Predictive Parameters of CyberKnife Fiducial-less (XSight Lung) Applicability for Treatment of Early Non-Small Cell Lung Cancer: A Single-Center Experience

doi:10.1016/j.ijrobp.2013.06.2048

International Journal of Radiation OncologyBiologyPhysics

Volume 87, Issue 3, 1 November 2013, Pages 583-589

https://doi.org/10.1016/j.ijrobp.2013.06.2048 Get rights and content

Purpose

To determine which parameters allow for CyberKnife fiducial-less tumor tracking in stereotactic body radiation therapy (SBRT) for early-stage non-small cell lung cancer.

Methods and Materials

A total of 133 lung SBRT patients were preselected for direct soft-tissue tracking based on manufacturer recommendations (peripherally located tumors ≥1.5 cm with a dense appearance) and staff experience. Patients underwent a tumor visualization test to verify adequate detection by the tracking system (orthogonal radiographs). An analysis of potential predictors of successful tumor tracking was conducted looking at: tumor stage, size, histology, tumor projection on the vertebral column or mediastinum, distance to the diaphragm, lung-to-soft tissue ratio, and patient body mass index.

Results

Tumor visualization was satisfactory for 88 patients (66%) and unsatisfactory for 45 patients (34%). Median time to treatment start was 6 days in the success group (range, 2-18 days) and 15 days (range, 3-63 days) in the failure group. A stage T2 (P=.04), larger tumor size (volume of 15.3 cm³ vs 6.5 cm³ in success and failure group, respectively) (P<.0001), and higher tumor density (0.86 g/cm³ vs 0.79 g/cm³) were predictive of adequate detection. There was a 63% decrease in failure risk with every 1-cm increase in maximum tumor dimension (relative risk for failure = 0.37, CI=0.23-0.60, P=.001). A diameter of 3.6 cm predicted a success probability of 80%. Histology, lung-to-soft tissue ratio, distance to diaphragm, patient's body mass index, and tumor projection on vertebral column and mediastinum were not found to be predictive of success.

Conclusions

Tumor size, volume, and density were the most predictive factors of a successful XSight Lung tumor tracking. Tumors >3.5 cm have ≥80% chance of being adequately visualized and therefore should all be considered for direct tumor tracking.

Introduction

In recent years, stereotactic body radiation therapy (SBRT) has been adopted as a standard treatment option in the management of inoperable early stage non-small cell lung cancer (NSCLC). Several studies 1, 2, 3 have shown excellent local control rates between 90% and 97% at 2-3 years' follow-up with minimal associated toxicities.

Lung SBRT is particularly challenging because of the constant respiratory motion. The CyberKnife (CK) is a noninvasive robotic system that allows delivery of SBRT using precise, near real-time image-guided tracking of a moving target. Three image guidance protocols are currently used in our center: Synchrony tracking of fiducial markers placed in proximity to the tumor, XSight lung tracking method that allows for direct tumor visualization—obviating the need for fiducial markers 4, 5, and the XSight spine method that tracks adjacent vertebrae.

There are several limitations to the use of fiducial markers, including the risk of pneumothorax after percutaneous positioning, fiducial marker misplacement, and/or migration and associated costs and treatment delays (6). Gold seed fiducial markers are most often placed percutaneously under computed tomography (CT) guidance by a radiologist. This procedure has been reported to carry a risk of pneumothorax between 13% and 45% 1, 2, 7, 8. In addition, a minimum of 3 markers should be implanted in or near the tumor in a specific geometric configuration. Fiducial marker misplacement and/or migration before or during treatment are frequent and may compromise tracking accuracy (6). On the other hand, the XSight spine technique does not track the movement of the tumor during treatment; therefore, it requires a larger treatment volume (internal target volume [ITV]) that accounts for the tumor location during the different cycles of the respiratory phase.

The recently developed XSight lung 5, 9 tracking system attempts to overcome these limitations. It allows for direct soft-tissue tracking by detecting and matching intensity pattern variation in the tumor 4, 6, 7. However, it requires adequate tumor visualization by the system.

XSight lung applicability is determined after a multistep process that includes: a 4-dimensional (4D) planning CT scan, a treatment simulation session, and a tumor visualization test. In the case of unsatisfactory visualization, patients must be considered for an alternate tracking method (fiducial markers or XSight spine) or an alternate treatment system capable of SBRT delivery (in our case, a c-arm linear accelerator with volumetric intensity modulated radiation therapy or a dedicated helical tomotherapy device). When the XSight lung test fails, the planning CT scan most often will need to be repeated on another day, causing additional treatment delay and resource utilization.

The aim of this study was to determine which parameters, available at the time of initial consultation, may predict satisfactory tumor visualization. Such predictions should help radiation-oncologists (ROs) make cost-effective decisions when orienting their patients toward an SBRT treatment method.

Section snippets

Patients

This is a retrospective analysis of 133 patients considered for stereotactic treatment using the XSight lung system at our institution between August 2009 and February 2012. Approval by our institutional board of ethics was obtained.

XSight lung tracking

CyberKnife version 8.5 and treatment planning system version 3.5 were used for this study. Gross tumor volume (GTV) was delineated using noncontrast planning CT scan with window width of 1600 Hounsfield units (HU) and window level of −600 HU as recommended by the

Patients' characteristics

One hundred thirty-three patients were considered for XSight lung treatment of a total of 215 patients (62%) (Table 1). Tumors were stage T1 for 88 (66%) patients and stage T2 in 45 patients (34%). Mean tumor volume was 12.2 cm³ (median, 8 cm³) and mean dimension was 2.6 cm (median, 2.7 cm), 2.7 cm (median, 2.7 cm), and 2.6 cm (median, 2.5 cm) in craniocaudal (CC), AP, and RL dimensions, respectively. Twenty-five patients (19%) had no histopathological diagnosis for the following reasons:

Discussion

This is the largest series reporting the experience with the XSight lung tumor tracking and the first study looking at the predictive factors for its applicability. When initially launched in 2007, it was estimated that the XSight lung was applicable for about 30% of patients considered for CK SBRT (6). Initial guidance from the manufacturer, based on image data analysis from more than 50 patients previously treated with CK, recommended XSight lung for peripherally located tumors of more than

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Physics ContributionPredictive Parameters of CyberKnife Fiducial-less (XSight Lung) Applicability for Treatment of Early Non-Small Cell Lung Cancer: A Single-Center Experience

Purpose

Methods and Materials

Results

Conclusions

Introduction

Section snippets

Patients

XSight lung tracking

Patients' characteristics

Discussion

Clin Lung Cancer

Radiother Oncol

J Thorac Cardiovasc Surg

Ann Thor Surg

Cancer Radiother

Physics Contribution
Predictive Parameters of CyberKnife Fiducial-less (XSight Lung) Applicability for Treatment of Early Non-Small Cell Lung Cancer: A Single-Center Experience