A Dose Accumulation Assessment of Alignment Errors During Spatially Fractionated Radiation Therapy

INTRODUCTION:Spatially fractionated radiotherapy (SFRT) techniques produce high-dose peaks and low-dose valleys within a tumor. Lattice stereotactic body radiation therapy (SBRT) is a form a SFRT delivered across five fractions. Due to the high spatial dose gradients associated with SFRT, it is critical for fractionated SFRT patients to be aligned correctly for treatment. Here we investigate the dosimetric impact of daily alignment uncertainty through a dose accumulation study. METHODS:Dose accumulation was retrospectively performed for ten patients enrolled on a Phase I trial. Lattice SBRT was completed in 5 fractions with 20Gy prescribed to the entire tumor and a simultaneous integrated boost of 66.7Gy prescribed to a set of regularly spaced high-dose spheres. Daily alignment error was quantified through manually selected landmarks in both the planning computed tomography (CT) scan and daily cone-beam CT. The dosimetric impact of alignment errors was quantified by translating the isocenter in the treatment planning system by the daily average alignment error. Large errors were simulated by translating isocenter 5mm and 10mm for one and two fractions, independently assessing errors in the superior-inferior and axial directions. The reduction of dose gradients was quantified using the dose ratio (DR) of the mean dose in the high-dose and low-dose spheres. RESULTS:The average alignment error was 1.8mm across the patient population resulting in minor smoothing of the high- and low-dose distributions in the dose accumulation. Quantitatively, the DR decreased from 3.42 to 3.32 (p=0.093) in the dose accumulation study. The simulated worst case was an inferior-superior shift of 10mm for two fractions where the average DR decreased to 2.72 (p=0.0001). CONCLUSIONS:The dose accumulation study revealed on average DR only decreased from 3.42 to 3.32. However, setup errors >5mm resulted in larger dosimetric degradation, reflecting a larger impact for individual high dose spheres within regions exhibiting larger displacements.