Room: ePoster Forums
Purpose: Both low-dose hypersensitivity for tumor cells and increased normal tissue repair for low-dose-rate irradiation have been explored in pulsed low-dose-rate (PLDR) radiotherapy. This work evaluates treatment planning criteria for PLDR radiotherapy using advanced delivery techniques.
Methods: IMRT and VMAT have been used for PLDR treatments of recurrent beast, pancreas, prostate, head and neck, and lung cancers and concurrent chemo-radiation prior to surgery for esophageal patients. PLDR plans were generated using the Varian Eclipse system with 6, 10 and 15MV photon beams for 60 patients. Each plan consisted of either 10 gantry angles for IMRT or two arcs for VMAT to achieve a daily dose of 2Gy in 10 sub-fractions (pulses). The dosimetry requirement was to deliver 20cGy/pulse with a 3min interval to achieve an effective dose rate of 6.7cGy/min. The maximum dose to the planning target volume (PTV) is limited to 0.4Gy/pulse considering beam attenuation/scattering and intensity modulation. This was achieved by avoiding beam angles that go through critical structures and increasing the priority of target dose uniformity. The maximum and minimum doses to the PTV were analyzed together with the dose volume histograms and isodose distributions.
Results: For most cases, especially those centrally located tumors, VMAT provided superior dose distributions with PTV doses between 8.56 and 31.2cGy/arc for breast, 12.9 and 27.5cGy/arc for pancreas, 12.6 and 28.3cGy/arc for prostate, 12.1 and 30.4cGy/arc for H&N, and 16.2 and 27.6cGy/arc for lung, respectively. IMRT provided more conformal overall dose distributions than VMAT for large, irregular or peripheral tumors with minimum doses above 8.2cGy and maximum doses under 0.4Gy/pulse.
Conclusion: Advanced PLDR techniques can provide superior target coverage and normal tissue sparing for late-stage, recurrent patients who have no alternative treatment options. Preliminary results of two phase-I trials showed excellent clinical outcomes (good local control with acceptable toxicities).