Vertebral Dose Sparing during Lung and Esophageal Radiation Therapy

Abstract

The vertebral bodies contain approximately 20-25% of all bone marrow in the body. Lymphopenia is a known negative prognostic factor in thoracic malignancies. Studies have previously demonstrated that patients receiving radiation to the lumbar spine and/or pelvis have shown increased fat deposition/bone marrow scarring following therapy, which leads to long term immunosuppressive effects. In anticipation for an open clinical trial, and based on previously published work, we explored whether retrospectively constraining the thoracic bone marrow is practically feasible and leads to minimal increase in dose to the remaining OARs (Organs at Risk). Patients with previously treated thoracic cancer were included in this study. The thoracic bone marrow was contoured using PET scan to verify avid areas and a new plan was created. Guideline constraints for the thoracic bone marrow were V10 (<300cc), V20 (<200 cc), and V30 (<150cc). If bone marrow optimization caused a violation of standard OAR guidelines, then the bone marrow constraints were not met. The median changes in thoracic bone marrow and OARs (lung, heart, spinal cord) were compared with the Wilcoxon sign rank test for non-parametric paired samples. We included 20 patients, 10 with esophageal cancer and 10 with NSCLC (Non-Small Cell Lung Cancer). Only three cases were able to meet the prespecified bone marrow constraints after optimization (15%). Bone marrow median V10 was reduced by 20cc (-8.16%). Larger changes were seen in V20 and V30 dose-volume reductions, with decreases of 35.3 and 32.3 cc (-16.4% and -21.3%), respectively. There was a significant reduction (p<0.001) for V30, V20, and V10. Though most experienced a decrease in mean heart dose, there was an overall median increase of 21.00 cGy, attributable to a single outlier (P = 1). Median cord dose was decreased by 418.5 cGy. There was a minimal non-significant increase in median Lung V20 of 0.76% as well as minimal median increase in the mean lung dose (7 cGy). This retrospective dosimetric analysis reveals that bone marrow can be optimized. While not able to meet thresholds predictive of minimizing Grade 4 lymphopenia, dose reduction can safely be done with minimal increase in values known to cause toxicity in the heart, lungs, and cord. Though radiation dosage to the thoracic vertebrae was not eliminated entirely, significant decreases in median dosages were achieved without compromising OAR dose. Bone marrow sparing should be considered when feasible and this planning technique is currently being used in an open prospective trial.