Abstract
Purpose: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 Dmean were 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 Dmin of −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.
Highlights
Obstructive lobar atelectasis, the collapse of lung tissue due to restricted airflow, commonly occurs in nonsmall cell lung cancer (NSCLC) patients with centrally located tumors.1 Initial atelectasis presentation rates for patients undergoing external beam radiotherapy have been reported to range between 10% and 40%.2–5 During the course of radiotherapy, tumor regression or progression can cause changes in atelectasis, either resolution or expansion
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
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 or 5% for bone and carina alignments, respectively
Summary
Obstructive lobar atelectasis, the collapse of lung tissue due to restricted airflow, commonly occurs in nonsmall cell lung cancer (NSCLC) patients with centrally located tumors. Initial atelectasis presentation rates for patients undergoing external beam radiotherapy have been reported to range between 10% and 40%.2–5 During the course of radiotherapy, tumor regression or progression can cause changes in atelectasis, either resolution or expansion. Resolution of atelectasis, in the case of full reaeration of whole lobe collapse, appears in computed tomography (CT) scans as a change from a uniform, high-intensity consolidated volume to a larger, lower-intensity region of normal parenchyma. This can produce large geometric changes in treatment anatomy which can cause baseline shifts in tumor position.. Anatomical variations have been shown to have a greater impact on target dose than either respiratory motion or baseline shifts (e.g., setup errors), highlighting the potential need for adaptive radiotherapy in patients with atelectasis changes. This can produce large geometric changes in treatment anatomy which can cause baseline shifts in tumor position. These large geometric changes impact dose to the target and critical structures and cannot be handled by treatment margins, instead requiring plan adaptation. Anatomical variations have been shown to have a greater impact on target dose than either respiratory motion or baseline shifts (e.g., setup errors), highlighting the potential need for adaptive radiotherapy in patients with atelectasis changes.
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