Dosimetric Prediction of Chest Wall Toxicity after Lung SBRT

To identify patient, tumor, and treatment related risk factors for the development of chest wall (CW) toxicity after thoracic stereotactic body radiation therapy (SBRT) for lung tumors. A prospective registry of 253 patients with 270 primary (n = 252) or metastatic (n = 18) lung lesions treated with lung SBRT at Mallinckrodt Institute of Radiology from 2004-2009 was explored to identify patients (1) treated to a dose of 54 Gy in 3 fractions or 50 Gy in 5 fractions, with (2) at least 3 months (mo) follow-up, and (3) no prior radiotherapy to the ipsilateral thorax or CW. One hundred seventy-six lesions in 167 patients were identified, and complete dosimetric data was available on 110 CWs. The ipsilateral CW (defined as a 3 cm outward expansion from the ipsilateral lung) and ribs were contoured on each patient. Patient related risk factors for CW toxicity were analyzed on all 167 patients. Univariate analysis and multivariate logistic regression modeling was used to correlate patient, tumor, and dosimetric factors (volume of CW or ribs receiving 5-80 Gy [V5-85] in 5 Gy increments, prescription isodose line, max dose at 2 cm from the PTV) to the development of CW toxicity. Spearman's rank correlation was used for measuring bivariate associations. Median follow-up was 11.5 months. Nineteen patients (11.4%) developed CW toxicity (8 with rib fracture, 11 with CW pain only). Median time to CW toxicity was 10.8 months (18.6 months for rib fracture, 7.9 months for CW pain only). Patient factors that correlated with CW toxicity on univariate analysis were prior or current tobacco use, connective tissue disorder, and elevated body mass index (p = 0.0308, p = 0.0348, and p = 0.0315, respectively). CW V5-85 all correlated positively with CW toxicity on univariate analysis. CW V65 > 0 cc was most predictive (p = 0.0046). The prescription isodose line correlated negatively with CW toxicity (p = 0.012), such that patients treated to a prescription isodose line of 60-69.9%, 70-79.9%, and ≥ 80% experienced a 20%, 15%, and 7% rate of CW toxicity, respectively. Both V65 and prescription isodose line correlations remained significant on multivariate analysis. V65 is the dominant variable in the development of CW toxicity in our dataset. Although adequate tumor coverage should not be compromised, we recommend limiting the max dose of the CW to < 65 Gy if possible.