Daniel G. Zhang, Abstract Co-Author: Nothing to Disclose

Vladimir M. Feygelman PhD, Abstract Co-Author: Nothing to Disclose

Eduardo G. Moros PhD, Abstract Co-Author: Nothing to Disclose

Kujtim Latifi, Abstract Co-Author: Nothing to Disclose

Geoffrey Zhang PhD, Presenter: Nothing to Disclose

Gd-containing materials used in MRI imaging as contrast agents for better delineation of target volumes can also potentially serve as radiosensitizers in radiotherapy. This study demonstrates that Gd can be used to enhance radiation dose in target volumes not only in HDR brachytherapy using Ir-192 source, but also in megavoltage external beam radiotherapy.

As MRI is often used in tumor localization for radiotherapy treatment planning, gadolinium materials are often used to enhance the contrast. Motexafin gadolinium, an avid electron acceptor, is also a radiosensitizer, depleting the pool of DNA repair substrates that become unavailable to repair the radiation-induced oxidative damage to DNA. It also increases intracellular oxygen levels, thereby potentially overcoming hypoxia and allowing “fixation” of radiation damage. This Monte Carlo study is the first quantitative investigation of Gd-induced dose enhancement in MV external beam therapy.

BEAMnrc, a radiotherapy Monte Carlo simulation package, was used to calculate dose enhancement as a function of Gd concentration. Published phase space files for the TrueBeam flattening filter free (FFF) and conventional flattened 6MV photon beams were used. High dose rate (HDR) brachytherapy with Ir-192 source was also investigated. The percent mean dose enhancement for the FFF beam is 0.52±0.49, 1.34±0.40, 2.41±0.30, 3.71±0.38 and 4.67±0.41 for Gd concentrations of 1, 5, 10, 15, and 20 mg/mL respectively. The values for the flattened beam are 0.25±0.08, 0.81±0.12, 1.23±0.28, 1.90±0.32, and 2.54±0.24. For Ir-192, they are 0.70±0.39, 2.99±0.30, 5.68±0.23, 8.32±0.20 and 10.9±0.19.  

The mean energies for the 6 MV FFF and flattened beams are 0.9 and 1.3 MeV, respectively. The mean energy difference causes the dose enhancement difference between the two beams. Since the Ir-192 photons have lower yet energy, the photoelectric effect in the presence of Gd leads to even higher dose enhancement in HDR. The Gd concentration in MRI imaging or used as radiosensitizer is usually less than 5 mg/mL. Higher Gd concentration is preferred to have higher dose enhancement.

Zhang, D, Feygelman, V, Moros, E, Latifi, K, Zhang, G, Monte Carlo Study of Radiation Dose Enhancement by Gadolinium in Megavoltage and HDR Radiotherapy.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14010878.html