Effect of various methods determining lung volume and fraction dose on the lung dose-volume parameters

Objective: To study the effect of various methods determining lung volume and fraction dose on the lung dose-volume parameters for lung cancer patients. Methods: Twenty patients with lung cancer were randomly enrolled into our study and the plan of three dimensional conformal radiation therapy (3DCRT) was designed by Varian Eclipse TPS. The lung volumes and the dose-volume parameters were measured under CT value of (-300) - (-980), (-400) - (-980) and (-500) - (-980). Under CT value of (-400) - (-980), total lung volumes were confirmed. The dose-volume parameters of V30, V20, V10 and mean lung dose (MLD) were reevaluated after GTV, CTV and PTV were subtracted from the total lung volumes and when the fraction dose was elevated from 2.0 Gy to 10.0 Gy. Results: When the CT value ranged from (-300) - (-980) to (- 500) - (-980), the median reduction of the total lung volumes (- 9.10%) was significantly higher than that of V30, V20, V10 and MLD (-3.18%, -1.13%, 0.82% and -0.79%, respectively). When the total lung volume was fixed at CT value of (-400) - (-980), the alterations of V30, V20, V10 and MLD became more apparent as the increase of the subtracted lung volume, among which the alteration of V30 was most significant while V10 the least. Among five cases with a fixed total dose of 60 Gy and PTV less than 140 cm3, the V30, V20, V10 and MLD were increased to a similar extent (about 40%) when the fraction dose was increased from 2.0 Gy to 10.0 Gy. MLD was increased (36%) when the fraction dose was above 6.0 Gy. Conclusions: When CT value ranges from (-300) - (-980) to (-500) - (-980), the total lung volume is influenced most. The alteration of V30, being statistically significant, might have some significance but is not enough to determine the plan of radiotherapy clinically. The alteration of V20, V10 and MLD is not statistically significant. When the overlapped target volume is subtracted from the total lung volumes, the alteration of V30 is the most significant while V10 the least. The fraction dose, being the most consuming factor (>10%) when comparing with the CT value and targeted volume, can significantly influence the dose-volume parameter. (authors)