The Dose Distribution Of Intensity-modulated Radiation Therapy (IMRT) In The Treatment Of Inoperable Non-small-cell Lung Cancer

Objective: To systematically evaluate intensity-modulated radiation therapy(IMRT) and three-dimensional conformal radiation therapy(3D-CRT) two different techniques of radiation therapy used to treat non-small-cell lung cancer and to determine their efficacy in meeting multiple normal-tissue constraints while achieving dose escalation, and evaluate the difference in the IMRT plans using five, seven, and nine beams. Methods: Between August 2003 and October 2004, 30 patients with Stageâ… to â…¢inoperable NSCLC were treated by radiotherapy. A retrospective treatment planning study was performed to compare IMRT and 3D-CRT for these patients. All of the patients were immobilized with thermoplast while quiet respiration. When the CT scan was performed, the slice thickness was 3mm and 5mm through the visible disease and the normal tissues outside the disease, respectively. Using the 3D-CRT planning system (CMS-Xio), CT images were transformed and restructured in three dimensions. Based on the CT images of the patients, the relationship between the target volume and high-risk organs was defined by delineating the gross tumor volume (GTV) and the organs at risk (OAR). The planning target volume (PTV) was defined as the GTV plus a 10mm¹5mm margin. Two kinds of radiation treatment plans were designed for each patient: the 3D-CRT plans and the IMRT plans. The 3D-CRT plan was designed on the basis of those used in the actual patient treatments. The planning principle was maximizing dose to the target volume and minimizing dose to the surrounding normal tissues. The therapy center was the center of the GTV or the beams. The beam configuration involved a template with 6MV coplanar photon beams. Each plans with an additional 5mm margin which accounting for the beam penumbra and PTV coverage. The 3D beam angles, weights, wedge angles were further reviewed to ensure adequate protection of normal tissues. The prescription for the 3D-CRT plan was coverage of 95% the PTV with 66Gy in 33 fractions. The IMRT plans were designed using the inverse planning component of the CMS-Xio treatment-planning system. Five beams IMRT plan was generated with the beam angles optimized manually considering the location of the tumors to minimize the lung dose. Seven and nine beams IMRT plans used equally spaced coplanar beams. The planning objectives for the IMRT plans reflected the following priorities: â‘ 95% of the planning target volume with the prescription dose; â‘¡keeping the maximal dose of the spinal cord ≤45Gy; â‘¢reducing the lung V₂₀(V₂₀<22%), lung V5 and lung V₁₀, the lung mean dose≤15Gy; â‘£reducing the volume of the planning esophagus receiving 50Gy (V50 < 25%) and mean dose ≤25Gy.Heterogeneity correction was applied to all dose calculations (maximum allowable heterogeneity within PTV 25%). To compare the 3D-CRT and IMRT plans, the dose-volume histograms (DVH), isodose distribution, esophagus V50, and spinal cord D01 were calculated. Also, to assess the plan quality, the conformity index (CI) and heterogeneity index (HI) were computed using the DVH of the PTV. Because the more heterogeneity of lung, the maximum dose within PTV, namely PTVDmax(%) was computed in order to evaluated the effect of IMRT. To assess the radiobiological implications of normal lung irradiation, the V₅, V₁₀, V₂₀, V₃₀ and lung NTCP for the normal lung were computed. Results: 1. The physical parameters of the PTV: (1) The CI of 3D-CRT plan and IMRT plans using 5, 7 and 9 beams was 0.64±0.04, 0.68±0.06, 0.70±0.06 and 0.70±0.07 respectively, we can found that IMRT plans were all associated with a greater degree of CI, the difference was statistically significant (F=5.347, P=0.002<0.05). But no significant difference was found in the three IMRT plans, P>0.05. (2) For 5, 7 and 9 beams IMRT plans, the HI had greater than those in 3D-CRT plan (1.18±0.03, 1.18±0.03, 1.19±0.03 and 1.12±0.03), also had statistical significance(F=30.341, P=0.000<0.05), but there was no statistical significance between three IMRT plans, P>0.05. (3) For 5, 7 and 9 beams I...