Proposal of a Clinical Protocol for the Implementation of Magic-f type or Equivalent GEL Dosimeter to Perform RTP Clinical Checks

When we receive a breast patient after mastectomy with an expandable prosthesis containing a valve with a rare-earth metal magnet a special protocol has to be designed. The magnet is used to guide the surgeon to the right spot to inject small quantities of saline solution to create a cavity that will be filled with a breast prosthesis in a restorative surgery. The problem is that during this period, the patient has also to undergo a radiotherapy treatment and the presence of the metal is a complicating factor. Several groups have being discussing this problem, by means of dosimetry with different detectors and Monte Carlo simulations. The results are not conclusive. We want to suggest the use of gel dosimetry, to help the understanding of this interface problem. Two phantoms made with glass were prepared according to a proposed design. At their bottom, tissue expander (SILIMED/470) was inserted yielding a 1.1 L and 0.89 L phantoms, both filled with Magic-f gel, that was prepared according to the published data. The phantoms were taken to a CT-scanner Brilliance Big Bore, (Philips Medical Systems, USA). Treatment planning using two opposed tangential asymmetric beams with a dose of 2 Gy at isocenter, for 15 MV (1.1 L) and 6 MV (0.89 L). The phantoms were irradiated in a Siemens Oncor linac. The relaxation rate (R2=1/T2) maps that are proportional to the dose were obtained by MRI imaging (Philips, Achieva, 3 T) of the phantoms. Results will be discussed.