Abstract Book

S333

ESTRO 37

Milan, Italy 20 IRCCS San Raffaele, Radiation Oncology, Milano, Italy Purpose or Objective To address the thus-far poorly investigated severity and duration of hematologic toxicity (Htox) from whole-pelvis (WP) radiation therapy (RT) in a cohort of chemo-naïve patients (pts) treated with different intensity modulated RT (IMRT) techniques, doses and fractionations. Material and Methods Pts enrolled in a multicenter observational trial aimed at the evaluation of Htox and quality of life after WPRT were included. Blood samples were obtained at baseline, at RT midpoint and end, and thereafter @ 3-6-12-18-24 months (mos). Selected clinical and dosimetric parameters were prospectively collected. Htox scoring followed CTCAE grading. Association between Htox and clinical/dosimetric features was evaluated through survival and logistic analysis. Results Data of 406 pts were available, median follow-up was 18 mos, mean age 68 years (range 43-84). 24% of pts were treated with exclusive RT, 76% after prostatectomy. 44% received conventional fractionation, while 56% hypofractionated RT (dose/fraction range 2.1- 2.7 Gy, mean 2.45 Gy). Mean prescribed dose (2Gy equivalent α/β=3 Gy) to prostate/prostatic bed was 74.5 Gy (range 64-93 Gy), with mean prescribed dose (2Gy equivalent α/β=3 Gy) to lymph nodes (LN) 52.8 Gy (range 48.4-68.7 Gy). LN volumes ranged between 338 and 1600 cc (median=1000 cc). 6% of pts were treated with static field IMRT, 43% with tomotherapy and 51% with Volumetric Modulated Arc RT. Neutrophils, platelets and hemoglobin Htox had very low incidence (≈1% acute, ≈3% late), while lymphocytopenia (Lymp) rate was important both in acute and late phase: 47%, 33% and 0.7%, acute G2, G3 and G4, respectively; late G2+ incidence @24mos was ≈30% with a median duration of late Lymp equal to 3 mos and the75° percentile equal to 6 mos; 38 pts suffered of G2+ Lymp for at least 1 year (See Figure 1a-1b-2a for longitudinal description of Htox). At multivariable logistic analysis (MVA), acute G3+ Lymp was associated to prostatectomy (OR=3.6), dose to LN (OR=1.12 for 1 Gy increase) and LN volume>900 cc (OR=1.9). MVA p=0.04. Incidence of late Lymp was associated (Kaplan-Meier) to prostatectomy (HR=1.7 p=0.07), LN volume>900cc (HR=2 p=0-03) and acute G3+ Lymp (HR=4.2 p<0.0001, see figure 2b). Late G2+ Lymp lasting at least 1 year was associated (MVA p<0.0001) to acute G3+ Lymp (OR=6), smoke (OR=4.2) and LN volume>900 cc (OR=3.6).

Conclusion Lymphocytopenia after WPRT was found to be less negligible and more prolonged than expected. Some treatment related (prostatectomy, LN dose and LN volume) and patient related (smoke) factors favouring Htox were identified. Awareness of this may be crucial when prescribing and planning/optimizing WPRT. The highly significant consequential effect between severe acute Lymp and prolonged late Lymp fosters the adoption of timely supportive care in pts exhibiting severe acute Lymp.