A Pilot Feasibility Trial of Diffusing Alpha-emitter Radiation Therapy (DaRT) for Malignant Skin and Superficial Soft Tissue Tumors

Diffusing Alpha-emitter Radiation Therapy is a form of interstitial brachytherapy which may be effective in treating malignant skin and superficial soft tissue tumors for several reasons. First, alpha-emitting radionuclides have a high linear energy transfer (LET), which produces a dense track of ionization events within cells and DNA, which produce complex DNA damage and more effective cell kill than more sparsely ionizing forms of radiation, such as gamma-rays or x-rays. Second, alpha-emitting radionuclides exhibit a higher relative biologic effect (RBE) compared to gamma-rays or x-rays, due to the relative insensitivity to cancer cellular radiosensitivity associated with cell cycling. Finally, alpha-emitting radionuclides are relatively insensitive to hypoxia, which is common as malignant skin and superficial soft tissue tumors become larger (Hall 2000).

Alpha particles have been considered unsuitable for treatment of solid tumors, since no practical method to effectively irradiate a macroscopic tumor with these short-range particles (penetration range in human tissue of ≤0.1 mm) has been found.

The "Diffusing Alpha-emitter Radiation Therapy (DaRT)", based on the interstitial intratumoral placement of an encapsulated Radium-224 source (3.7 days half-life), is described in this study. These sources release short-lived alpha-emitting atoms into the tumor microenvironment by recoil. These atoms are dispersed in the immediate surrounding tumor microenvironment both by convection through tumor interstitial fluid and by thermal diffusion. The decay of Radium-224 in the capsule results in the release of the short-lived (half-life of about 1 minute) radioisotope Radon-220. Radon-220 migrates in the tumor microenvironment until it decays. This is followed by the decay of its daughter radioisotope, Polonium-216. Lead-212, the result of this last decay, gives rise to Bismuth-212, which emits yet another alpha particle. The result of these decay events is the release of alpha particles (Kelson 1994 and Kelson 1995). DaRT will be used in this study for the treatment of malignant skin and superficial soft tissue tumors.

This study has been designed to determine the feasibility of performing the procedure required to deliver radiotherapy to the tumor. At the same time, the safety of the treatment, tumor response, radiation safety, device placement stability, and patient reported quality of life, and molecular and histologic effects on the tumor will be assessed.

The DaRT sources will be inserted using preplanned radiotherapy parameters, with a specified number and size of DaRT sources. Approximately 2-3 weeks after placement of the DaRT sources, the placement of the sources will be reassessed by volumetric imaging, and then they will be removed. Tumor response to DaRT will be assessed periodically 3 months after removal of the device.