Abstract
The purpose of this study was to assess the impact of respiratory gating on tumor and normal tissue dosimetry in patients treated with SBRT for early stage non‐small cell lung cancer (NSCLC). Twenty patients with stage I NSCLC were studied. Treatment planning was performed using four‐dimensional computed tomography (4D CT) with free breathing (Plan I), near‐end inhalation (Plan II), and near‐end exhalation (Plan III). The prescription dose was 60 Gy in three fractions. The tumor displacement was most pronounced for lower peripheral lesions (average 7.0 mm, range 4.1–14.3 mm) when compared to upper peripheral (average 2.4 mm, range 1.0–5.1 mm) or central lesions (average 2.9 mm, range 1.0–4.1 mm). In this study, the pencil beam convolution (PBC) algorithm with modified Batho power law for tissue heterogeneity was used for dose calculation. There were no significant differences in tumor and normal tissue dosimetry among the three gated plans. Tumor location however, significantly influenced tumor doses because of the necessity of respecting normal tissue constraints of centrally located structures. For plans I, II and III, average doses to central lesions were lower as compared with peripheral lesions by 4.88 Gy, 8.24 Gy and 6.93 Gy for minimum PTV and 0.98, 1.65 and 0.87 Gy for mean PTV dose, respectively. As a result, the mean single fraction equivalent dose (SFED) values were also lower for central compared to peripheral lesions. In addition, central lesions resulted in higher mean doses for lung, esophagus, and ipsilateral bronchus by 1.24, 1.93 and 7.75 Gy, respectively. These results indicate that the tumor location is the most important determinant of dosimetric optimization of SBRT plans. Respiratory gating proved unhelpful in the planning of these patients with severe COPD.PACS numbers: 87.55.‐x, 87.55.kd, 87.90.+y
Highlights
Stereotactic body radiation therapy (SBRT) is a noninvasive alternative to surgery for patients with medically inoperable, early stage non-small cell lung cancer (NSCLC) with tumor control rates ranging from 80% to 95% at two to three years
The linear quadratic model (LQM) may over estimate cytotoxic effects of SBRT.[1,2,3,4] A universal survival curve (USC) model has recently been proposed, combining the LQM for low-dose region and the multitarget model asymptote for high-dose region.[5]. Based on the USC concept, the single fraction equivalent dose (SFED) was introduced as the dose delivered in one fraction that would cause the same biological effect as any dose-fractionation scheme in question.[5]
The purpose of this study was to quantify the range of motion of lung tumors relatively to their location within the lung parenchyma, and subsequently assess the impact of respiratory gating on tumor and normal tissue dosimetry in patients treated with SBRT
Summary
Stereotactic body radiation therapy (SBRT) is a noninvasive alternative to surgery for patients with medically inoperable, early stage non-small cell lung cancer (NSCLC) with tumor control rates ranging from 80% to 95% at two to three years. Radiobiological modeling has been used to predict the biological impact of radiation therapy and to compare treatments utilizing various dose-fractionation schemes, mostly based on the linear quadratic model (LQM).(3) for fraction doses of 5 Gy or greater, actual cell survival curves appear linear instead of down bending as LQM would predict. There is increasing evidence that gating respiration results in decreased volumes of irradiated normal lung.[6] This may prove to have significant impact in limiting pulmonary toxicity when few large radiation fractions are used such as in SBRT. It may be used to identify the optimal phase of respiratory cycle where the healthy lung may potentially be spared of excessive radiation exposure, minimizing the risk of toxicity. 4D CT may be useful for tumors of the lower lung as their motion may be more pronounced due to their proximity to the diaphragm.[7,8,9,10]
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