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W. Fan



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    P1.08 - Poster Session 1 - Radiotherapy (ID 195)

    • Event: WCLC 2013
    • Type: Poster Session
    • Track: Radiation Oncology + Radiotherapy
    • Presentations: 1
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      P1.08-025 - How to Determine Internal Margin for Clinical Practice? - A Study of 44 Lung Cancer cases with 4D-CT Imaging. (ID 3059)

      09:30 - 09:30  |  Author(s): W. Fan

      • Abstract

      Background
      Internal margin was a margin to compensate for expected physiologic movements and variations in size, shape, position of the target during therapy. The purpose of our study was to identify the particular tumor characteristics to determine suitable internal margin.

      Methods
      43 patients with 44 lung tumors who underwent 4DCT based radiotherapy were included. GTVs on 10 respiratory phases were contoured by single radiation oncologist. Internal gross tumor volume (IGTV) was obtained by combining the GTVs at ten phases of the respiratory cycle. No separate margins were used to account for microscopic tumor extension. Additional isotropic setup margin of 3mm to derived the PTV-4D from the IGTV. In order to encompass PTV-4D, there were four expansion approaches to derive PTVs by adding uniform expansion in step of 1mm in each three direction: (1)PTV(lowest), derived from a margin to the GTV whose phase corresponding to the movement of lowest; (2) PTV(highest), derived from GTV at peak positon of the breathing cycle with an appropriate margin; (3) PTV(20%), derived from GTV on phase 20% with a suitable margin; (4) PTV(20%80%), derived from the composite GTV on phase 20% and 80% with a fit margin. The GTV centroid motions were collected. The association between margin expansion and tumor motion was analyzed. Using multivariate logistic regression, image-based risk factors for the presence of narrow margin (≤8mm) in four expansion approach were identified, and a prediction model was developed based on these factor.

      Results
      For the cases of GTV centroid motion less than 3mm, an isotropic margin of 10mm from any GTV can fully covered PTV-4D (IGTV+3mm). For the cases of GTV centroid motion exceeding 5mm, the dominant directions for GTV and margin expansion were not always in accordance. The former was almost all in SI, while the latter may be affected by tumor shape heavily. Even an irregular tumor in very small mobility, probably needed a large margin expansion. A model of three steps to screen the tumors with margin less than 8mm: As an initial step, tumors with fSI >0.5 and fAP >0.6 were filter out. In a second step, the small tumors (<45 cm[3]) with fSI 0.4-0.5 and fAP 0.5-0.6 were kicked out. In a third step, the irregular tumors were eliminated. (fSI or fAP was the relative fractional location in the lung in AP or SI direction. fSI equaled to 0 when tumor located in the apex of the lung ,and equaled to 1 when tumor in the lowest of the lung. fAP was the same, equaling to 0 meant front edge of the lung and equaling to 1 meant at the back. )

      Conclusion
      In selected patient group, fewer internal margins could be applied. Individual internal margin is necessary for high-mobility tumors. More cases were needed to confirm our model.

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    P3.08 - Poster Session 3 - Radiotherapy (ID 199)

    • Event: WCLC 2013
    • Type: Poster Session
    • Track: Radiation Oncology + Radiotherapy
    • Presentations: 1
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      P3.08-025 - Lung Tumors with Big Size or Irregular Shape or High-mobility can Better Benefit from Four-dimensional Radiotherapy (ID 3054)

      09:30 - 09:30  |  Author(s): W. Fan

      • Abstract

      Background
      Consideration of respiration-induced motion based on 4DCT for lung cancer yields individualized margin. The purpose of our study was to quantify the gain from 4DCT based radiotherapy in lung cancer and to identify the tumor characteristics of better benefit.

      Methods
      51 patients with 52 lung tumors who underwent 4DCT based radiotherapy were included. GTVs on 10 respiratory phases were contoured by single radiation oncologist. Internal gross tumor volume (IGTV) was obtained by combining the GTVs at ten phases of the respiratory cycle. No separate margins were used to account for microscopic tumor extension. Additional isotropic setup margin of 3mm to derived the PTV-4D from the IGTV. PTV-3D was generated by adding a isotropic margin of 10mm to the GTV in a single phase (20%) for upper or middle lobe tumor, and a margin of 10,10,15mm in later, AP and SI direction for tumor in lower lobe. The target coverage on PTV-4D and PTV-3D were compared respectively. Using linear regression, clinical and anatomic factors for the reduction of PTV-4D vs PTV-3D were identified.

      Results
      PTV-4D was significantly smaller than PTV-3D, by 89cm[3] on average. Approximately 17% PTV-3D (in 9 of 52 tumors) were not fully covered PTV-4D with up to 2%-18% slices lost. For the cases of target missed in PTV-3D, tumors were either irregular or had a larger mobility of ≥ 1cm in SI direction. For the cases of target encompassed in PTV-3D (43 tumors), the volume reduction of PTV-4D vs PTV-3D was associated with the GTV size and the tumor movement in SI direction. Taken median GTV (48cm[3], the corresponding diameter of around 4.5cm) as a cutoff, big or small tumors had an average PTV reduction of 141 cm[3 ](77-304 cm[3]) or 42cm[3] (11.5-107 cm[3 ]), respectively.

      Conclusion
      4D plan has a more accurate and safe target coverage than that of empirical estimated margins in 3D plan. Patients with tumor character one of irregular shape, big size or high-mobility can better benefit from 4DCT based radiotherapy. It will be of clinical significance when administration a higher dose to a bulky tumor at equal normal tissue constraint, or high-precision radiation was delivered. A small benefit in a large PTV should be paid more attention especially when 4D based radiotherapy transforming a palliative intent to a curative one.