1. A method for treating a soft tissue organ, tumor or tumor bed of a patient with a radiation beam, using a beam delivery setup, wherein a prescribed dose is delivered in one or more fractions, comprising the steps of a) inserting one or more markers into soft tissue of the organ, tumor or tumor bed, the one or more markers having a proximal end, a distal end and continuous intervening length, the one or more markers configured to be held by the soft tissue and being visible under at least one imaging modality, the one or more markers markers comprising a single flexible wire in the form of a helical coil which is substantially linear just after insertion, the helical coil having a length and flexibility along the longitudinal axis of the marker and flexibility transverse to the longitudinal axis which flexibilities are effective to permit the marker to be responsive to changes of shape of the organ, tumor or tumor bed and to follow movements of the organ, tumor, or tumor bed after insertion into the soft tissue and while implanted in a patient; b) positioning the patient for receiving the radiation beam; c) forming one or more images of the one or more markers under one or more of the imaging modalities selected from the group consisting of x-ray, radiography, CT scan, ultrasonography, and fluoroscopy; d) inferring position, shape and change of shape of the organ, tumor or tumor bed from the one or more images; e) adjusting patient position and/or radiation beam delivery setup in order to direct the radiation beam towards the organ, tumor or tumor bed; f) delivering one of the treatment beam fractions; and g) repeating steps (b) to (f) until the prescribed dose is reached, wherein the marker comprising a non-radioactive helical coil having a length, diameter, a proximal end, a distal end, the coil configured for insertion into soft tissue inside the body and held by the soft tissue, the marker substantially straight just after insertion inside the soft tissue, the interstitial marker having a continuous intervening length, at least a portion of the intervening length of the marker being visible under at least one imaging modality selected from the group consisting of x-ray, radiography, CT scan, ultrasonography, and fluoroscopy, wherein the marker is a single flexible wire in the form of the helical coil having an outer diameter of from about 25 μm to about 2500 μm, a ratio of coil length to coil diameter of 10 or more and a wire diameter of from about 10 μm to about 2500 μm when the wire has a circular cross section or a wire rectangular cross section of from about 10 to about 500 μm, and a ratio of wire cross section width to helical pitch of about 1:1.2±about 0.2, the coil length, wire diameter, ratio of coil length to coil diameter and wire cross section effective for providing flexibility along the longitudinal axis, of the coil and flexibility transverse to the longitudinal axis which flexibilities are effective to permit the marker to be responsive to changes of shape of the organ, tumor or tumor bed and to follow movements of the organ, tumor, or tumor bed after insertion as a substantially straight coil, but after at least one of treatment, natural growth or stresses from the soft tissue while implanted in the mammalian body.