• Carol McGibney, Ola Holmberg, Brendan McClean, and John Armstrong.
• Department of Radiation Oncology, St Luke's Hospital, Highfield Road, Dublin 6, Ireland.
• Radiother Oncol. 2003 Jan 1; 66 (1): 87-93.

Background And PurposeAppropriate planning target volume (PTV) definition is critical for local disease eradication in the treatment of non-small cell lung cancer (NSCLC). When margins are added to the gross tumour volume (GTV) in the standard way, the PTV formed may be too large to facilitate dose escalation due to normal tissue tolerance. To increase the feasibility of dose escalation with 3-dimensional conformal radiotherapy (3DCRT), this study examines an alternative method for the formation of the PTV in NSCLC. This strategy is based on the reduced probability of tumour cells from the GTV outwards and on the associated lower dose requirements to eradicate such subclinical disease.Materials And Methods3DCRT plans were generated from the CT scans of 15 patients with NSCLC (stages Ib to IIIb). Each PTV was formed by adding a margin for geometric uncertainties directly onto the GTV. The success of this approach is dependent on the volume immediately outside this smaller PTV, the Rind volume, receiving 50 Gy, the minimum dose requirement that is considered sufficient for eradication of the reduced tumour cell density in this volume. While optimizing the treatment plans for each PTV to 70 Gy, the dose distribution in the Rind volume, and the factors affecting it, were assessed.ResultsOne hundred percent of each PTV received a minimum of 95% of the prescribed dose. The percentage of the Rind volume receiving 50 Gy or more (V50) had a median value of 94%. The minimum dose in this volume, however, ranged from 5.6 to 32.1 Gy. The V50 was highest for apical tumours (96.1%) and lowest for peripheral tumours (86%) and correlated positively with the size of the PTV (Kendall's rank correlation (Kt)=+0.3, P=0.05) and the number of beams used (Kt=+0.3, P=0.03) but not with the conformity index. The average volume outside the Rind which still received >/=50 Gy (the Wasted 50 Gy) increased significantly with the V50 of the Rind volume and was inversely proportional to the Rind <50 Gy, correlating significantly with the dose to the organs at risk.ConclusionsUsing this strategy with standard 3DCRT, all PTVs were irradiated to the required dose with this approach, but none of the corresponding Rind volumes had an acceptable dose distribution. The addition of dual volume planning or the use of intensity modulated radiation therapy may achieve an appropriate dose distribution in the Rind volume while not increasing the dose to the organs at risk and may thereby facilitate dose escalation.

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