Does MLC With Finer Leaf Width Improve the Treatment Plan Quality in Stereotactic Body Radiation Therapy of Lung Cancers

Abstract

To investigate the impact of leaf width on treatment plan quality in stereotactic body radiation therapy (SBRT) of lung cancers. 4DCT scans were acquired for 10 patients. The GTV was delineated on the maximum intensity projection CT, and it was expended by a uniform margin of 5mm to create the PTV. Volumetric Modulated Arc Therapy (VMAT) treatment plans were generated on the average intensity projection CT. The prescription dose was 48 Gy delivered in 4 fractions. For each patient two VMAT plans were created: the first plan (Plan1cm leave) used the 1 cm leaf width MLCi on Elekta Synergy, and the second plan (Plan0.5cm leave) utilized the 0.5 cm leaf width Agility MLC on Elekta Infinity. The output factors, beam energy and profiles of the two treatment machines were matched to within 1%. The two VMAT plans were analyzed and compared using the following dosimetric parameters: For target dose coverage, Quality of Coverage (QI), Homogeneity Index (HI), and Conformity Index (CI) were calculated; For organs at risk, the maximum dose of cord, heart and ribs, and the mean/V5/V10/V20 for the total lung dose were analyzed; The treatment time in terms of MU was also compared. For the dose coverage of GTV, the averaged CI, HI and QI of Plan1cm leave and Plan0.5cm leave are 0.980±0.046 / 0.970±0.043, 1.079±0.053 / 1.084±0.056, and 0.915±0.218 / 0.960±0.092, respectively. The corresponding p-values between the two plans are 0.250, 0.245 and 0.442. For PTV dose coverage, the averaged CI, HI and QI of Plan1cm leave and Plan0.5cm leave are 0.949±0.079 / 0.953±0.076, 1.142±0.083 / 1.142±0.085 and 0.865±0.141 / 0.862±0.154, respectively. The corresponding p-values are 0.465, 0.943 and 0.594 respectively. The mean MU is 2334.3±395.0 for Plan1cm leave and 2442.8±548.5 Plan0.5cm leave, with the p-value of 0.135. For the total lung, the mean lung dose, V5, V10, and V20 of Plan1cm leave are 4.53±1.88 Gy, 24.83±10.70%, 13.97±6.00%, 6.64±3.53% respectively. The corresponding values for Plan0.5cm leave are 4.48±1.79Gy, 25.06±11.53%, 14.27±6.27%, 6.67±3.65% respectively. The p-values for mean lung dose, V5, V10, and V20 are 0.825, 0.751, 0.410, and 0.864 respectively. The averaged maximum dose for cord, heart and ribs are 8.69±4.40, 16.84±13.68, 42.31±4.06 Gy respectively for Plan1cm leave versus 10.29±5.03, 16.46±13.68, 41.67±3.62 for Plan0.5cm leave. The corresponding p-values are 0.034, 0.530 and 0.322 respectively. Treatment plan quality using 0.5 cm leave width MLC is not statically better than the plan with 1 cm leave MLC. This can be explained by the fact that in most SBRT lung cases the tumor has smooth edge and both 1 cm and 0.5 cm leave MLCs are able to conform to the target edges smoothly. However, the maximum cord dose with Agility MLC is statistically lower than that with MLCi because Agility MLC offers lower MLC transmission.