ROC7080: RADIATION THERAPY PHYSICS LABORATORY LAB A : MECHANICAL MEASUREMENTS LINAC QA

Transcription

1 ROC7080: RADIATION THERAPY PHYSICS LABORATORY LAB A : MECHANICAL MEASUREMENTS LINAC QA GROUP I SPRING 2014 KEVIN JORDAN GRADUATE STUDENT, RADIOLOGICAL PHYSICS KARMANOS CANCER CENTER WAYNE STATE UNIVERSITY

2 Table of Contents I. Introduction... 3 II. Laboratory Procedures & Methods... 3 III. Questions & Discussion... 3 IV. Appendix C: References... 9 JORDAN,KEVIN 2

3 I. Introduction In this laboratory, the uses and mechanisms of determining LINAC quality assurance measurements have been explored. The proceeding sections can be found to include Laboratory Procedures & Methods, Questions & Discussion along with the appendices, which include the experimental data, sample calculations and references. II. Laboratory Procedures & Methods This laboratory allowed the group to gain experience in light field and radiation field alignment, collimator/table/gantry isocentricity, laser alignment, optical distance indicator accuracy, light field & digital readout field size agreement and block tray alignment all to ensure the accuracy of the LINAC mechanical setup and gain experience in the quality assurance process. Questions and discussion given the exercises demonstrated in the lab can be found in the proceeding section. III. Questions & Discussion 1. How does one adjust the position of the light field so that it matches the radiation field? How does one adjust the size of the light field to match the radiation field? Recall, the light field is supposed to represent the area to be irradiated by the x-ray beam. This means that the center of the light field and x-ray field should coincide. Because of how the light is projected, it can be affected by the position of the light bulb, the bulb type, along with the position and angle of the mirror. One may need to keep in mind the beam steering and positioning if the radiation field deviated significantly to one side or overall on the film, keeping in mind the positioning of the bending magnet as well. A nice pictorial representation of this can be seen below. JORDAN,KEVIN 3

4 2. How could you use the two films taken at 5 and 10 MU to quantitatively check the light field size vs. radiation field size? One could take these two films and analyze their optical density, by using a scanning densitometer, in order to determine the true edge of the radiation field. It is difficult for the blind eye to determine the 50% line, so thus it is best to utilize the scanning densitometer to verify this most accurately. Recall, we truly applied 100 MU at two different energies, 6MV and 10MV photons. 3. How did the check of collimator isocentricity done with the star shot compare with that done using the light field? Estimate the uncertainty of the methods. The star shot is certainly more precise and accurate, as it represents a static hold on the radiation that was seen during the beam on time. The Light field method is more of a quick and dirty method, providing some level of accuracy, based on the crosshairs put in the middle of field and then measuring deviation about certain angles along the axis of rotation. Recall, the light field was based of a ruler measurement with 1 mm resolution and thus +/- 0.5 mm uncertainty. The manual scanning densitometer manual states that it has an uncertainty of +/- 0.02D. Since we did not have the film scanned it is hard to speculate on comparative uncertainties, but these values can be used for respective analyses. JORDAN,KEVIN 4

5 4. If a physicist found that the collimator star shot were perfect, but the crosshair shadow moved in a circle when the collimator rotated, what could be in need of adjustment? One might need to look at the angle of the mirror for the light field or the possibility that the entire collimator head itself is slightly out of plane. If either of these, or both, were not level and true with the table surface, it would cause a shadow that would then rotate with the collimator as it rotated. 5. How did the check of gantry isocentricity done with the star shot compare with that done using the light field? Estimate the uncertainty of the methods. Recall how the film was held upright in the phantoms, perpendicular to the axis of rotation, as the gantry rotated around this setup. Again, it seems that the uncertainty of the methods again can be quantified by the scanning densitometer, +/- 0.02D and then the ruler resolution of 1mm and thus an uncertainty of +/- 0.5 mm. Also, we used the front pointer to rotate about the isocenter. All of these methods suggested no more than a 0.5 mm deviations +/- 0.5 mm, thus meeting the required gantry tolerance standards. 6. What are the limitations of star shots? Can star shots determine deviations in all three dimensions? Are there any considerations about using star shots with asymmetric jaws? One can intuitively determine that, like film, star shots are 2D and thus can only be used for one plane at a time. Star shots cannot determine 3D distribution; another setup for an additional plane would have to be done. Asymmetry offers something unique in that the light and radiation distribution will be located at different portions along the film, and not necessarily at isocenter, TG-50 provides some nice instructions on this and MLC s in general. So the rotation about a circle at the center may not be viable for this given asymmetry. Recall at the end of the laboratory we were able to conduct both the picket fence and interdigitation tests, showing these respective results on film, one could analyze with a scanning densitometer the results of the picket fence, for example, and verify the optical density consistency within each fence post where the collimators were opened and adjusted with the beam on for the same field size at each fence post. JORDAN,KEVIN 5

6 7. Routine weekly or monthly quality assurance tests are often made with the light field, rather than by shooting star shots. Are there any requirements on the order in which these checks are made? While conducting this laboratory, we all assisted the instructor with the monthly QA. The specification sheet has in order: review of daily QA, front pointer calibration, laser alignment, treatment couch positioning, wedge/electron cone, and then field light intensity. Additionally, the data should be recorded as: symmetric jaws, asymmetric jaws, digital angle indicators, optical distance indicators, isocentricity. It seems intuitive to have these prescribed orders as they provide for a lock step and linear method for achieving the end isocentricity. One may have to repeat certain steps if they overall broad picture of the QA process was not addressed and then getting more specific, somewhat of a hierarchy and logical flow chart to provide for timely and most accurate end results of the LINAC QA. 8. How does the isocentricity of this linear accelerator compare to recommended standards? Overall, one desires to meet no more than 1 mm deviation between the gantry, collimator and couch. From our best estimations using the tools used, it seemed that the deviation of the gantry, collimator and couch deviated no more than 0.5 mm +/- 0.5 mm, and thus met the recommended standards. 9. During an acceptance test, what additional tests of light field/radiation field alignment would you recommend? There are some additional mechanical system tests worth mentioning. These would include verifying the patient support system and also the beam modifier systems, including the electron applicators, as deviations with these items also change the nominal field and radiation sizes and these are complimentary tests to be done while doing the other acceptance tests. This laboratory conducted an assessment of the alignment of the collimator axis and collimator jaws. The adjustable collimator jaws, when closed, must maintain symmetry about the collimator assembly axis while being rotated. A mechanical front pointer grasped by the jaws can be used and then extended from isocenter. Consequently, the rotation of the collimators will allow the front pointer to trace out any misalignment on a sheet of paper any misalignment between the collimator axis and the jaw faces. Additionally, the light field and radiation field congruence and coincidence should be checked. JORDAN,KEVIN 6

7 10. How would you verify that the lasers were properly aligned at points away from isocenter? One could use a rudimentary method of using a piece of paper and yardstick at different locations along the beam to determine the alignment. The values should be consistent along this path. Recall, that the laser alignment should be within 2 mm, as talked about in question During an acceptance test, what additional tests of optical distance indicator accuracy would you recommend? Some additional testing in reference to the optical distance indicator accuracy might include both manual and automatic tooling for verifying readout values with that of ones recorded during the acceptance test. For example an electronic tape measure and a level may be used for comparing distance and angles respectively at different points along the couch, gantry angle, collimator rotation angle to have a double redundant immediate and accurate values for the machine upon acceptance. 12. How closely did the indicated field size agree with the measured? What is the uncertainty of the measurement? What would be the maximum allowable deviation? All in all, the field sizes were all off by about 1mm, so 10 x 10 cm 2 field size was approximately 10.1 x 10.1 cm 2, thus presenting a 1% error/side (10.1/10 = 1.01 = 1%). The ruler which was used for the measurement had a resolution of 1 mm, and thus the uncertainty was +/- 0.5 mm. The maximum allowable deviation is 2 mm, as prescribed by TG How frequently, and by whom, the following items should be checked: Laser Alignment Daily, 2mm tolerance Gantry Isocentricity Annual, 2mm diameter circle tolerance Light/X-ray alignment Monthly, 2mm or 1% on a side tolerance ODI accuracy - Daily, 2mm tolerance Tray alignment Monthly, 2mm tolerance These frequencies and tolerances can all be found on pp. 12 of Report 46, for TG- 40. All these items should be verified by the physicist, even if someone else like the field service engineer did some of these checks already. JORDAN,KEVIN 7

8 14. In this lab you check several mechanical alignments individually. Many such alignments affect the accuracy of patient treatments. Think about the process that a patient is taken through to prepare and deliver a treatment consisting of two parallel- opposed fields (i.e. simulation, block preparation, and treatment). Assume the patient is simulated AP and is to be treated with an AP: PA pair. List as many mechanical parameters as you can think of which could affect the ultimate location of one edge of the PA field. If each were held to a tolerance of 2 mm, and if the errors were distributed randomly, what would be the average error? What would be the maximum error if no misalignments canceled? Can you suggest a test which would check the overall accuracy of the treatment delivery system? So there are many variables that can add up to be a potentially unfavorable tolerance stack up. First off, the patient was simulated AP, the PA side could easily have differentiation and asymmetry from the AP side. Additionally, the makeup of this tissue could be slightly different as well, but that is another discussion. The simulation environment versus the treatment day may have been different as well. The couch sag, respiratory gating and bladder retention all will vary to an extent, hopefully recognized by the TPS upon setup during time of treatment. One could hopefully assume that if the errors were distributed randomly that they would cancel out; so +2mm on tolerance, say the couch and another tolerance of -2 mm, say on the patient bolus profile, negated this, then the average error would be zero ( / 2 = 0), but with an uncertainty based upon the measurement device. Let one assume this to be 1mm resolution again, therefore an uncertainty of +/- 0.5 mm/variable. Recall, that the root sum square method can be used for quantifying the cumulative uncertainty. Cumulative Uncertainty Quantification (RSS) = Uncertainties! Fiducials would be a likely candidate for usage to ensure the accuracy of the tumor location, minimizing the issue with the peripheral mass of the person and the setup differences. Treatment delivery based on the simulation would then be validated by the true locations of these fiducials for current and future, realtime alignment. JORDAN,KEVIN 8

9 IV. Appendix C: References TG-40 TG-45 TG-142 Burmeister, Jay. Radiation Dosimetry Coursepack Rakowski, Joe. Radiation Therapy Physics Course Notes JORDAN,KEVIN 9