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

Abstract

In lung stereotactic body radiotherapy (SBRT), variability of intrafractional target motion can negate the potential benefits of four‐dimensional (4D) treatment planning that aims to account for the dosimetric impacts of organ motion. This study used tumor motion data obtained from CyberKnife SBRT treatments to quantify the reproducibility of probability motion function (pmf) of 37 lung tumors. The reproducibility of pmf was analyzed with and without subtracting the intrafractional baseline drift from the original motion data. Statistics of intrafractional tumor motion including baseline drift, target motion amplitude and period, were also calculated. The target motion amplitude significantly correlates with variations (1 SD) of motion amplitude and baseline drift. Significant correlation between treatment time and variations (1 SD) of motion amplitude was observed in anterior‐posterior (AP) motion, but not in craniocaudal (CC) and left‐right (LR) motion. The magnitude of AP and LR baseline drifts significantly depend on the treatment time, while the CC baseline drift does not. The reproducibility of pmf as a function of time can be well described by a two‐exponential function with a fast and slow component. The reproducibility of pmf is over 60% for the CC motion and over 50% for the AP and LR motions when baseline variations were subtracted from the original motion data. It decreases to just over 30% for the CC motion and about 20% for the AP and LR motion, otherwise. 4D planning has obvious limitations due to variability of intrafractional target motion. To account for potential risks of overdosing critical organs, it is important to simulate the dosimetric impacts of intra‐ and interfractional baseline drift using population statistics obtained from SBRT treatments.PACS number: 87.55.‐x

Retrospective evaluation of CTV to PTV margins using CyberKnife in patients with thoracic tumors

“…Indeed, since both parameters seem to have a small standard deviation, a patient parameter can be obtained to estimate the range of movement. The results seem to be similar to those obtained by previous authors for the range of movement 4 , 6 . The median and mean values excluding the 5% smaller and 95% higher values were similar to the ${\mathrm{R}}_{\mathrm{m}}$ in each fraction, with differences lower than 1 mm in most situations (results not shown).…”

“…Mean cycle‐to‐cycle motion amplitude ± standard deviation was $6.3\pm 5.2\text{mm}$ (SI direction), $1.2\pm 1.0\text{mm}$ (LAT direction), and $2.3\pm 2.1\text{mm}$ (AP direction) — that is, close to the means of $6.0\pm 4.6\text{mm}$ and $5.0\pm 1.6\text{mm}$ reported by Chan et al (4) and Suh et al, (6) respectively, in the craniocaudal axis. Since interfraction variation of ${\mathrm{R}}_{\mathrm{m}}$ and ${\mathrm{R}}_{95\%}$ is under 2 mm in most patients, it seems that, although the movement for a thoracic lesion changes during a fraction, some parameters for each fraction remain stable, with a small standard deviation.…”

“…This motion is not predictable by location, 5 , 36 as we observed in specific cases. Although intrafraction motion has been demonstrated, 4 , 6 , 27 we observed that ${\mathrm{R}}_{\mathrm{m}}$ and ${\mathrm{R}}_{95\%}$ were stable estimators during a treatment for each patient. The collaborative CT scan could be used as a basis to obtain a general “feel” for the ${\mathrm{R}}_{95\%}$ peak‐to‐peak amplitude of breathing motion for a specific patient, to eliminate obvious seeds not correlated with GTV movement, and to determine an estimated upper limit for each step by basing Step 1 on ${\text{CoM}}_{\text{CT}}$ and tumor position distance and Steps 3 and 4 in the association between uncertainties and range movement (Fig.…”

“…Although this step is not independent of Step 2, we considered them independent and conservatively assumed that all the uncertainty for this step was due to changes in the respiratory cycle from the external markers, variations in patient respiratory cycles, correlation between skin and internal fiducials, and unknown sources 4 , 5 , 6 , 29 , 30 . We assumed that the variation in the seed configuration during treatment had no effect on the correlation model.…”

“…During recent decades, understanding the movement of the tumor in the thorax and its reproducibility during treatment has been an area of concern 1 , 2 , 3 , 4 , 5 , 6 . A variety of methods have been proposed to manage respiratory motion, (1) and the margins necessary to compensate for geometric uncertainties have been reduced, depending on the technique 7 , 8 .…”