This chapter describes the setup of the water phantom system for measurements in the beam. It is assumed that the equipment has been installed and verified according to Chapter 4. Instructions for performing measurements are provided by the myQA Accept Instructions for Use.
IMPORTANT NOTICE: AWARENESS
Personnel using the water phantom system must read this chapter, as well as Chapter 10 and Chapter 4, and be fully aware of its contents.
Water Phantom Setup
Step 1: Place the Water Phantom on the Lift Table
- Place the water phantom on the lift table. Move the lift table to position the water phantom under the treatment head. Choose the orientation according to the application.
- Switch off the room light and switch on the light field of the radiation device. Open the field as much as possible.
- Move the lift table to adjust the lines marked on the bottom of the water phantom to align with the lines of the light field so that the center of the water phantom matches to the central beam axis.
Water phantom marker at bottom and light field aligned
- Lock the lift table by fastening the parking brakes.
- Use the lateral adjustment handles for fine adjusting the position. Ensure that the light field crosshairs and the crosshairs on the bottom of the water phantom are superimposed.
- Place the spirit level on the platform of the lift table and turn the tilt adjust hand wheels to adjust the phantoms to a perfect horizontal position.
CAUTION: COORDINATE SYSTEM
The phantom coordinate system are indicated in one lower corner of the phantom. Choose the orientation according to the application.
IMPORTANT NOTICE: MARKERS ON WATER PHANTOM
The alignment markers at tank bottom are intended for the initial alignment of the tank only.
The references for the horizontal detector movement on the main axes are the light field crosshairs.
Step 2: Prepare the Water Reservoir
- Position the water reservoir carriage close to the water phantom.
- Lift the filling hose out of the tube support at the water reservoir.
- Connect the quick coupling of the water reservoir hose (i) to the counterpart in water phantom (ii), which is located at one bottom corner of the phantom; push in until it snaps on.
Detail of quick coupling at water phantom
- When the desired water height is reached, press the Stop button.
CAUTION: DO NOT REMOVE FILLING SYSTEM IF PUMP IS SWITCHED ON
Do not remove the quick coupler from the phantom while the pump is switched on.
CAUTION: DO NOT MOVE THE PHANTOM WHEN IT IS FILLED WITH WATER
It is recommended to not move the phantom when it is filled with water, especially when the water occupies more than half of the tank. Doing so, could lead to electrical damage of the side-mounted control box due to direct exposure to water.
CAUTION: DO NOT FILL IN ADDITIONAL WATER
Do not fill the water phantom with additional water when the former proper filled water reservoirs tank is empty to avoid an overflow of the water reservoir when the phantom is drained.
IMPORTANT NOTICE: DELAY IN FILLING MODE
If the Fill button is pressed the first time after the water reservoir has been connected to the mains outlet, there is an incident waiting time of approximately 15 seconds. During this time, the electromagnetic valve is opened in order to remove the air between the pump and the electromagnetic valve.
Step 3: Adjust SSD (Source Surface Distance)
- Take care that the parking brakes of the lift table are fastened.
- When an electrical lift table is used, press the Up or Down button to lift or lower the lift table.
If a mechanical lift table is used, turn the hand wheel to move the platform up- or downwards. - Switch off the room light and switch on the distance indicator of the radiation device. Open the field as much as possible. Alternatively, a SSD verification tool can be used.
- Adjust the high of the equipment with the lifting table until the water surface has achieved the desired SSD (use a piece of paper on the water surface if the read-out of the current SSD is done by the light distance indicator or a SSD verification tool is used to indicate the contact with the water surface).IMPORTANT NOTICE: REPEAT SSD CHECK
Check the Source Surface Distance before and after measurements or in-between long sessions. The tank material is absorbing water or the water volume gets less by evaporation.
Step 4: Connect Water Phantom to CCU and Computer
- Place the computer in the control room, close to the accelerator control terminal.
- Place the CCU in the treatment room. It is convenient to store the CCU on the top plate of the Water Reservoir or treatment couch. However, before performing measurements, it is recommended to place the CCU as far away as possible from the primary beam, since leaking radiation and neutron contribution may damage the electronics.
- Connect the drive control cable between the CONTROL CABLE connector at the CCU and the CONTROL CABLE connector at the water phantom.
- Connect the Ethernet cable between the NETWORK connector CCU to the NETWORK connector on the computer, direct or via the local network.
CAUTION: UNEXPECTED MOVEMENT
Make sure that the CCU is switched off before connecting the drive control cable between the CCU and the water phantom.
WARNING: DO NOT CONNECT TO A CHAMBER USED FOR PATIENTS
The measuring assembly (including the CCU) shall not be connected to a chamber assembly which is used in contact with a patient.
IMPORTANT NOTICE: NETWORK CONNECTION
If the CCU is connected to the computer via the local network, ensure that the TCP/IP settings have been configured before. For the first time it is recommended to use a direct Ethernet connection between CCU and computer to get a first fast overview of the correct communication function.
Warming up the CCU
At this point of the equipment connection, the CCU can begin warming up by turning on the power to save time.
For the CCU, the warm-up and stabilization time is 30 minutes. If ionization chambers are used, the high
voltage should be applied during the warm-up time. If semiconductors are used, do not apply a high voltage.
To turn on the high voltage, click the HV Setup button in the Controller panel of myQA Accept SW:
Step 5: Detector Setup for BP SMARTSCAN
CAUTION: SET HIGH VOLTAGE TO ZERO
Always set the high voltage to zero before connecting / disconnecting detector extension cables or detectors. Of course, this is also fulfilled if the CCU is switched off.
CAUTION: USE SPECIFIED DETECTORS
Performance of the system cannot be assured if detectors or adapters other than those specified for the BP SMARTSCAN are used. Unsuitable adapters may pose a safety hazard, as high-voltage may appear on adapter or detector surfaces.
Two detectors, a field and a reference detector, are required to use the BP SMARTSCAN in continuous measurement mode under a linear accelerator. Field and reference detector should be of the same type, or if this is not possible, have at least approx. the same active volume (please note, this does not apply to the Stealth Chambers). The standard field and reference detectors for the BP SMARTSCAN are the CC04 ionization chambers (or diodes) and the Stealth Chamber as reference detector.
Using the continuous measurement mode of the BP SMARTSCAN (myQA Accept software option) has two significant advantages compared to the systematic (integrating) measurement mode:
- It significantly reduces scanning time.
- It delivers more data points in a given time than in systematic (step-by-step) mode.
A linear accelerator produces pulsed radiation. If only a field detector would be connected and continuous measurement would be used, the pulsing of the beam would directly appear on the curve. The signal of a reference detector is needed in order to remove the accelerators pulsing from the curve, using real-time division of the field detector signal by the reference detector signal.
Step 5a: CC04 Setup
- Insert the detector holder’s dovetail groove into the counterpart of the scan mechanic (top side of slider at X-axis). Fasten the screw when the holder is in the right position, indicated by a noticeable “snapping”.
- Mount the field chamber in the detector holder and fasten it with the screw at the side.
- Insert the detector cable into the self-gluing cable clamp on the BP SMARTSCAN's metal frame or outer side of the tank (for pull-relief of the detector). Cable clamps are part of the tool kit, delivered with the water phantom system.
- Connect the detector cable to the chamber extension cable. Close the connector protection caps by screwing them together.
- Connect the extension cable to the field sensor input of the CCU.
- Insert the reference chamber into the reference holder tube and fasten the chamber with the nylon screw at the front end. When using a Stealth Chamber attach it to the collimator of the linear accelerator.
Left: reference detector support mount at one corner; Center: CC04 mount at reference holder tip; Right:
Square Stealth Chamber with detector holder on a linac head
- Mount the reference holder at a comfortable place at the BP SMARTSCAN mechanic frame (preferably a corner). When using a Stealth Chamber attach it to the collimator of the linear accelerator.
- Insert the detector cable into the clamp on the BP SMARTSCAN's metal frame (for pull-relief of the detector, see before).
- Connect the detector cable to the chamber extension cable. Close the connector protection caps by screwing against each other.
- Connect the extension cable to the reference sensor input of the CCU.
- Place the reference chamber with the chamber tip towards to the focus at the field’s edge (the light field helps to find the correct location).
IMPORTANT NOTICE: CORRECT CONNECTOR HANDLING
Connectors on detector and extension cables are susceptible to contamination and mechanical damage. Keep cables covered by the protective caps when not in use.
When connecting TNC connectors, keep the connector body steady and only rotate the swivel nut (screwing ring).
IMPORTANT NOTICE: LOCATION FOR REFERENCE CHAMBER
The reference detector should be placed well in the field as this reduces the effect of unstable field-asymmetry. On the other hand, care should be taken not to shadow the field detector with the reference detector.
The reference detector must always be within the primary radiation field to provide a stable signal. Therefore, it is recommended to review the placement of the reference detector from time to time, especially, when the field size has been changed.
When the Stealth Chamber is used in the appropriate holder it is ensured that the filed is covered and the chamber is centered. There is no need for adjustment of the detector when the filed sizes are changed.
Step 5b: Diode Setup
- Insert the detector holder’s dovetail groove into the counterpart of the scan mechanic (front side of
slider at X-axis). Fasten the screw when the holder is in the right position, indicated by a noticeable “snapping”.Universal detector holder at X-axis
- Mount the field diode in the detector holder and fasten it with the screw at the side. Take care about to select the right diode with respect to the used energy (yellow = photon, blue = electron, green = stereotactic).
- Insert the detector cable into the self-gluing cable clamp on the water phantom's metal frame or outer side of the tank (for pull-relief of the detector). Cable clamps are part of the tool kit, delivered with the water phantom system.
- Connect the detector cable to the detector extension cable. Close the connector protection caps by
screwing against each other. - Connect the extension cable to the field sensor input of the CCU.
- Insert the reference diode (RFD) into the reference holder tube until it appears at the other end. Put the reduction tube at the RFD stem, move both backwards into the reference holder tube and fasten them with the nylon screw at the front end. The reduction tube can be found as a part of the tool kit, delivered with the water phantom system.
Reference diode including reduction tube mounted at tip of reference detector holder
- Mount the reference holder at a comfortable place at the water phantom mechanic frame (preferably a corner).
- Insert the detector cable into the clamp on the water phantom's metal frame (for pull-relief of the detector, see before).
- Connect the detector cable to the detector extension cable. Close the connector protection caps by screwing against each other.
- Connect the extension cable to the reference sensor input of the CCU.
- Place the reference diode with the white dot at the chamber tip towards to the focus at the field’s edge (the light field helps to find the correct location).
Reference diode positioning
IMPORTANT NOTICE: ORIENTATION OF REFERENCE DIODE
At the reference diode’s tip, there is a small white dot printed. This point indicates the best position for the beam entrance, i.e. the point should be oriented towards to the focus.
Step 6: Fine Leveling of the Water Phantom
After filling the water phantom with water, the 3-D servo frame must be leveled to the water surface. This fine leveling is done with the leveling pins mounted at the upper four corners of the phantom (Section
5.1.6.1), with the CC13 chamber (Section 5.1.6.2), or diode detectors (Section 5.1.6.3).
IMPORTANT NOTICE: INCLINE CORRECTION
The slider at the X-axis does have an option for incline corrections. The horizontal alignment of the detector can be performed with this tool easily. It is recommended to check the orthogonality of the slider (holder) every time before defining the water surface, see Section 8.1.
Step 6a: Fine Leveling with Leveling Pins
The leveling pins are used to fine-level the frame to water surface. There are two sets of pins available: the pin length = 6.2 cm (a) and 15.5 cm (b). The water should be filled to the tip of the pins.
left: two leveling pin sets (a & b); right: water filled to the tip of the pins
With a lift table, both sets can be used for normal LINACs at any SSDs. However, for a Halcyon LINAC, the long set is for SSD 100 cm and the short set is for SSD 90 cm. For special cases that the water is filled to other levels, please use a CC chamber (Section 5.1.6.2) or diode detector (Section 5.1.6.3) for frame leveling.
Prerequisites
- To have maximum adjustment lengths in both directions (~ 5 mm), check that the lengths of the leveling screw bolt above (1) and below (2) the rail are about the same.
Leveling screw
- Since the adjustment height is about +/-5 mm, please fill water to about 1 mm below the tip of the pin.
IMPORTANT NOTICE: START THE LEVELING WITH THE PIN ABOVE THE WATER SURFACE
Because the movement of the water makes the alignment difficult, it is recommended to start the leveling with the pin above the water surface.
Leveling procedure
- Look at the tip of the leveling pin from the side of the phantom. When the tip is exactly at the water surface, the tip and its mirror image form a perfect cross.
Leveling pin at the water surface (seen from above the water surface line); 1: Locking screw; 2: Leveling screw
- If the leveling pin is not exactly at the water surface, loosen the locking screw at this corner of the servo frame. Turn the leveling screw to adjust the frame vertically so that the leveling pin and its mirror image at the water surface form a perfect cross. Then tighten the locking screw. IMPORTANT NOTICE: LOSS OF ORTHOGONALITY
Do not loosen any one of the remaining three locking screws while adjusting one position; otherwise, the mechanic orthogonality might be lost. - Repeat the above two steps for adjusting the remaining three corners of the phantom.
Exchange pin sets (optional)
- Remove the old pin by pushing the pin up from the bottom and then removing it from the top.
- Install the pins. The ring of the hold has a spring ball that locks the pin (a). Push the pin down until the pin is locked (b). The head of the pin should be flat with the holder surface (c).
Inserting the pins
IMPORTANT NOTICE: FIXATION OF LEVELING PINS
Please ensure that the top of the leveling pin is aligned with the top of the Leveling Pin
Holder.
CAUTION: DO NOT REMOVE LEVELING PIN HOLDER FROM FRAME
The leveling pin holders are aligned to each other. After the leveling procedure, do not remove the leveling pin holder. If removing the holders, the leveling pins must be releveled to avoid wrong alignment of the mechanical frame.
IMPORTANT NOTICE: REPEAT PHANTOM ALIGNMENT CHECK
After the phantom is filled with water, the gravity point becomes much higher; it could happen that the equipment is slightly tilted and thus the phantom alignment is no more correct against the CAX. Therefore, please perform a crosscheck light field vs. markers at phantom bottom every time you lift the phantom up and make a correction by using the Lift Table lateral alignment screws.
Step 6b: Fine Leveling with CC Chamber
This section describes the fine leveling of the water phantom's mechanical frame, using a CC chamber and the alignment cap.
IMPORTANT NOTICE: ALIGNMENT CAP DESIGN
The delivered alignment cap should be only used with the CC04 chamber. However, for the Z-direction alignment, e.g., water surface levering, it can be also used with other CC chamber .
- CC chamber is already mounted in the phantom. Have the white alignment cap ready (part of the tool kit delivered with the water phantom system).
- Unfasten the CC chamber and insert the chamber as far as possible into the alignment cap, usually
until the thin black ring. The active volume (the black part) of the chamber points away from the X-axis, towards the phantom's front wall. - Slide the chamber with the mounted alignment cap backwards until the rear edge of the alignment cap touches the field chamber holder. This should be done by pushing the alignment cap and a soft support at the chamber cable. Do not pull at the cable only; otherwise, it may happen that the chamber is no more at the right position in the alignment cap.
- Secure the chamber in the holder by closing the detector locking screw. Leave the alignment cap
mounted.Alignment cap mounted on the CC04
- Switch on the CCU control unit.CAUTION: UNEXPECTED MOVEMENT
Switching on the CCU may cause unexpected movements of the water phantom mechanics. Keep your hands away from the water phantom mechanics and take care that no cables or foreign parts are squeezed, which causes damages. - With the remote control, position the chamber into one of the corners of the water phantom, approx. 5 cm away from each wall and at the water surface.
- To check whether the chamber is really at the water surface, turn the alignment cap so that the
crossline on the front of the cap are at 45° towards the water surface. Look at the cap from below the water surface. The exact water surface position (1) is reached if the lines form a precise cross (see the figure below).CC04 exactly at water surface (seen from below the water surface line); Blue line represent water lineIMPORTANT NOTICE: START OF LEVELING
Always begin the leveling adjustment from below the water surface. - Ensure that at the starting corner the chamber is exactly at the water surface. Do not use the leveling mechanism here, only position the chamber with the remote control.
- Store the current chamber position as position A on the remote control.
- With the remote controls joystick, move the chamber horizontally to the next corner of the water phantom.IMPORTANT NOTICE: NO MOVEMENT OF SERVO DURING LEVELING
Do not move the servo vertically during the leveling procedure. - Store this position as position B on the remote control.
- Look at the alignment cap. If the chamber is not exactly at the water surface, loosen the locking screw at this corner of the servo frame. Turn the leveling screw to adjust the mechanical frame vertically so that the lines on the cap again form a precise cross. Then tighten the locking screw.
- Repeat the aligment procedure for the remaining corners of the water phantom.
Please note, the remote control provides just three positions for storage, the water surface position can be used at the fourth corner.
IMPORTANT NOTICE: USAGE OF STORED POSITIONS
You can use the stored positions to approach easily the leveling points again.
Step 6c: Fine Leveling with Diode
- Diode and detector holder are already mounted in the phantom.
PFD diode mounted at water phantom detector support
- Switch on the CCU control unit.
- To check whether the diode is really at the water surface, look at the top of the detector from below the water surface. The exact water surface position is reached if the mirror image of the detector meets the top of the detector.
EFD diode exactly at water surface (from below the water surface line (1)
- Ensure that at the starting corner the diode is exactly at the water surface. Do not use the leveling mechanism here, only position the diode with the remote control.
- Store the current chamber position as position A on the remote control.
- With the remote controls joystick, move the detector horizontally to the next corner of the water phantom.
- Store this position as position B on the remote control.
- Look at the diode top and mirrored image. If the diode is not exactly at the water surface, loosen the locking screw at this corner of the servo frame. Turn the leveling screw to adjust the mechanical frame vertically so that the diode head and mirrored image meet at the water surface. Then tighten the locking screw.
- Repeat the aligment procedure for the remaining corners of the water phantom.
Due to that, the remote control provides just three positions for storage, the water surface position can be used at the fourth corner.
Step 7: Define Water Surface and Isocenter
- With the remote control, move the CC13 field detector to the actual water surface in the central beam (use alignment cap). If a diode is used, proceed in the same way until the mirror image of the detector meets the top of the detector.
- Check horizontal alignment – if required perform incline correction.
Wheel for horizontal incline correction at X-slider
- With the remote control, define the water surface.
- With the remote control, move the field detector to the isocenter position in the central beam and define the isocenter position at this point.
- When a CC13 with alignment cap is used: Turn the alignment cap at the water surface position by 45° until the line at the front of the alignment cap is equal/parallel to the water line. Now move the alignment cap to the CAX (light field cross) until the crossed lines at the alignments top are matching with the cross hair of the light field – this shall be the Isocenter position.
CAX verification at CC13 alignment cap
- When a diode is used: The light field’s cross hair need to be placed exactly at the center of the diodes round top surface if this area is equal with the water surface – this shall be the Isocenter position in this case.
CAX verification at EFD diode
Step 8: Ready for Measurement
After the previous steps have taken place, the water phantom setup is complete and device connection should be the same as the diagram shown in Section 4.4.2.
IMPORTANT NOTICE: CCU WARM-UP AND BACKGROUND SUBTRACTION
For the CCU, observe a warm-up and stabilization time of 30 minutes. If ionization chambers are used, the high voltage should be applied during the warm-up time. If semiconductors are used, do not apply a high voltage.
Background subtraction should be done initially after the warm-up time, and each time the sensitivity settings have changed.
For absolute dose measurements, background measurements shall be done repeatedly every 30 minutes after warm-up, and each time the sensitivity settings have been changed.
From this status onwards, measurements with myQA Accept can be started. Please refer to myQA Accept Instructions for Use for instructions.
IMPORTANT NOTICE: INFLUENCE OF WAVES
In case that there should be profile scans close to the water surface, it might be recommendable to reduce the scanning speed (see myQA Accept Instructions for Use, Measurement Queue Setup) to avoid waves, which could have negative influence to the scan quality.
CAUTION: LOCATION OF CONTROL
When the remote control is not used, do not hang it up at the phantom servo mechanic to avoid a blockage during movement.
Measurement tips
IMPORTANT NOTICE: BACKGROUND SUBTRACTION
For the CCU, background subtraction should be done initially after the warm-up time, and each time the sensitivity settings have changed. For absolute dose measurements, background measurements should be done repeatedly every 30 minutes after warm-up, and each time the sensitivity settings have been changed.
IMPORTANT NOTICE INFLUENCE OF WAVES
In case that profile scans are close to the water surface, it is recommendable to reduce the scanning speed (see myQA Accept Instructions for Use, Measurement Queue Setup) to avoid waves, which could have negative influence on the scan quality.
CCU Electrometer Setup
Important for good measurement results are the right, well-adapted selection and constellation of detectors, and the associated electrometer setup in the myQA Accept software.
The CCU electrometer panel provides three different, independent selectable sensitivity ranges for the Field and Reference channels:
- High
- Medium
- Low
Table 5.1. Input current ranges of sensitivities
| Sensitivity | Current [nA] |
| High | 0.0004 to 0.4 |
| Medium | 0.25 to 40 |
| Low | 25 to 4000 |
To choose an appropriate sensitivity setting, the operator needs to estimate the measured current, in regards to the dose rate used.
To calculate the input current, use the following formula: I (nA) = sensitivity (nC/Gy) x dose rate (Gy/s)
With the calculated input current for the electrometer, and the known sensitivity of the selected detector, the applicable sensitivity range for the CCU electrometer channel can be selected.
The associated myQA Accept Instructions for Use contains additional details and instructions concerning the CCU control panel.