PULSE LOW DOSE RATE RADIATION THERAPY FOR RE-TREATMENT OF THE SAME DISEASE SITE WHICH NORMAL TISSUES TOLERENCES ARE AT OR NEARLY REACHED

Purpose: As patients live longer with cancer, we have seen many patients come back to us with cancer in the same or an adjacent disease site in near proximity to their previously irradiated disease site. Often times re-irradiation would not be possible because critical structures, such as the spinal cord have been treated to near max dose and cannot receive more radiation. Pulse Low Dose Rate therapy is a method that allows us the ability to control tumor growth, provide necessary pain relief in a metastatic disease site and not cause a catastrophic normal tissue side effect to an area that was previously treated with a critical structure that has reached what would be considered normal tissue dose tolerance.

Methodology: Low Dose radiation less has been shown to allow normal tissue repair and is more pronounced in dose rates of 1-100cGy/min. Tumor cells have shown a hypersensitivity to radiation at doses < 0.3Gy. These phenomena can be achieved by dividing the number of radiation beams into pulses with each individual beam delivering less than the tumor transition dose but greater than the normal tissue transition dose ensuring radiation repair is activated in normal tissues but not in tumor cells.

Results: Our experience is to primarily plan using volumetric modulated arc therapy planning techniques, although IMRT and 3-d radiation therapy beams are also possible. In order to plan and meet the necessary constraints for pulse low dose rate therapy we must first optimize on the simulation CT scan with a contoured planning target volume using only two beams initially. During the planning process meeting these constraints can be somewhat challenging and prioritizing target coverage and getting a homogenous dose distribution is the top priority. Once the planning metrics are met, we then duplicate the beams until there are a total of ten with each beam delivering less than 40cGy max to the planning target volume and a mean of 20cGy per beam pulse delivered with ten beam pulses.

Conclusions: Treating patients that have been previously treated with external beam radiation therapy with normal tissue structures that are at or nearly at radiation tolerance, and are in need of further treatment due to a local recurrence or metastatic cancer in an adjacent area are now able to receive another course of radiation using pulse low dose rate therapy, which previously this was not possible without completely sparing the normal tissue structure, which also means sparing the recurrent tumor as well. This therapy has shown to be effective for normal tissue repair as well as cancer cell kill, which allows us to re-irradiate disease sites in close proximity to the initial disease site and hopefully extend the life of the patient while offering pain relief as well.

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