Room: ePoster Forums
Purpose: Pulsed Low Dose Rate (PLDR) treatment technique uses a normal target dose divided into a series of pulses, each followed by a 3-min pause. Recently, this treatment regimen has been enhanced using advanced planning techniques, specifically Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy(VMAT). These planning techniques often use Image Guided Radiation Therapy(IGRT) to ensure accuracy of treatment delivery. Since the PLDR patients are in the treatment position for 20-30 minutes, a review of intra-pulse motion after the initial IGRT for these patients was performed.
Methods: The IMRT delivery is divided into 10 pulses. IGRT was performed before pulses 1, 4, and 8. VMAT PLDR delivery consist of 5 separate 1-2 IMRT arcs delivered as a pulse with a 3-minute pause between each pulse. IGRT was employed before pulses 2 and 4. The tolerance for patient realignment was 3 mm.
Results: The maximum deviations for IMRT and VMAT were 9.6mm, and 5.6mm, respectively. The IMRT plans maximum deviation was 9.6 mm in the vertical direction. Both deliveries showed a decrease in the 3rd KV versus the 2nd KV imaging session. The VMAT plan DVH comparison between the original recalculated plans using the maximum deviation of only 4.6mm shows a significant difference.
Conclusion: Our evaluation of the intra-pulse motion shows that during PLDR delivery, the deviation from the original plan coordinates are greater than our tolerances of 3 mm. Since the PLDR method relies on accurate coverage of the PTV for each pulse, deviations from accurate delivery will fail to take advantage of the radiobiology of reduced dose rate delivery. By using a minimum of 2 IGRT sessions during the pulse delivery, patient realignments could minimize intra-pulse motion during delivery of PLDR treatment.