WASHINGTON (JTA) — More than 500 rabbis urged the release of Alan Gross, citing the possibility that he has a cancerous growth.

“We would urge your government to release Mr. Gross on humanitarian grounds,” said the letter sent Sunday to Cuban President Raul Castro. “Alternatively, if despite his and his family’s suffering over the past three years in prison you remain determined to detain him, we would urge you to allow a doctor of his choosing to evaluate and treat him for whatever medical conditions that he may have.”

The letter cites an assessment last month of Gross’ Cuban medical records by Dr. Alan Cohen, a radiologist in Bethesda, Md., that said what Cuban doctors identified as a hematoma behind Gross’ right shoulder is likely a tumor.

“A soft tissue mass in an adult who has lost considerable weight must be assumed to represent a malignant tumor unless proven to be benign,” Cohen wrote.

Most of the rabbis signing the letter are American and represent all streams, including leaders of religious umbrella bodies. A number of the rabbis are from Israel and Canada.

Gross, 63, of Potomac, Md., was sentenced last year to 15 years in prison for “crimes against the state.” He was arrested in 2009 for allegedly bringing satellite phones and computer equipment to members of Cuba’s Jewish community while working as a contractor for the US Agency on International Development.

Gross’ wife, Judy, was due Sunday to join a rally for his release to take place outside a performance in West Palm Beach, Fla., by the Cuban Symphony Orchestra.