[Comparison of three dosimetric techniques for lung tumor irradiation]

Comparison of three dosimetric techniques of lung tumor delineation to integrate tumor motion during breathing.

Nineteen patients with T1-3N0M0 malignant lung tumor were treated with definitive chemoradiotherapy (14 cases) or pre-surgery chemoradiation. Doses were, respectively, 66 and 46Gy. CT-scan for delineation was performed during three phases of breathing: free breathing and deep breath-hold inspiration and expiration. GTV (gross tumor volume) was delineated on the three sequences. The classic technique included GTV from the free-breathing sequence plus a CTV (clinical target volume) margin of 5 to 8mm plus a PTV (planning target volume) margin of 7 to 10mm (including ITV [internal target volume] margin and set-up margin). The gating-like technique included GTV from the deep breath-hold inspiration sequence plus a CTV margin of 5 to 8mm plus a PTV margin of 2mm. The three-volume technique, included GTV as a result of the fusion of GTVs from the three sequences plus a CTV margin of 5 to 8mm plus a PTV margin of 2mm. Dosimetry was calculated for the three PTVs, if possible, with the same fields number and position. Dose constraints and rules were imposed to accept dosimetries: firstly spinal cord maximal dose less than 45Gy, followed by V95 % for PTV greater than or equal to 95 %, and V20 GY(Gy) for lung less than or equal to 30 %, V30 GY(Gy) for lung less than or equal to 20 %.

GTVs were not statistically different between the three methods of delineation. PTVs were significantly lower with the gating-like technique. V95% of the PTV were not different between the three techniques. With the classic-, the gating-like- and the 3-volume techniques, dosimetry was considered as acceptable, respectively in 15, 18 and 15 cases. Comparisons of constraint values showed that the gating-like method gave the best results. In the case of pre-operative management, the gating-like method allowed the best results even for the V95% values. However, in the absence of gating device or without the possibility to use it, the 3-volume method allowed to take into account more precisely the organ motion than the classical technique.

The 3-volume method can be done. It is a good method to take into account the organ motions. However, the gating-like method gives the best results leading to propose its use even for pre-operative patients with upper tumors.