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Pulsed Low Dose Rate Radiotherapy Using Advanced Treatment Methods: A Novel Technique for Patient Re-Irradiation

M Lin*, R Price, S Koren, J Li, C Ma, Fox Chase Cancer Center, PHILADELPHIA, PA

SU-E-T-383 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

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.

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