It’s hard to envision gender reassignment surgery as a cosmetic medical procedure. But that’s the IRS’s position in a test case that should determine whether a medically prescribed sex-change qualifies for a federal tax write-off.

The trial in the case is scheduled to begin this week in Boston before Judge Joseph H. Gale of U.S. Tax Court. At issue is the gender reassignment of Rhiannon O’Donnabhain, a 63-year-old Coast Guard veteran and construction worker. O’Donnabhain claimed the write-off on her federal income tax return for 2001 — the year of her sex-change surgeries — after what she describes as years of internal torment over her gender. But the IRS audited and rejected the write-off on the grounds that the operation was a purely cosmetic procedure.

Gay & Lesbian Advocates and Defenders (GLAD), a legal advocacy group based in Boston, is representing O’Donnabhain in her lawsuit against the IRS. In 1996, a therapist diagnosed O’Donnabhain — who had divorced four years earlier after fathering three children — with gender identity disorder, a persistent conflict between a person’s birth sex and gender identity. The condition didn’t improve, and in 2001 she sought surgery on the recommendation of a physician.

After spending about $25,000 for several surgeries, O’Donnabhain claimed the deduction and received her refund check in June 2002. Six months later she was audited.

GLAD says that after an initial indication by an appeals officer that the IRS would allow the deduction, the local IRS office decided to seek a decision from Washington. IRS Chief Counsel Donald L. Korb denied the deduction in October 2005. Arguing that Congress had not signaled any clear intent to include sex change operations as allowable write-offs, he concluded that the expenses O’Donnabhain claimed “were not incurred for treatment of a disfiguring condition arising from a congenital abnormality, personal injury, trauma, or disease” — and therefore her surgery was not deductible.

GLAD attorney Jennifer Levi contends that the finding amounts to a discriminatory double standard. In most cases of surgical deductions, she says, the taxpayer need show only that the costs are “responding to a medical need.”

Still, both Levi and O’Donnabhain say they want the case to make a bigger point than highlighting an alleged inconsistent application of the tax code. Levi says she looks for the case to “clarify the reality of Gender Identity Disorder, and the importance of people being able to access medical care.” O’Donnabhain concurs: “While the money is significant to me, this is really about the underlying principle,” she says. “I hope this case sends a clear message that transgender people deserve dignity, respect and equal treatment.”