(Sunday, 7/29/2018) 1:00 PM - 1:55 PM
Room: Room 209
Purpose: To investigate the best radiation treatment technique for the treatment of the rare case of positive margins on the tenon fascia after resection of mucoepidermoid carcinoma (MEC) of the conjunctiva.
Methods: IMRT, VMAT, double-scattering (DS) proton, and reverse eye plaque low dose rate (LDR) brachytherapy treatment plans were created. The dosimetry comparison was based on tumor coverage and normal tissue limiting dose guidelines. The prescription dose was 66 Gy given in 30 fractions for the external beam plans, and 148 hours for LDR brachytherapy. All plans were normalized to give 100% dose coverage to 95% of the treatment volume. Doses were calculated for the photon, proton and LDR plans using Varian’s AAA, PBC and TG43 based algorithms, respectively.
Results: The target volume proved too large for an eye plaque application. Using the largest plaque available (22mm), the minimum dose to the PTV and skin was 57% and 1000% of the prescription dose, respectively. The proton plan completely spared the left ocular structures, but had a higher dose to the macula than the photon plans and uncertainty in the dose through the nasal cavities was large. IMRT and VMAT plans produced similar outcomes, but VMAT minimized the dose to the ipsilateral eye. Clinical follow-up of the patient during the course of radiation therapy was recorded.
Conclusion: Due to the uniqueness of this case, a thorough study of the available options was deemed necessary. All of the external beam treatment options produced usable plans, though replacement of the right lens will likely be necessary. A reverse eye plaque treatment was not appropriate for this case specifically due to the long and irregular target shape. VMAT treatment is able to wrap the dose around curved structures such as the eyeball particularly well, making it the best treatment for this case.
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