Physics quality assurance

X-ray, laser, and couch alignment check

There are two objectives for this test: testing communication and data flow as well as checking the functionality of the x-ray imaging system. In this procedure, a stored plan in MOSAIQ is sent to the Hitachi HMI (Hitachi machine interface) computer. This will initiate the MOSAIQ site setup task, which sends the two-dimensional (2D) orthogonal reference images from MOSAIQ to the Hitachi imaging system, which is called PIAS (Patient Positioning Image and Analysis System). In the case of actual patient treatment, the reference images are compared with a set of two setup verification x-ray images. In the case of the QA patient, the purpose of sending the reference image from MOSAIQ to PIAS is to ensure that two systems are communicating as expected. Fig. 6.2 shows the x-ray setup consisting of an acrylic jig and a Varian On-Board Imaging (OBI) cube that has a 2-mm metal sphere in its center. The imaging cube is tightly secured to the jig, which is also tightly snapped onto the treatment couch using a ball-and-bearing screw system. This setup ensures reproducible placement of submillimeter accuracy of the center of the x-ray imaging cube. The couch is then moved to its expected calibrated coordinates ( x, y, and z ) using the Hitachi couch control pendant. This brings the metal sphere into the gantry isocenter, and the room lasers should be aligned to the crosshair in the cube if no change has taken place in the couch calibration and laser alignment. If a misalignment outside the 1-mm tolerance is observed, the cause of the misalignment is investigated, and corrective actions have to be taken before the patient treatment can start. A major reason for misalignment has been found to be a collision between the treatment head and the couch that affects the couch positioning calibration.

During routine daily QA, the location of the center of the imaging cube is compared with the center of the x-ray system using two orthogonal x-rays. The daily tolerance level for this test is 1 mm, which is the radius of the sphere. The PIAS system has a pixel size of 0.3 mm, but it allows shifts in increments of 0.1 mm (therefore, some shifts do not move the image). If the tolerance is exceeded, but within 2 mm, the covering medical physicist will be notified, and further tests will be required at a later time. If those tests confirm the deviation, a Hitachi engineer will have to recalibrate the couch coordinates. As a side note: the absolute couch coordinates are actually more important for correct QA setup to get accurate dose readings (see subparagraphs later) because if the table origin has shifted, the setup of the ionization chamber will not be correct. For patient setup, however, this is will not be a problem because the patient’s daily position is shifted according to the result of the x-ray and is not based on absolute couch coordinates. The daily shifts are recorded, and the two orthogonal images are sent from the PIAS system via couch pendant back to MOSAIQ. The correct data transfer of those images is part of the daily QA check. In addition, safety tests are performed every day. This includes the check of the audio system, video-TV system, the room search audio warning signal, the radiation monitor (inside/outside), the beam pause button, as well as the door interlock.

Proton pencil beam range check

The way the range check is performed has changed over time, but it can be divided into two periods: before 2016 and after 2016. The measurement devices and the idea behind the measurement remained the same: A different energy spot pattern has been used on each weekday. The corresponding proton pencil beam ranges vary from 28.8 to 5.1 cm. The amount of buildup (Solid Water, Gammex, RMI) is fixed for a particular weekday for the proton beam checked on that day.

Range check before 2016

The original range check was a convolution of positioning as well as dose output. Thus dose readings were obtained, but not actual ranges. This test consisted of that monoenergetic spots that are centered to the positions of the ion chamber. Their energies were selected in such a way that dose values at a depth close to the Bragg peak and at two more depths, one proximal to the Bragg peak and other distal to Bragg peak, were measured. The proximal dose and D _max were an average of two chamber readings, whereas the distal one was only from one ion chamber. The method was able to detect output changes, setup shifts, and range changes (see the green symbols in Fig. 6.3 from February 2014 to December 2015).

Range check since 2016

Similar to the procedure used before 2016, each weekday, a different set of energies and a different amount of buildup are used. However, the revised range check now uses five different energies daily compared with three energies before. In addition, the spots are not delivered on a single point over the ion chamber as was done before 2016, but instead on a 3 × 3 cm ² square using optimized weights in such a way that the dose within this square is uniform. Thus this new pattern makes the dose reading not sensitive to any phantom setup variation. The five monoenergetic energies resulting in different dose readings since the buildup remain constant for each 3 × 3 cm ² square delivery, and the measurement depth is always close to the Bragg peak. An example of this test is shown in Fig. 6.4 . The first derived data point is used as a verification of the output check. The tolerance for this is 3%. Then, from the first four dose readings, a third-order polynomial fit is performed, connecting all four measured points with a line. This polynomial fit creates a maximum value that can be used to derive its position (second green circle) as well as the maximum dose value. From the dose maximum, the 80% dose value (third green circle) and the 0% dose values (fourth green circle) are estimated by interpolation between the fourth and fifth measured dose values.

The new method allows measuring the ranges indirectly by deriving them from charge readings. The trend of those measurements is shown in Fig. 6.5 . The output has been within 1% over more time shown. The changes in the positions Pmax, P80%, and P0% have remained constant over the last 2 years, being within 0.6 mm of the baseline values. However, in May 2017, a noticeable jump in the trend was observed. This was caused by the installation of a new primary dose monitor whose water-equivalent thickness differed from the one it replaced by 0.2 mm. Because the change was rather small, the baseline data were not changed.

Spot position accuracy check

For this test, five monoenergetic spots are delivered at the central axis and at ±10 cm along the x -axis and along the y -axis. The highest available energy of 221.8 MeV is used because it provides the smallest spot size available for our scanning proton pencil beam. This spot size, however, is increased by 15% because of the use of a 20-cm solid water buildup, which is used to increase the dose reading by more than 50% to about 112 cGy. The spot position check has been performed since 2008 on every treatment day, and its baseline value was not changed until May 2017. The change was necessary because of the replacement of the primary dose monitor, which was built using thinner copper foils and thus resulted in a reduced spot size of about 10%. This change in spot shape increased the dose reading by 7% after replacement of the hardware. This system is capable of detecting both spot size changes as well as spot position change, as shown in Fig. 6.6 and Table 6.2 . Although the overall deviations in Fig. 6.6 are fairly constant, there are, however, some jumps, for example, October 2010, February 2011, August 2012, August 2013, March 2015, and February 2016. Most of those deviations are attributed to misalignment of the absolute couch coordinates after a collision between nozzle and couch pedestal, resulting in a drop of dose value because the detector is not anymore located at the maximum dose value. This is corrected by recalibrating the couch by the Hitachi engineer.

Volumetric dose check

The volumetric dose check was initially done as part of the weekly dose output check using a cube-like water tank. However, because this required an additional setup, we decided in 2012 to use the same tracker detector (see previous paragraphs) for this test as well. We used three different volumetric spot scan patterns (all using a field size of about 10 × 10 cm ² and a spread-out Bragg peak [SOBP] of 10 cm) with maximum ranges of 30.6, 20.0, and 14 cm. The first pattern is our standard calibration file that delivers 1 cGy/monitor unit (MU) as described in the paper by Gillin et al. Other files are derived from different versions of the Eclipse treatment planning system (TPS). Every day, one volumetric spot pattern file is used for the volumetric dose check, and over the course of the week, each pattern is delivered at least once. Fig. 6.7 below shows the deviation in the daily volumetric dose check results. All data points are within ±2%. However, there were some outliers in August and December 2013, when the dose monitor cable was changed. In addition, there were two dose monitor exchanges, one in January 2016 and one in May 2017. The first change resulted in a different energy response, especially for the R30 file (purple symbols) that now exhibits a larger fluctuation, as well as an offset to the R20 file (yellow symbol). Again, the baseline data were not changed over the 5-year period.

Mechanical quality assurance checks

The couch translation is checked by moving it to predefined positions (−10, −3, 0, +3, +10 cm) in x, y, and z directions. The position of the laser is measured using a ruler, and the deviations from the expected shift are recorded. The tolerance is less than 1 mm, which was always the case during the time of operation of the scanning beam gantry. The movable snout of the PTC is checked similarly using a ruler and laser. The snout position is set to 38, 23, and 8 cm, and the actual position is recorded. The deviation was always within 1 mm of the expected value. In addition to the gantry angle, the six-dimensional couch, as well as the snout rotation, is measured using a spirit level.