Recurrent Sarcoma: Given the patient's history of radiation to the back for sarcoma, a new neck mass could be a recurrence of the primary tumor, which has metastasized to the neck. The history of radiation therapy increases the likelihood of this diagnosis.

Radiation-Induced Fibrosis: Radiation therapy can cause fibrosis in the treated area, potentially leading to a palpable mass. This condition is a common sequela of radiation therapy and could explain the new neck mass.
Benign Neck Mass (e.g., Cyst, Lipoma): Benign masses such as cysts or lipomas are common in the neck and could present as a new mass, especially if they have grown recently or become more noticeable.
Lymphadenopathy due to Infection: Although the labs are normal, an infectious cause of lymphadenopathy (enlargement of the lymph nodes) could still be considered, especially if the patient has been exposed to or has symptoms of an infection.

Metastatic Disease from Another Primary Site: It is crucial not to miss the possibility of a new primary malignancy that has metastasized to the neck. This could include cancers such as lung, breast, or thyroid cancer, among others. Early detection is key for treatment and prognosis.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy, including in the neck. Given the potential for serious consequences if missed, lymphoma must be considered in the differential diagnosis.

Branchial Cleft Cyst: These congenital anomalies can present at any age, including adulthood, and may become more noticeable due to infection or other factors.
Carotid Body Tumor: A rare tumor of the carotid body, which could present as a neck mass. Although uncommon, it is an important consideration in the differential diagnosis of a neck mass.
Castleman Disease: A rare disorder that can cause lymphadenopathy, which might be considered if other diagnoses are ruled out and the clinical presentation is suggestive.