Normal lung dose-volume histogram varieties in the former and later period of three dimensional conformal radiation therapy plans and their clinical significance in non-small cell lung cancer

Objective: To analyze the normal lung dose-volume histogram (DVH) varieties in the former and later period(P1 and P2) of three dimensional conformal radiation therapy(3DCRT) plans and the compound(Pc) plan in non-small cell lung cancer(NSCLC), and to access the feasibility to modify the target volume during the treatment course. Methods: Twenty-one NSCLC patients who had received accelerated hyper-fractionation 3DCRT in P2 were included in the study. Both of the P1 and P2 plans were redesigned to a total dose of 70 Gy with V20 smaller than 35%. When the target volume was modified and P2 plan was redesigned using accelerated hyper-fractionation 3 DCRT of 30 Gy after P1 plan of 40 Gy, the Pc plan was compounded by transmitting the parameters (such as target volume, irradiation field and dose) of P1 plan into P2 plan. Total lung volume and target volumes (GTV, PTV) of P1 and P2 were evaluated. MLD, V5, V10, V20 and V30 of P1, P2 and Pc were calculated. Results: The total lung volume in P1 and P2 plans was not significantly different (t=0.19, P=0.850). The volumes of GTV, PTV in P2 were obviously smaller than P1 (t=2.88, P= 0.009; t=4.01, P=0.001). When comparing P2 with P1, MLD were 16.5 Gy Vs 17.8 Gy (t=2. 60, P=0.017), V30 was significantly decreased (t=2.19, P=0.041), but V5, V10 and V20 had no significant difference. Similar differences were found in MLD, V5, V10, V20 and V30 when comparing Pc to P1. P2 plans had significantly smaller MLD, V5, V10, V20 and V30 than Pc plans. Fourteen patients with decreased PTV were further analyzed. The V30 and MLD decreased significantly(t=3.00, P=0.010; t=2.38, P=0.033), but V5, V10, V20 had no difference when comparing P1 and P2 plans. Among these 14 patients, the V10 and V30 decreased significantly(t=2.76, P=0.033; t=3.60, P=0.011) when P2 plans were generated using the same field number and beam angles in P1 plans in 7 patients. The parameters were similar in P1 and Pc plans, but increased significantly when comparing to P2. Various parameters were the same among P1, P2 and Pc plans when P2 plans were designed using 1-2 different fields and angles in the other 7 patients. The differences were not significant between P1 and P2 plans in 7 patients with the same or increased PTV. Expansion or contraction of PTV significantly influenced MLD and V20 (r=-0.62, P=0.03; r=0.48, P=0.029). Conclusions: When the tumor regresses, the high dose volume of the lung decreases with modifying the target volume and re planning in the later period using accelerated hyper-fractionation 3DCRT. The low dose volume of the lung may decrease if the field orientations are same throughout the treatment. It is rational to evaluate the normal lung DVH of the whole plan when the physical parameters of the later period plan are the same as the former one. (authors)