ESTRO 2020 Abstract Book

S1013 ESTRO 2020

structure and both a pre-treatment (collected within one year of radiotherapy start) and an End-of-Radiotherapy (EoRT) peripheral blood count (collected within six months after radiotherapy ended). Kernel-weighted polynomial smoothing of all available blood counts within 12 months before and after radiotherapy start was used to illustrate kinetics. The association of the volume of the body irradiated with the degree of hematological toxicity was analyzed. For blood components markedly affected by irradiation, linear regression was used to evaluate the association of square root transformed EoRT counts with age, gender, Charlson comorbidity score, chemotherapy (prior, sequential and concomitant), fractionation schemes and volume of the body exposed to at least 2 (V2) and 40 (V40) Gy, adjusting for pre-treatment blood counts and cancer diagnoses. Results We included 4,314 patients with available pre-treatment and EoRT platelet and leukocyte counts, of whom 2,479 patients also had neutrophil and lymphocyte counts. All blood components declined after start of radiotherapy. Decline of platelet, leukocyte and neutrophil counts was modest and recovered rapidly after radiotherapy, whereas the lymphocyte count decline was clinically significant and remained low (Fig 1). The higher the volume of the body exposed to radiation (both low dose and high dose), the higher the percentage of patients who experienced hematological toxicity in the first 3 months after radiotherapy, but this association only persisted for longer periods for lymphocytes (Fig 2 for V2). Older age (≥70 vs. <50 years old [coef. -0.037, 95% CI: -0.072 to -0.001, p=0.046]), female gender (-0.076, -0.100 to -0.051, p<0.001), number of fractions (per 1 fraction increase: - 0.003, -0.006 to -0.001, p=0.004), dose-volume (V2 ≥11 vs. <3 L [-0.258, -0.311 to -0.204, p<0.001]) and concomitant use of chemotherapy, particularly the use of platinum compounds vs. none (-0.192, -0.233 to -0.150, p<0.001) were independently associated with a lower EoRT lymphocyte count. Estimates were similar for V40, though volumes were smaller (V40 ≥2.1 vs <0.4 L [-0.183, -0.228 to -0.139, p<0.001]). Comparable results may have been due to correlation between V2 and V40 (Spearman’s rho=0.73, p<0.001).

Conclusion An increase of T-cells, specific cytotoxic cells, in surrounding non-irradiated environment when an higher dose per fraction was delivered to primary tumor, reveals that these fractionations are more effective in terms on antitumor immune response. Althought the small number of patients, our study seems to mean that a better immunologic antitumor response, in terms of cytotoxic lymphocytes, to patients treated with single fraction SRS versus fractionated schedules. Futures paths can be establish a more accurate threshold in fractionation dose, as well as, the durability and potency of that effect. Funded by ISCIII PI17/01735 grant (cofunded by FEDER). PO-1817 Kinetics and dose-volume effect of radiotherapy on leucocyte and platelet blood cell components C. Terrones Campos 1 , B. Ledergerber 1 , I. Vogelius 2 , M. Helleberg 1 , L. Specht 2 , J. Lundgren 1 1 Rigshospitalet, Centre of Excellence for Health- Immunity and Infections CHIP- Department of Infectious Diseases, Copenhagen, Denmark ; 2 Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective We aimed to assess the kinetics and dose-volume association of external beam irradiation on platelets, leukocytes, neutrophils and lymphocytes in a large well- characterized cohort of patients with solid tumor diagnosis undergoing their first course of radiotherapy with curative intent. Material and Methods Patients were included from 2009 to 2016 at a single site, with available dosimetric data for the delineated body