Conformal treatment planning using 4DCT can decrease ipsilateral lung dose and improve tumor coverage: A prospective 4DCT treatment planning study

Abstract

To determine the full range of lung tumor motion in non-coached, free-breathing lung cancer patients and to use the data to evaluate 3D conformal radiation treatment planning techniques. 20 patients were enrolled in a prospective study of lung tumor motion. Each patient was immobilized in the treatment position and underwent quantitative spirometry during a high resolution CT scan. The CT scanner was operated in cine mode and acquired 15 sets of twelve contiguous 1.5 mm slices. The spirometer measured tidal volume and was compared with the imaged air volume using a semiautomatic autothreshold to correct for measurement drift. Each image was associated a tidal volume measurement, subdivided by inhalation or exhalation phase. Inhalation, mid-inhalation, exhalation, and mid-exhalation image datasets were fused with a standard helical CT scan acquired during the same session. The inhalation and exhalation tumor volumes (GTV) were combined to produce an internal target volume (ITVl) that implicitly assumed linear motion. A second ITV (ITVh) was produced by combining the ITVl and the mid-inspiration and mid-exhalation GTVs. This enabled an evaluation of the effects of internal hysteresis on the ITV definition. 3D conformal radiation therapy plans were generated to 1) prescribe 70 Gy to the isocenter of the GTV defined using the standard helical scan (GTVs) using a 2 cm block margin; and 2) prescribe 66.5 Gy (95% of 70 Gy) to cover 100% of ITVl and 3) prescribe 66.5 Gy to cover 100% of ITVh using customized margins for 2) and 3). To date, 12 patients have been analyzed. 8 patients had upper lobe tumors and 8 patients had peripheral tumors. The tumors moved an average of 0.9 (range 0.3 to 1.5) cm. The ratio of ITVl to GTVs was 1.0–4.0 (mean 1.95, median 1.70) and the average ratio of ITVh to ITVl was 1.16. This indicated that the GTVs inaccurately modeled the moving tumor, while incorporation of hysteresis did not substantially impact the definition of ITV in all but one patient. Block margins required to cover the ITVh and ITVl ranged from 1.0 to 1.5 cm. The 2 cm margin around the GTVs overcompensated for tumor motion in at least one direction in all patients. This resulted in a 5% decrease (range 2% - 8%) in V20 of the ipsilateral lung for the ITV-based plans. In 3 patients, the GTVs was an inaccurate model for the ITV and even the 2 cm margin was insufficient to adequately cover the tumor (Figure 1). Motion analysis can decrease the ipsilateral lung dose in most patients by allowing margin reduction compared to standard clinical practice. It also shows inadequate tumor coverage in some patients. In certain patients, full tumor motion may not be adequately represented by inhalation and exhalation reconstructions.