ESTRO 38 Abstract book

S406 ESTRO 38

0915 and 0813. Kaplan-Meier curves, log-rank analysis and cox regression were performed to assess for survival differences associated with severe TRL in SPSS. We have developed a simulation for thorax RT to model circulating blood in treatment planning. In this we considered radiation dose to circulating blood by coupling the time- dependence of the radiation delivery with a blood flow transport model that considers the transient time in the regional structures as well as the mixing of irradiated and non-irradiated blood volumes. Results On average, the patients saw a sharp decrease in absolute lymphocyte counts from 0- 25 days following SBRT to lung. By day 30, the mean ALC was only 58% of baseline. When the post RT lymphocyte count reduction falls in to (0.75 - 1.0) x10 9 cells/L category, hospitalization within 5 months of SBRT increased by a factor of two compared to the lower lymphocyte depletion cohorts. Patients with severe TRL (less than 0.5x10 9 cells/L ) had worse overall survival (log rank, Ӽ 2=7.62, p=0.006). On multi-variate cox regression, minimum absolute lymphocyte count was directly related to survival, with a hazard ratio of 0.613 (p=0.034). Predictive model results: a) predict the post treatment absolute lymphocyte value to better than 16% across all variables of interest: age, pre-Tx LYA value, post Tx blood draw day, treatment delivery time, tumor volume size, and location of the tumor. b) model has a sensitivity and a specificity to predict a patient having a post RT lymphocyte value of <0.8x10 9 cells/L with an the area under the curve (AUC) of Receiver Operating Characteristic (ROC) of 0.83 .

Material and Methods Patients with primary NSCLC treated with salvage SBRT for local recurrence after previous definitive SBRT for early- stage NSCLC were identified using a prospective institutional review board-approved SBRT registry. Both initial and salvage SBRT courses were given in 3 to 5 fractions with a biologically effective dose (BED) of at least 100 Gy. Local failure was defined as either failure in the involved lobe within 1 cm of the initial planning target volume (PTV) or with significant treatment overlap of the ≥25% isodose lines. Lobar failures >1 cm beyond the PTV, without overlap of at least the 25% isodose lines, or with other sites of recurrence were excluded from this study. Kaplan-Meier analysis was used to estimate survival outcomes. Results We identified 21 patients who received salvage SBRT for salvage of local recurrence after initial SBRT for non- metastatic NSCLC treated between 2008 and 2017. Interval from initial to salvage SBRT was a median of 23 months (range, 7 - 52 months). The median age at salvage was 75 years (range, 59 – 89), median Karnofsky performance status was 80 (range, 60 - 100), and median age-adjusted Charlson comorbidity index was 6 (range, 2 – 11). Median tumor diameter was 1.4 cm (range, 1.0 – 2.5 cm). Median follow-up from time of salvage SBRT was 23 months (range, 2 – 59 months). Median survival after salvage SBRT was 24 months (range, 3 - 60 months). After salvage SBRT, two-year primary tumor control was 81%, distant control was 75% and overall survival was 68%. Four patients developed new lung primaries; 3 of these were new NSCLC while 1 patient developed small cell lung cancer. Grade 2 pneumonitis was observed in 2 patients (9%) and grade 2 chest wall toxicity was observed in 3 patients (14%). No grade 3+ toxicity was observed in this cohort. Conclusion Reirradiation with SBRT for salvage of local failures after initial definitive SBRT for early-stage, medically- inoperable NSCLC appears to be a safe treatment modality associated with low rates of treatment-related toxicity and encouraging rates of tumor control observed in our series. PO-0785 Clinical significance of treatment related lymphopenia in lung SBRT and a method to ameliorate them K. Wijesooriya 1 , J. Colen 2 , T. McMullen 2 , S. Liyanage 3 , C. Alonso 1 , K. Romano 1 , S. Peach 1 , P. Read 1 , J. Larner 1 1 University of Virginia, Radiation Oncology, Charlottesville, USA ; 2 University of Virginia, Physics, Charlottesville, USA ; 3 Albemarle High School, High School, Charlottesville, USA Purpose or Objective Treatment related lymphopenia (TRL) is a side-effect of radiation therapy. Moderate to severe TRL decreases immunological function, causing suppression of immunoresponse to tumors as well as increased susceptibility to infection. Severity of TRL has been shown to correlate with inferior outcomes for patients in multiple prior studies. Even though SBRT provides over 90% local tumor control for patients with early stage inoperable non-small cell lung cancer (NSCLC), long-term OS at 3 years post SBRT remains suboptimal at 38%. Thus there is opportunity for improvement. This work focusses on TRL due to lung SBRT, and its effects on hospitalizations, and overall survival. This work also describes the first algorithm for predicting TRL for lung SBRT treatments as a potential strategy to minimize TRL. Material and Methods We conducted an Institutional Review Board (IRB) approved retrospective analysis of 92 patients treated with Lung SBRT in the absence of chemotherapy where, RT treatment plans met all dosimetric criteria from RTOG

Conclusion TRL due to lung SBRT treatments can have a clinically significant impact on overall survival and frequency of hospitalizations due to immune suppression. The predictive algorithm we have developed has an accuracy that could enable treatment plan design and optimization to reduce TRL.