Effect of atelectasis changes on tissue mass and dose during lung radiotherapy.
OBJECTIVE
To characterize mass and density changes of lung parenchyma in non-small cell lung cancer (NSCLC) patients following midtreatment resolution of atelectasis and to quantify the impact this large geometric change has on normal tissue dose.


METHODS
Baseline and midtreatment CT images and contours were obtained for 18 NSCLC patients with atelectasis. Patients were classified based on atelectasis volume reduction between the two scans as having either full, partial, or no resolution. Relative mass and density changes from baseline to midtreatment were calculated based on voxel intensity and volume for each lung lobe. Patients also had clinical treatment plans available which were used to assess changes in normal tissue dose constraints from baseline to midtreatment. The midtreatment image was rigidly aligned with the baseline scan in two ways: (1) bony anatomy and (2) carina. Treatment parameters (beam apertures, weights, angles, monitor units, etc.) were transferred to each image. Then, dose was recalculated. Typical IMRT dose constraints were evaluated on all images, and the changes from baseline to each midtreatment image were investigated.


RESULTS
Atelectatic lobes experienced mean (stdev) mass changes of -2.8% (36.6%), -24.4% (33.0%), and -9.2% (17.5%) and density changes of -66.0% (6.4%), -25.6% (13.6%), and -17.0% (21.1%) for full, partial, and no resolution, respectively. Means (stdev) of dose changes to spinal cord Dmax, esophagus Dmean, and lungs Dmeanwere 0.67 (2.99), 0.99 (2.69), and 0.50 Gy (2.05 Gy), respectively, for bone alignment and 0.14 (1.80), 0.77 (2.95), and 0.06 Gy (1.71 Gy) for carina alignment. Dose increases with bone alignment up to 10.93, 7.92, and 5.69 Gy were found for maximum spinal cord, mean esophagus, and mean lung doses, respectively, with carina alignment yielding similar values. 44% and 22% of patients had at least one metric change by at least 5 Gy (dose metrics) or 5% (volume metrics) for bone and carina alignments, respectively. Investigation of GTV coverage showed mean (stdev) changes in VRx, Dmax, and Dminof -5.5% (13.5%), 2.5% (4.2%), and 0.8% (8.9%), respectively, for bone alignment with similar results for carina alignment.


CONCLUSIONS
Resolution of atelectasis caused mass and density decreases, on average, and introduced substantial changes in normal tissue dose metrics in a subset of the patient cohort.
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298.
Christopher L
CL
Guy
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298.
Elisabeth
E
Weiss
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298.
Nuzhat
N
Jan
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298.
Leonid B
LB
Reshko
Department of Electrical and Computer Engineering and Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
Gary E
GE
Christensen
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298.
Geoffrey D
GD
Hugo
D002289:Carcinoma, Non-Small-Cell Lung / Q000150:complications / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D006801:Humans; D008175:Lung Neoplasms / Q000150:complications / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D058958:Organs at Risk / Q000528:radiation effects; D001261:Pulmonary Atelectasis / Q000150:complications*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D036542:Tomography, Spiral Computed; D047368:Tumor Burden / Q000528:radiation effects