Harpell Associates - Radiation Oncology, Medical Physics, Diagnostic Imaging, Nuclear Medicine, PET, Radiology, Ultrasound, Radiation Safety

Matrix Thermoplastic Shield as a Variable Depth Treatment

A “compensation” shield could be fabricated either within the accelerator tray or directly on the patient, depending on mode of therapy. By “stacking” the shielding material, the physicist could create multiple MeV energy levels of treatment to appropriate tumor field without multiple dosing or overdosing a particular area.

For example, our superficial tumor requires a 4MeV beam; however, the associated lymphatic system is depper and requires a 9MeV beam dosage. A full 10cmx10cm thermoplastic shield is constructed so as to reduce a 12MeV beam to our desired 9MeV to the lymphatics: 12-9=3MeV to filter x 1.2mm (shield thickness per 1.0MeV) = 3.6mm thermoplastic shield measuring 10cm x 10cm.

The superficial tumor to receive 4MeV is now receiving only 9MeV due to the 3.6mm filter. The filter can now have 9MeV – 4MeV = 5MeV filtered by adding 5MeV x 1.2mm = 6mm of thermoplastic to area to be treated with the 4MeV beam. Within this 10cm x10cm field, we have treated our tumor with 4MeV and its associated lymphatics with 9MeV in one dosage. Sparing 9MeV – 4MeV = 5MeV = approximately 2.5cm of tissue. This 2.5cm (or one inch) could be critical in spinal, oropharyngeal, esophageal, oral, and reproductive cell considerations. The 2.5cm translates to a volume of 22,500mm3 healthy tissue spared. (30mm x 25mm x 30mm = 22,500mm3)

The matrix shields can be stacked and contoured to compensate for tissue contour and density variables to minimize “overdosing” particular depths. For our illustration on electron energy; 2.4mm Matrix 10mm soft tissue. Matrix can be cut, shaved, milled, or molded for tissue equivalence and contour. The matrix syringe may be helpful in incremental placement of additional material in specific areas. Matrix may be exactly applied to the involved anatomy while in the thermoplastic state and allowed to harden or mounted in the accelerator tray depending upon the mode of clinical treatment.

The tumor treatment field can be transposed from the radiograph to carbon paper, the outline transcribed directly on the manufactured shield and then “hot” cut to tumor configuration, lymphatics involved etc. This direct technique would not employ styrofoam nor mold procedures, as the matrix shield is manufactured in 10cm x 10cm x 5mm sheets.

Abstract:
Matrix Thermoplastic Shield as a Variable Depth Treatment. TECtimes Newsletter.



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