Evaluation of Gross Tumor Volume Changes Prior to the First Delivered Fraction in Primary and Metastatic Lung Tumors Treated By Stereotactic Body Radiation Therapy: Does Time from Simulation to Start Matter?

Abstract

To quantify potential volumetric discrepancies in gross tumor volume (GTV) from time of simulation to the delivery of the first radiation fraction and to identify predictors of size change for patients with primary and metastatic lung tumors treated by image-guided stereotactic body radiotherapy (SBRT). The cone beam computed tomography (CBCT) scan obtained at the first fraction of radiation for 14 primary early-stage non-small cell lung cancer tumors and 10 metastatic lung tumors treated with thoracic SBRT were rigidly registered to the initial computed tomography (CT) scans obtained at simulation. All patients were treated with four-dimensional CT (4DCT)-based image-guided 3-5 fraction SBRT using abdominal compression and body-fixation. The GTV was delineated on both the mean intensity projection (MeanIP) data set of the CT simulation scan and the CBCT scan, and percent change in GTV volume was calculated from time of CT simulation to first fraction of treatment. The most common prescription dose and fractionation regimen was 50 Gy in 5 fractions (n = 21). The fraction size ranged from 8 to 18 Gy (median, 10 Gy). The correlation between percentage GTV change and treatment variables, tumor characteristics, and clinical outcome (median 45 months, range 25 to 77 months) were statistically evaluated using linear regression to test significance. The median patient age was 72 years (range, 43 to 90 years). GTV and planning target volume (PTV) were 7.8 cc (range, 0.8 to 53.8 cc), and 31.2 cc (range, 7.5 to 118.2 cc), respectively. For all treated tumors, GTV increased a median of 19.9% (range, -4.4 to 45.9%) over a median time interval of 13 days (range, 2 to 39 days). When stratified according to tumor status, primary lung tumors exhibited a median percent GTV change of 16.2% (range, -4.4 to 39.49%), and metastatic lung tumors GTV size increased a median of 23.4% (range, -3.7 to 45.9 %). The degree of percentage GTV change was inversely related to initial tumor volume on CBCT scan for all treated tumors (P = 0.004). The observed change in GTV volume and treatment interval for all tumors approached significance (P = 0.07) with tumors treated less than or equal to 10 days exhibiting smaller % change in volume compared to those treated after 10 days. Right sided lung tumors were more likely to experience increase in GTV size (P = 0.03). Primary vs. metastatic status, histology, and central vs. peripheral tumor location did not correlate with observed tumor volume change (P > 0.05). Degree of GTV change did not correlate with clinically observed local control rates (P > 0.05). Target localization during SBRT is critical, and our results emphasize the value for accurate image guidance techniques for all thoracic tumors treated with hypofractionated courses, particularly in smaller tumors in which larger changes gross tumor volume may be observed. Decreasing time interval from CT simulation to first fraction to 10 days or less may minimize the degree of volumetric tumor change.