Accuracy and sensitivity of four‐dimensional dose calculation to systematic motion variability in stereotatic body radiotherapy (SBRT) for lung cancer

Abstract

The dynamic movement of radiation beam in real‐time tumor tracking may cause overdosing to critical organs surrounding the target. The primary objective of this study was to verify the accuracy of the 4D planning module incorporated in CyberKnife treatment planning system. The secondary objective was to evaluate the error that may occur in the case of a systematic change of motion pattern. Measurements were made using a rigid thorax phantom. Target motion was simulated with two waveforms (sin and cos4) of different amplitude and frequency. Inversely optimized dose distributions were calculated in the CyberKnife treatment planning system using the 4D Monte Carlo dose calculation algorithm. Each plan was delivered to the phantom assuming (1) reproducible target motion, and (2) systematic change of target motion pattern. The accuracy of 4D dose calculation algorithm was assessed using GAFCHROMIC EBT2 films based on 5%/3 mm γ criteria. Treatment plans were considered acceptable if the percentage of pixels passing the 5%/3 mm γ criteria was greater than 90%. The mean percentages of pixels passing were 95% for the target and 91% for the static off‐target structure, respectively, with reproducible target motion. When systematic changes of the motion pattern were introduced during treatment delivery, the mean percentages of pixels passing decreased significantly in the off‐target films (48%; p < 0.05), but did not change significantly in the target films (92%; p=0.324) compared to results of reproducible target motion. These results suggest that the accuracy of 4D dose calculation, particularly in off‐target stationary structure, is strongly tied to the reproducibility of target motion and that the solutions of 4D planning do not reflect the clinical nature of nonreproducible target motion generally.PACS numbers: 87.53.Ly, 87.55.km

Quantifying variability of intrafractional target motion in stereotactic body radiotherapy for lung cancers

“…( 2 , 22 ) It has been demonstrated that target tracking techniques such as dynamic MLC tracking, robotic respiratory tracking, and tumor trailing strategy based on the moving average algorithm have the ability to adapt the baseline variation, but such adaptation is hardly reflected in the 4D dose calculation. Recently, Chan et al (23) have demonstrated that the sensitivity of 4D dose calculation to the variation of target motion differed by organs of interest in target‐tracking SBRT, with the greatest dosimetric errors observed in structures that were static or moved asynchronously with respect to the tracking object. Since planning risk volumes (PRV) have the limitations in serially functioning organs such as the spinal cord, (24) we suggest to simulate the intrafractional (and also interfractional) baseline drifts using the population data during treatment planning to avoid potential risks of overdosing these critical organs.…”

Feasibility study of robotic hypofractionated lung radiotherapy by individualized internal target volume and XSight Spine Tracking: A preliminary dosimetric evaluation

“…[15] On the contrary, our group has performed experimental investigations of the intrafractional target motion for the CyberKnife focusing on the RTS. [16,17] While it is hypothesized that XST based lung tumor treatment may be beneficial for a subset of patients who are medically inoperable, unsuitable for invasive fiducial implantation and whose tumors are not visible on the x-ray tracking system or attached to the vertebral column with limited motion range, experimental evaluations of this delivery technique have never been reported despite its increasing clinical applications. [18][19][20] In this study, we aimed to evaluate the adequacy of using the XLS-based strategy by studying the dose delivered to a moving tumor.…”

Dosimetric Effect of Intrafraction Tumor Motion in Lung Stereotactic Body Radiotherapy Using CyberKnife Static Tracking System

“…14 Chan et al have performed experimental investigations of the intrafractional target motion during CyberKnife treatment, focusing on real-time tracking and XST. [15][16][17] However, the influence of beam sizes and that of the interplay effect have not been experimentally studied.…”