Long Term Radiographic and Clinical Course in Chest Wall Toxicity following Thoracic Stereotactic Body Radiation Therapy

Abstract

Stereotactic Body Radiotherapy (SBRT) for malignant lung lesions is known to potentially cause chest wall (CW) toxicity, however the long term course of these side effects is not well described. We hypothesize that radiation-induced rib fractures frequently do not radiographically heal fully and are closely associated with prolonged CW pain in areas of high doses. One-hundred ten lung tumors treated with SBRT in 101 patients (61 female, 40 male) were retrospectively analyzed. Only patients with a minimum follow up of 2 years were included. The most common treatment regimen was 48GY/4fx. The incidence, clinical course, and related dosimetric and clinical factors of CW toxicity are analyzed. In addition, the radiographic appearance of radiation induced rib fractures and their healing process are characterized. Statistical results were obtained using Mann-Whitney U-test and Chi-Square test. Mean follow up was 48 months (range 24-100), mean age at treatment of 68 years (range 47-91). Overall, 29% (32/110) of treatments resulted in rib fractures (cumulative 50 fractured ribs) after on average 23 months (range 1-46). Of the 32 treatments resulting in rib fracture, 66% (20/32) developed CW pain and 56% (18/32) fractured more than one rib, including 10 treatments with 3 or more resulting rib fractures. 31% (34/110) treatments resulted in CW pain occurring at a mean time of 12 months (range 1-50) with mean duration of 22 months (range 2-63); 38% (13/34) had ongoing pain at time of last follow up . Interestingly only 62% (20/34) treatments with CW pain had associated rib fractures. Clinically, women and African Americans were more likely to develop rib fractures (p<0.01 and p=0.01 respectively); while older patients and women were more likely to develop CW pain (p=0.02 and p=0.006 respectively). CW V20Gy, V30Gy, V40Gy, Dmax and D30cc were higher in patients with rib fractures (p<0.05 in all). The PTV was abutting CW in all but 1 patient who developed rib fractures while the mean distance to CW in patients without rib fracture was 1cm (range 0-4.4); p=0.016. Radiographically, only 16% (8/50) ribs were fully healed at last imaging, 76% (38/50) displayed soft tissue fibrosis and 20% (10/50) had commuted fractures. Bone Density was measured but no differences were found between patients with or without rib fracture or CW pain (p>0.05). Thoracic SBRT is associated with a non-negligible risk of rib fracture and CW pain which can occur late after RT. Radiation induced rib fractures are unlikely to fully heal radiographically and can subsequently result in CW pain with long duration. High doses to the CW are most likely to put patients at risk and should be minimized as much as possible, particularly in females.