SU‐GG‐T‐153: Scaling the Prescription Dose: How Accurate Is the Varian Delivery System?

Abstract

Listen

Translate article

Purpose: The prescribed dose of an IMRT treatment plan may need to be scaled without changing any other parameters of the plan (e.g. relative fluence‐maps). Examples include changing the prescription as ordered by the physician, or rescaling the dose to avoid overexposure of a dosimeter. In these instances, it may be advantageous and most efficient to rescale the dose without recalculating fluence maps and leaf‐sequencing, as is possible with the Varian delivery system. This work investigates the dosimetric consequences of such dose‐scaling. Method and Materials: A complex 9‐field IMRT treatment plan was created in the Eclipse planning system to meet the RPC head‐and‐neck IMRT credentialing test, which prescribes a dose of 6.6Gy to the primary PTV. A modified RPC head phantom incorporating a PRESAGE 3D dosimeter required a maximum dose of 4Gy to the PTV to avoid over exposure. The prescription dose was correspondingly scaled in the Eclipse system without changing any other aspects of the plan. Other plans were also created scaling the prescription dose to deliver 10 Gy, 4 Gy, 3 Gy, 2 Gy and 1 Gy to the primary PTV. These plans were delivered to the MapCHECK QA device to determine dosimetric errors. DynaLog files were collected to investigate changes in leaf positioning errors and beam hold‐offs. Results: As the prescription dose was decreased, the number of beam hold‐offs and the error in leaf position increased. Reducing the prescription dose by a factor of 6.6 resulted in an average leaf position error of 0.5 mm and caused 17.5% of MapCHECK diodes to fail a relative dose difference threshold of 0.3% with a maximum relative dose difference of 2.2%. Conclusion: These results indicate that scaling the dose prescription by <40% has a negligible effect on dosimetric measurements. However, reducing the prescription dose by >40% can introduce significant errors to measurements.