo derived modeling for primary and scattered extra-focal photons. The functional shapes of the fundamental physical expressions in the AAA enable analytical convolution, which significantly reduces the computational time. The AAA was originally conceived by Dr. Waldmar Ulmar and Dr. Wolfgang Kaissl. The development of the algorithm culminated in the publication of the triple-Gaussian photon kernel model in 1995. Important improvements have been made to the AAA dose calculation algorithm in the areas of treatment unit and tissue heterogeneity modeling, and increasing the accuracy of the scattered dose calculation. The AAA accounts for tissue heterogeneity anisotropically in the entire three-dimensional neighborhood of an interaction site, by using photon scatter kernels in multiple lateral directions. The final dose distribution is obtained by the superstition of the dose calculated with photon and electron convolutions.
Fig: 08. Treatment Unit Components, Broad Beam Division
Central Axis Percentage Depth Dose Curves
The first commissioning measurements are of the central axis Percentage Depth Dose (PDD).
Central axis dose distributions inside the patient or phantom are usually normalized to Dmax = 100% at the depth of dose maximum Zmax and then referred to as the PDD distributions. The PDD is thus defined as follows:
PDD(z, A, f , hµ ) = 100DQ/DP =100DQ /DP
Where DQ and DQ are the dose and dose rate, respectively, at point Q at depth Zon the central axis of the phantom and DP and DP are the dose and dose rate, respectively, at point P at Zmax on the central axis of the phantom.
Fig: 09. The Geometry of PDD Definition
The geometry for PDD definition is shown in above Fig. 08. Point Q is an arbitrary point at depth Zon the beam central axis; point P represents the specific dose reference point at Z= Zmax on the beam central axis. The PDD depends on four parameters: depth in a phantom Z, field size A, SSD f and photon beam energy hµ. The PDD ranges in value from 0 at Z ? to 100 at Z= Zmax.c
To measure the PDD, the surface of the water phantom was placed at the nominal SSD or at the isocenter. The vertical depth of the ionization chamber in the water phantom was determined by measuring from the bottom of the meniscus of the water to the centre of the chamber. Central axis PDD values were measured over the range of field sizes from 4×4 cm2 to 40×40 cm2. Increments between field sizes was no greater than 5 cm, but are typically 2 cm. Measurements were made to a depth of 35 or 40 cm. Chambers of 0.1 cm3 typically had diameters of 3 to 4 mm, the length of the order of 1.5 cm are used. A 0.1 cm3 chamber orientated with its central electrode parallel to the central axis of the beam was used in a water phantom.
Beam Profiles
The transverse photon beam profiles were measured to determine the off-axis dose distribution of photon beams. These profiles were measured in a water phantom with a small volume ionization chamber. The surface of the phantom was placed at 100cm SSD and the ionization chamber was scanned perpendicularly to the central axis.
Quality Index
We have measured the quality of the beam by using the 3D water phantom and OmniPro software. The central axis depth dose profile of 10×10 cm2 field size at each 2 mm increment of up to 30 cm depth. The depth dose ratio of PDD20/10 represents the beam energy of the photon radiation.
Tissue Maximum Ratio
Fig: 10. Geometry for measurement of TPR (d, AQ, hv).
(a) The geometry for the measurement of DQ at depth Z in a phantom;
(b) The geometry for the measurement of DQref at depth Zref in a phantom.
The distance between the source and the point of measurement, as well as the field size at the point of measurement, is the same for (a) and (b).
TMR (Z, AQ, hv) = DQ/DQmax = DQ/DQmax
Where DQ and DQ are the dose and dose rate, respectively, at point Q at a depth Zin a phantom and DQmax and DQmax are the dose and dose rate, respectively, at point Q at Zmax.
TPR (z, AQ, hv) = DQ/DQref = DQ/DQref
Where DQ and DQ are the dose and dose rate, respectively, in a phantom at arbitrary point Q on the beam central axis and DQref and DQref are the dose and dose rate, respectively, in a phantom at a reference depth Zref on the beam central axis.
Output Factors
The radiation output at Zmax, in cGy/MU for a linac, increases with an increase in collimator opening or field size. This increase in output was measured at Zmax of each field size. Alternatively, the increase in output was measured at a fixed depth for each field size and the output at Zmax determined by using the appropriate central axis PDD values.
Tray Factors
Shielding blocks are mostly made of lead. The thickness of lead required to provide adequate protection of the shielded areas depends on the beam quality and the allowed transmission through the block. For that purpose a block holding device called shielding tray was used for the radiotherapy treatment of cancer patient. The thickness of the shielding tray ????less than depth of Dmax of the respected photon beam radiation.
Wedge Factors
Wedges are used to shape the dose distribution of radiation treatment fields. The central axis wedge transmission factor is the ratio of the dose at a specified depth on the central axis of a specified field size with the wedge in the beam to the dose for the same conditions without the wedge in the beam. Central axis wedge transmission factors determined for one field size at one depth are frequently used to calculate beam-on times or MU settings for all wedged fields and depths.
To measure the central axis wedge transmission factor for a given field size at one depth the ionization chamber was placed on the central axis of the beam with its axis aligned parallel to a constant thickness of the wedge. Measurements were performed with the wedge in its original position and with the wedge rotated through 180º. This set of measurements verified that the wedge and the ionization chamber were correctly positioned. The wedge position may be rotated through 180º by rotating the collimator or by rotating the wedge itself.
Results
Acceptance Tests Data
Radiation Survey
Table: 04. Radiation Survey Data
| Gantry Position | Survey Point | Survey Value |
| Gantry=900 | Consol Control | 1.7µSv/h |
| Door | 0.3µSv/h | |
| Simulator | 0.5µSv/h | |
| Stairs | 1.5µSv/h | |
| X-ray Room | 3.5µSv/h | |
| Gantry=1800 | Store | 1.6µSv/h |
Mechanical Checks
Collimator Rotation
Table: 05. Collimator Rotation
| Collimator Angle Digital Readouts | ||
| Collimator Angle | Digital Specification | Result |
| 90° | ±0.5° | Pass |
| 180° | ±0.5° | Pass |
| 270° | ±0.5° | Pass |
Gantry Rotation
Table: 06. Gantry Rotation
| Gantry Angle Digital & Mechanical Readouts | ||||
| Gantry Angle | Digital Specification | Result | Mechanical Specification | Result |
| 0° | ±0.5° | Pass | ±1.0° | Pass |
| 90° | ±0.5° | Pass | ±1.0° | Pass |
| 180° | ±0.5° | Pass | ±1.0° | Pass |
| 270° | ±0.5° | Pass | ±1.0° | Pass |
| 360° | ±0.5° | Pass | ±1.0° | Pass |
Couch Rotation
| Mechanical Isocenter Measurements | ||
| Axis | Specification | Actual Result |
| Collimator rotation | ?1.0 mm radius | Pass |
| Gantry rotation w/Counterweight | ?1.0 mm radius | Pass |
| Couch rotation w/Counterweight | ?1.0 mm radius | Pass |
Table: 07. Couch Rotation
Field Light and Crosshair Alignment
Table: 08. Field Light & Crosshair Alignment
| Field Light & Crosshair Alignment | ||
| Optical Test | Specification @ 100cm TSD | Result |
| Field light run-out | ?1.0mm | Pass |
| Crosshair run-out | ?1.0mm | Pass |
| Radial crosshair parallelism | ||
(MLC leaf calibration reference line)?2.5mmPassTransverse crosshair parallelism
(m3 leaf calibration reference line)?2.5mmPass
Table: 09. Asymmetric Mode
| Independent Jaw Digital Readouts | |||
| Jaw | Position | Specification | Result |
| Y1 | -8cm | ±2mm | Pass |
| Y1 | 20cm | ±2mm | Pass |
| Y2 | 20cm | ±2mm | Pass |
| Y2 | -8cm | ±2mm | Pass |
| X1 | -1cm | ±2mm | Pass |
| X1 | 20cm | ±2mm | Pass |
| X2 | 20cm | ±2mm | Pass |
| X2 | -1cm | ±2mm | Pass |
Table: 10. Symmetric Mode
| Symmetric Jaw Digital Readouts | |||
| Field Size | Specification | Result | |
| X Jaws | Y Jaws | ||
| 6×6cm | ±2mm | Pass | Pass |
| 30×30cm | ±2mm | Pass | Pass |
Couch Mechanical Motions
Table: 11. Couch Mechanical Motions
| Couch Angle Digital & Mechanical Readouts | ||||
| Couch Angle | Digital Specification | Result | Mechanical Specification | Result |
| 90° | ±0.5° | Pass | ±1.0° | Pass |
| 180° | ±0.5° | Pass | ±1.0° | Pass |
| 270° | ±0.5° | Pass | ±1.0° | Pass |
Table: 12. Couch Longitudinal Readouts
| Couch Longitudinal Digital Readouts | ||
| Longitudinal position | Specification | Result |
| 20cm | ±1mm | Pass |
| 150cm | ±1mm | Pass |
Table: 13. Couch Lateral Readouts
| Couch Lateral Digital Readouts | ||
| Lateral Position | Specification | Result |
| 77cm | ±1mm | Pass |
| 100cm | ±1mm | Pass |
| 123cm | ±1mm | Pass |
Table: 14. Couch Vertical Readouts
| Couch Vertical Digital Readouts | ||
| Lateral Position | Specification | Result |
| 65cm | ±1mm | Pass |
| 100cm | ±1mm | Pass |
| 100cm | ±1mm | Pass |
Optical Distance Indicator
Table: 15. Optical Distance Indicator
| ODI Optical Display | ||
| TSD | Specification | Result |
| 80cm | ±5mm | Pass |
| 100cm | ±1mm | Pass |
| 130cm | ±5mm | Pass |
Radiation Isocenter Test
Table: 16. Radiation Isocenter Test
| Gantry Radiation Spoke Shot | ||
| Test Condition | Specification | Result |