International Journal of Radiation Oncology*Biology*Physics
Physics contributionFour-dimensional CT scans for treatment planning in stereotactic radiotherapy for stage I lung cancer
Introduction
The local control rates after curative radiotherapy in patients with medically inoperable Stage I non–small-cell lung cancer (NSCLC) have been disappointing (1). Even the use of three-dimensional (3D) conformal radiotherapy to doses of up to 70 Gy has failed to improve on these results, with local failure rates still as high as 40–60% at 3 years (2). Approaches for dose-escalation using conventional, hyperfractionated and hypofractionated schemes have been investigated as a means of improving local control. Recent studies have reported unacceptable toxicity with conventionally fractionated doses exceeding 90 Gy (3, 4). In contrast, several Phase I/II studies using hypofractionated stereotactic radiotherapy (SRT) for Stage I NSCLC have shown that high local control rates of 90% or more can be obtained with only limited toxicity, although the follow-up is relatively short in some series (5, 6, 7, 8, 9, 10, 11, 12).
The application of high-dose radiotherapy, however, requires meticulous attention toward ensuring an optimal target definition. Recent studies have shown that individualized (i.e., “patient-based”) margins, as opposed to standard “population-based” margins, are required for radiotherapy of lung tumors (13, 14, 15). Approaches used for determining individualized margins include “slow” computed tomography (CT) scans (16, 17), and the coregistration of expiratory and inspiratory target volumes (13, 18, 19, 20).
When the extracranial SRT program for Stage I NSCLC commenced at our center in April 2003, the option of performing “slow” CT scans on the CT scanner was not available. Therefore, target definition was based on the use of six rapid unmonitored spiral multislice CT (MSCT) scans to generate an internal target volume (ITV), a lengthy approach that requires laborious coregistration. Recently, a 16-slice CT scanner became available, and it allowed for four-dimensional (4D) or respiration-correlated CT scans to be performed. 4DCT scans generate spatial and temporal information on mobility in a single investigation and represent a major breakthrough in imaging for radiotherapy planning (21, 22). In this technique described as retrospective gating, the respiratory waveform is synchronously recorded during CT acquisition, and multiple CT slices are acquired at each table position for at least the duration of one full respiratory cycle (23). This yields CT datasets for 10 phases of the respiratory cycle.
We evaluated the use of 4DCT scans as the sole technique for generating ITVs for peripheral lung tumors and compared this with target volumes generated using our routine six-scan approach. The dosimetric consequences of treatment planning using both methods of target definition were evaluated, and compared with the use of standard planning target volumes (PTVs).
Section snippets
Methods and materials
Ten consecutive patients with peripheral Stage I NSCLC, in whom both CT scanning techniques were performed for planning hypofractionated SRT, were included in this analysis. All patients underwent staging using an (18F)fluorodeoxyglucose-Positron Emission Tomography (FDG-PET)scan and only involved-field radiotherapy was performed. Tumor characteristics are summarized in Table 1. Five patients had lesions located in the lower lobes.
Volumetric analysis
The mean GTVs derived using both CT techniques were comparable, ranging from 1.1 to 36.8 cc for multiple MSCT scans, and 1.5 to 38.8 cc for 4DCT scans (Table 2). To estimate the extent of tumor mobility, the ratio between the mean GTV6 CT and ITV6 CT was calculated for each patient. For completely immobile tumors, this ratio will be 1. The mean ratio GTV6 CT/ITV6 CT was 0.58 ± 0.17, and was particularly low in Patients A (0.36) and G (0.26), both of whom had tumors of the lower lobe.
The volume
Discussion
Far higher rates of local control have been reported in early-stage NSCLC with the use of hypofractionated SRT than was achieved using other radiotherapy techniques (5, 6, 7, 8, 9, 10, 11, 12). Most authors used standard, population-based margins for target definition (5, 6, 9, 11). However, an accurate individualized 3D definition of target volumes is a more appropriate prerequisite in view of the high doses per fraction and steep dose gradients characteristic of SRT. This is particularly the
References (27)
- et al.
The role of radiotherapy in treatment of stage I non-small cell lung cancer
Lung Cancer
(2003) - et al.
Has 3D conformal radiotherapy improved the local tumor control in stage I non-small cell lung cancer?
Radiother Oncol
(2002) - et al.
Acute and late toxicity results of RTOG 9311: A dose escalation study using 3D conformal radiation therapy in patients with inoperable non-small cell lung cancer [Abstract]
Int J Radiat Oncol Biol Phys
(2003) - et al.
Extracranial stereotactic radioablation: Results of a phase I study in medically inoperable stage I non-small cell lung cancer
Chest
(2003) - et al.
Stereotactic body frame based fractionated radiosurgery on consecutive days for primary or metastatic tumors in the lung
Lung Cancer
(2003) - et al.
Stereotactic radiosurgery for lung tumors: Preliminary report of a phase I trial
Ann Thorac Surg
(2003) - et al.
Computed tomography-guided frameless stereotactic radiotherapy for stage I non-small cell lung cancer: A 5-year experience
Int J Radiat Oncol Biol Phys
(2001) - et al.
Stereotactic hypofractionated high-dose irradiation for patients with stage I non-small cell lung carcinoma: Clinical outcomes in 241 cases of a Japanese multi-institutional study [Abstract]
Int J Radiat Oncol Biol Phys
(2003) - et al.
Stereotactic single-dose radiotherapy of stage I non-small-cell lung cancer (NSCLC)
Int J Radiat Oncol Biol Phys
(2003) - et al.
Respiratory-driven lung tumor motion is independent of tumor size, tumor location, and pulmonary function
Int J Radiat Oncol Biol Phys
(2001)
Tumor location cannot predict the mobility of lung tumors: A 3D analysis of data generated from multiple CT scans
Int J Radiat Oncol Biol Phys
Digital fluoroscopy to quantify lung tumor motion: Potential for patient-specific planning target volumes
Int J Radiat Oncol Biol Phys
Multiple “slow” CT scans for incorporating lung tumor mobility in radiotherapy planning
Int J Radiat Oncol Biol Phys
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