Quality Assurance and Commissioning of New Radiotherapy Technology

Safety Interlocks

The testing of safety interlocks can vary from a simple test of the door interlock by opening the door to a complex test of the symmetry interlock by steering the beam until the interlock trips and then measuring the symmetry. The more complex tests may not be necessary if a risk analysis is performed showing that the probability of a failure is low. See Chapter 12 for a more detailed discussion of risk-based QA program development.

Mechanical Checks

Testing of mechanical parameters is fairly straightforward and well described in vendor documents. The sequence of testing, however, is very important as adjustments of one parameter may affect another. A sample sequence could be as follows:

• 1.

  Set gantry level for gantry angle of 0. Do this first as all other checks will hinge on it. This can be checked using methods described in the literature. Once true zero is established, identify the surface on the collimator face where a level can be placed to reliably confirm zero. A high quality level with 0 and 90 degree indicators is a required tool for a medical physicist. Digital levels can be used but the accuracy must be validated. The accuracy of any level can be checked by placing it on a level surface and then rotating the level 180 degrees to confirm the reading.

• 2.

  Confirm gantry angle accuracy at other angles. Use the previously identified surface on the collimator face and the level to check 90, 180, and 270 degrees.

• 3.

  Confirm radiation field and light field congruence. Check by marking the light field on film and then exposing the film. Perform for small, medium, and large field sizes for all photon energies. Ensuring a very tight tolerance (<1 mm) at this step will allow for the use of the light field in checks farther down the list. There are commercially available plates with radio-opaque field markings embedded that eliminate the need for manual marking of the light field (Sun Nuclear and Civco). Some of these can be used with EPID devices instead of film.

• 4.

  Confirm jaw concentricity and position accuracy. Check by marking the light field on a piece of paper and then rotating the collimator 180 degrees to confirm that the light field still coincides with the marks.

• 5.

  Confirm crosshair centering. This can be checked at the same time as Step 3 by marking the crosshair center. Also check that the crosshair center projects along a true vertical line. This can be done by transferring where the crosshair projects at isocenter to the floor using a plumb bob and then confirming that the crosshair projection matches.

• 6.

  Confirm mechanical isocenter. This is typically done by using the mechanical front pointer. Place a pointer stick on the treatment couch extending off the superior edge. At gantry angle 0, set the mechanical pointer to 100 cm. Align the pointer on the couch with the front pointer. Rotate the gantry through the full range and confirm that the maximum deviation is within specifications. This also confirms the accuracy of the front pointer. If there is a consistent offset, the front pointer needs to be adjusted. At this point, align the lasers to the mechanical isocenter.

• 7.

  Confirm radiation isocenter and radiation/mechanical isocenter congruence. Check with film using the traditional star shot technique. Mark the mechanical isocenter location on the film before exposing. When evaluating the film there are two parameters to measure. First, check that the intersection of the beams from various gantry angles is within specification. Second, check that the mechanical isocenter coincides with the radiation isocenter. If there is a small but acceptable difference, set the lasers to the radiation isocenter.

• 8.

  Set lasers to radiation isocenter. Care must be taken to ensure that the lasers are level and orthogonal and that opposing lasers match. The use of a plumb bob can assist in checking level. Another tool that can assist in laser setup is a three-direction, self-leveling construction laser. It can be placed at isocenter and used to project level lines onto the walls, the floor, and the ceiling. The device should be checked for proper calibration before using.

• 9.

  Confirm collimator isocenter. This is also measured with film using a star shot technique. Mark the crosshair center on the film before exposing. Check both the concentricity of the beams and the agreement of the center with the crosshair.

• 10.

  Confirm collimator angle accuracy. With the gantry at 0, set one of the jaws to 0 in the inplane direction. Open the other jaws all the way. Tape a piece of paper to the couch top and mark the beam center. Rotate the gantry in both directions while observing the light field. The jaw edge will remain on the center mark across the entire field if it is exactly parallel to the gantry motion, indicating a true zero collimator angle. Collimator angles of 90, 180, and 270 degrees can be similarly tested. Intermediate angles can be tested by aligning a protractor to the center and observing the angle indicated by the crosshair.

• 11.

  Confirm collimator position accuracy. Vary the collimator jaw/leaf setting and observe the light field compared with a calibrated ruler. MLC position accuracy can also be checked with the electronic portal imaging device (EPID) or film.

• 12.

  Confirm couch vertical motion is parallel to the beam. Tape a piece of paper to the couch top and mark the beam center. Set the gantry angle to zero using a level to ensure accuracy. Lower and raise the couch while observing the light field. If the couch is traveling in a true vertical path, the crosshair will remain on the center mark.

• 13.

  Confirm couch vertical position accuracy. Raise the couch top so that the lasers are just skimming the top of the couch. Confirm that the couch vertical reads 0. Tape a calibrated ruler to the side of the couch in a vertical direction. Confirm that the couch vertical readout matches the ruler by observing where the laser hits the ruler.

• 14.

  Confirm couch isocenter. This is measured with film using a star shot technique. Mark the crosshair center on the film before exposing. Check both the concentricity of the beams and the agreement of the center with the crosshair.

• 15.

  Confirm that the couch travels in a straight line when being extended toward the gantry. Tape a piece of paper to the couch top and mark the beam center. Extend the couch top in and out while observing the light field. If the couch is traveling in a straight line the crosshair will remain on the mark throughout the range of travel.

• 16.

  Confirm couch angle accuracy. Confirm couch angle 0 using the test in Step 15. Couch angles of 90 and 270 degrees can be similarly tested. Intermediate angles can be tested by aligning a protractor to the center and observing the angle indicated by the crosshair.

• 17.

  Confirm couch travel accuracy. Place a calibrated ruler on the couch top and move the couch known distances by observing the crosshairs. Check both direction of travel across the full range of motion. Evenly distributed weight should be added to the couch top to simulate clinical conditions. This can be done with solid water or anthropomorphic phantoms.

• 18.

  Confirm couch sag for various table extensions. Tape a ruler to the side of the couch in a vertical direction. Align the laser to a convenient position of, for example, 10 cm on the ruler. Add weight to the couch as described in Step 17, and observe the change in where the laser hits the ruler.

• 19.

  Confirm optical distance indicator (ODI) accuracy. Place slabs of precise thickness on top of the couch top. Adjust the table vertical until the lasers just skim the top of the slabs. Confirm that this is 100 cm source-to-surface distance (SSD) using the mechanical front pointer. Confirm that the ODI reads 100 cm within specifications. Remove the slabs one by one and confirm that the SSD reads 100 + total slab thickness removed for each. To check SSD <100 cm, adjust couch vertical so that the couch top is at 100 cm and then add the slabs one at a time while checking the SSD at each step.

This entire process could easily take an entire day, particularly if adjustments need to be made. The commissioning process is the time to make all parameters as accurate as possible. There are uncertainties in the measurements, and the equipment performance will likely degrade in the future. For these reasons, parameters should be adjusted to be well within specification so that adjustments are not needed in the near future. This may take some diplomatic negotiation with the installation engineer to continue adjusting a parameter even if it is barely within specifications. Commercial devices are available that can make some of this testing more efficient.