Thank you for reaching out.

The Torah (Vayikra 19:28) explicitly prohibits tattooing, unless there is no medical alternative. Therefore, since it seems like both options would be medically acceptable, it would be preferable to choose the marker.

The mark created by the marker is not a chatzitzah because it has no substance (it is just color absorbed into the skin, not a barrier between your body and the water), and because it will last for a prolonged period during which you want it to be there.

You can read more about cancer and halacha here. We also have an article on tattooing for radiation therapy in the Jewish Women's Health section of our site, adapted for medical professionals, here.

We wish you a successful treatment and a refuah shleimah!