SU-E-T-383: Pulsed Low Dose Rate Radiotherapy Using Advanced Treatment Methods: A Novel Technique for Patient Re-Irradiation.

PURPOSE

Pulsed low dose-rate radiotherapy (PLDR) re-irradiation has the potential to reduce late normal tissue toxicity while still providing significant tumor control for recurrent cancers. In contrast to conventional treatments delivered at dose-rates of 400-600cGy/min, PLDR treatments deliver 20cGy pulses separated by 3-minute intervals to achieve an effective-dose-rate of 6.7cGy/min. This work aims to investigate the planning strategy and delivery quality of PLDR treatment using IMRT and RapidArc techniques.

METHODS

Twenty cases (10 treated with PLDR IMRT, 10 for evaluation purposes) were recruited in this study including prostate, pancreas, lung, head-and-neck, breast and pelvis. IMRT and the RapidArc treatment plans were generated using the Eclipse TPS. For IMRT treatment, each plan consisted of 10 fields to achieve a daily dose of 200cGy. The breast IMRT and the RapidArc plans consisted of two fields/arcs, respectively (40cGy/plan) and were delivered 5 times. The dose contribution from each field to the planning target volume (PTV) was analyzed to evaluate the feasibility for PLDR treatment. Machine-operation- dose-rate and plan quality was also investigated. Dose delivery accuracy was assessed using a cylindrical diode array.

RESULTS

Throughout the six treatment sites, the mean PTV dose ranged from 16.1 to 26.1cGy/arc for RapidArc plans and 10.3 to 36.7cGy/field for IMRT plans. For IMRT, the PTV dose contribution from each field strongly depends on the beam arrangement and optimization parameters. With very low dose for a full rotation (∼ 20cGy/arc), the machine-operation-dose-rate of RapidArc plans significantly affects plan quality and deliverability. A machine-operation-dose-rate of 100 MU/min results in superior delivery accuracy (>97.7% gamma-passing-rate for 3%/3mm criteria) for both IMRT and RapidArc plans.

CONCLUSIONS

PLDR radiotherapy using IMRT and RapidArc techniques Result in both dosimetric and radiobiological benefits, which may have great potential for those previously-irradiated patients who have historically done poorly.