Quantitative analysis for the interfractional volumetric changes of stereotactic ablative radiotherapy for early-stage or oligo-metastatic lung tumors

Objective: To measure the volumetric changes precisely during stereotactic ablative radiotherapy for early-stage and oligo-metastatic lung tumors and optimize the treatment plan timely. Methods: From October 2011 to October 2014, 66 patients with 71 early-stage or oligo-metastatic lung tumors received SABR. Median age was 66 years. To measure the volume of tumors, the verification images were registered before each treatment fraction with stimulation images by reference to bone structure. Tumors volume was measured by the first verification images, and were defined as the reference when evaluating the trend of tumors volume change during SABR treatment. Generalized estimated equations were used to analyze the trend of the change of tumors volume over time with several possible predictors. The primary plan (P-plan) was modified when the biological effective dose (BED) of a tumor reached 60 Gy and volume change reached 25%. The modified plan was named as M-plan. Paired t-test was used to compare the dose of organs at risk (OAR) between M-plan and P-plan. Results: In 71 tumors, 49(69%) tumors showed volumetric shrinkage, 21(30%) tumors showed enlargement and 1 tumor showed invariance. Generalized estimated equation showed no statistical significance (P = 0.281) for the volumetric shrinkage of lung tumors. M-plan was made in 26 tumors. Of these tumors, 21 tumors decreased over 25% and the result of paired t-test showed V₅ of lung, D_max and D_{1.2 cm³} of spinal cord, D_max and D_{5 cm³} of esophagus and D_{30 cm³} of chest wall were statistically different between two plans(t = 3.139∼11.939, P < 0.05). 5 tumors enlarged over 25% and the result of paired t-test showed V₅ and V₂₀ of lung, D_max and D_{1.2 cm³} of spinal cord, D_max of esophagus and D_{30 cm³} of chest wall were statistically different between the two plans(t = -10.436--2.518, P < 0.05). Conclusions: Size of lung tumors changed dynamically during SABR, but it is unnecessary to modify treatment plans for all tumors. The tumors which showed obvious volumetric change may benefit from modifying treatment plans. (authors)