ESTRO 36 Abstract Book

S418 ESTRO 36 2017 _______________________________________________________________________________________________

S. Wegener 1 , A. Spiering 1 , O.A. Sauer 1 1 University Hospital, Radiation Oncology, Würzburg, Germany Purpose or Objective Modern radiation therapy aims to minimize negative side effects on healthy tissue by tailoring the dose distribution as accurately as possible to each individual tumor. This leads to a progressively increasing complexity of the treatment plans and demands a very high precision of all involved components. Even small errors can significantly compromise treatment techniques which require such an extensive precision as stereotactic radiation therapy. A suitable quality management for such techniques should include a regular end-to-end test that closely mimics the entire procedure of the actual patient treatment while being able to reliably detect a variety of possible errors e.g. in calculation, positioning and movement, spatial precision and absolute dose application. We present a test that was introduced into the clinical workflow and evaluated its sensitivity to those errors. Material and Methods Prior to the irradiation, a custom-built phantom insert for the ArcCHECK (Sun Nuclear, USA) allowed for automatic registration of the cone beam CT to reference data. A 12- field plan including gantry and table rotations targeting a spherical volume of approx. 2 cm diameter was measured weekly using a Synergy accelerator with an Agility MLC (Elekta, Sweden). Signals were obtained from all diodes along the cylinder surface of the ArcCHECK and additional dose was measured with an ionization chamber in the phantom center. For each measurement the plan was compared to the calculation of the treatment planning system via gamma evaluation and every diode reading was compared to the averaged diode readings from previous weeks. Additionally, errors were induced to test the sensitivity for phantom malposition, machine geometry problems and MLC positional inaccuracies. Results Due to the phantom set up according to the cone beam CT registration, the measurements were very reproducible without any observable user-to-user differences. The typical dose map for the diode cylinder is shown in fig. 1. For all diodes, mean values with small standard deviations were obtained from many consecutive measurements. Any diode deviation observed for the correct application of the test plan never exceeded three standard deviations, while much larger discrepancies could be detected for all induced errors (example: fig. 2).

Results The measured absolute dose rates agreed with the values quoted in the calibration certificates of the plaques within the experimental uncertainty, with typical differences below 5%. The relative standard uncertainties obtained were of 3.8% for dose distributions measured at planes perpendicular to the symmetry axis at 5 mm from the surface of the plaque, and of 7.4% for planes containing the symmetry axis. These values are comparable to those reported by other authors using plastic phantoms, but avoiding the uncertainties associated to the conversion from dose–to–plastic to dose–to–water. A good agreement was obtained between measurements and simulations, improving upon published data (see figures for data of depth-dose curves, and lateral profiles at 5 mm from the surface of the plaque, for the CCX plaques).

Conclusion We developed a practical experimental method to measure with the EBT3 radiochromic film the dose distributions in water produced by 106 Ru/ 106 Rh ophthalmic plaques. The obtained results were of similar or better quality than those obtained using solid phantoms. These setups may ease the quality assurance procedures to the users of these plaques. PO-0794 Comprehensive quality assurance test for high precision teletherapy