Room: AAPM ePoster Library
Purpose: The Pulsed Low Dose Rate (PLDR) technique uses a 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 Volumetric Modulated Arc Therapy (VMAT). Often Image Guided Radiation Therapy (IGRT) is used to ensure accuracy of the delivery. Since PLDR patients are in the treatment position for 20-30 minutes, a review of the dosimetric changes from intra-pulse motion after the initial IGRT for these patients, were performed to determine suggested robustness values.
Methods: VMAT PLDR delivery consists of 5 separate 1-2 arcs delivered as a pulse, with a 3-minute pause between pulses. IGRT was employed before pulses 2 and 4 with a realignment tolerance of 3 mm. Retrospective re-planning of two cranial and one head and neck PLDR cases were performed using documented shift data. Each fraction was adjusted for the max shifts based on the IGRT before pulses 2 and 4. These plans were compared to the original.
Results: The maximum shift for all cases was 7mm, which is more than double the tolerance of 3mm. All cases had fractions where shifts exceeded the CTV to PTV expansion, leading to a reduction in CTV coverage for that fraction. The TCP re-planned is lower than the TCP planned for the PLDR cases. This in turn reduces the expected increase in TCP from the PLDR delivery.
Conclusion: Our evaluation of the intra-pulse motion shows that during PLDR delivery, the deviation from the original plan coordinates are greater than our institutional tolerance. 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 PLDR delivery. This evaluation proves the necessity of using robustness in treatment planning of PLDR cases.