Longitudinal Shift Limitation for Long-Field Delta4 Measurements
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PO-GePV-T-180 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: This work aims to determine the limits to longitudinal shifts in measuring patient IMRT QA plans using the Delta4+ device. This is applicable to extended field measurements on radiotherapy plans longer than 40 cm, such as for total body irradiation plans on a Tomotherapy treatment delivery system.
Methods: An IMRT plan was created on an anonymized CT scan of a patient, where a uniform dose distribution was optimized in a target, 60 cm long. This was to ensure that the dose distribution was longer than the length of the phantom. An IMRT QA plan was created for with a Delta4+ phantom within the RayStation treatment planning system. Three separate measurements were then performed with the phantom centered at three different locations with respect to the plan, shifted -8.0 cm, 0.0 cm and +8.0 cm. A comparison of dose accumulation in a horizontal line of the plan was performed to determine the effect of the amount of phantom material near the measurements.
Results: Figure 1 shows the setup to measure a line profile with different shifts of the Delta4+ phantom. It was determined that the center line profile of the dose distribution measured with the phantom centered is equivalent to the measurement performed when the phantom was shifted 8 cm superiorly. However, the measurement for the same line was less when the phantom was shifted 8 cm inferiorly. This shows that the amount of phantom material preceding a line in the dose distribution affects the accumulated measured dose.
Conclusion: Delta4+ phantom position affects dosimetry in the IMRT QA process in plans whose dose distribution is longer than the Delta4+ phantom, limiting the amount one may shift the phantom. There is an amount of needed phantom material, above which measurements remain equivalent. Additional measurements are needed to establish this amount.
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