Why must a radiation-induced secondary cancer have a different histology from the original tumor?

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linear accelerator; external beam radiation therapy — radiation therapy External beam radiation therapy (also known as external beam teletherapy, or long-distance therapy) being delivered using a machine known as a linear accelerator.

The premise that a radiation-induced secondary cancer must have a different histology is a diagnostic criterion used by clinicians to distinguish a new, treatment-related malignancy from a recurrence or metastasis of the original tumor.

For a malignancy to be classified as radiotherapy-induced, it must meet specific clinical criteria:

Distinct Histology: The secondary tumor must be of a different histological type than the initial cancer. This ensures that the new growth is a primary malignancy induced by the radiation exposure rather than a late recurrence or spread of the original disease.1
Location: The new tumor must arise within the specific field that received the radiation therapy.1
Latency Period: There must be a sufficient interval between the initial treatment and the appearance of the new cancer, typically five or more years, which is the time required for radiation-induced carcinogenesis to occur.1

By requiring a different histology, medical professionals can ethically and accurately differentiate between the long-term toxicities of treatment and the natural progression of the patient's primary diagnosis.1