In order to ensure safe patient treatment, every radiation therapy treatment plan has to be verified for each patient by a dose measurement. Usually these verifications are performed as point or planar dose measurements in phantoms with simplified geometrical shapes. A more realistic approach are 3D printed heads, based on the geometry of a patient, as seen on a CT scan. These heads have an insert for an ionization chamber, film pieces and dosimetry gel. The gel inserts allow to perform three-dimensional (3D) dose measurements since they contain a monomer gel, which polymerizes when interacting with ionizing radiation and thereby changes its signal in an MR scanner.
In this project (funded by the FöFoLe commission of the Medical Faculty of LMU Munich) three such heads are used to evaluate the influence of the underlying image data on the process of treatment plan generation. The image data typically acquired in radiation therapy are the initial computed tomography (CT) images needed for treatment planning, the in-room cone-beam computed tomography (CBCT) images acquired with an X-ray imager mounted at the treatment device and the pre-treatment magnetic resonance (MR) images. After a conversion of the MR images into pseudoCT images and an intensity correction of the CBCT images proton treatment plans are optimized with the same fixed constraints on all three image sets. These plans are then irradiated on different gel inserts in the same 3D printed head to evaluate differences in the dose distributions originating from the differences in the image sets used for planning.
This collaborative project is a continuation of work initiated at the Department of Medical Physics of the Faculty of Physics of the LMU Munich, which has an established collaboration with the phantom manufacturer RTsafe, and where gel dosimetry was proved valuable to establish proton range in 3D.
- Hillbrand M, Landry G, Ebert S, Dedes G, Pappas E, Kalaitzakis G, Kurz C, Würl M, Englbrecht F, Dietrich O, Makris D, Pappas, E, Parodi, K. Gel dosimetry for three dimensional proton range measurements in anthropomorphic geometries. Zeitschrift für Medizinische Physik. 2019 May 1;29(2):162-72.