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To investigate the dosimetric factors associated with acute hematologic toxicity in rectal cancer patients treated with preoperative short course radiotherapy (SCRT)

We analyzed 20 patients with clinical stage T3-4, N0- 2 who were eligible for preoperative SCRT, from January 2020 to September 2022. Three-dimensional (3D) conformal radiotherapy was applied to the whole pelvis with total dose of 25Gy in 5 fractions. All patients underwent blood cell counts before and after RT (prior to sequential chemotherapy). Cell count ratios were calculated by dividing the cell counts post-RT by pre-RT cell counts. The patient acute hematologic toxicity was recorded according to RTOG scoring scale. Pelvic bone marrow (BM) was defined as the region extending from the iliac crests to the ischial tuberosities, including the os coxae and the lumbosacral spine extending from the superior border of the L5 vertebra to the coccyx. PTV volume, BM volumes receiving 8 and 17.5 Gy (V8 and V17.5 respectively) and mean of dose (Dmean) were evaluated.

The median age was 56 years (41-81). All patients had advanced clinical stages (stage III or above) at the time of diagnosis.

Overall, V8, V17.5 and Dmean to BM were 81.8% (57.3% -97%), 44.6 % (24%-66%) and 15.6Gy (11-22.5) respectively. PTV volume was 1032 cc (715 -1625).

The mean hemoglobin, neutrophil, lymphocyte and platelets cell count ratio were 0.95, 0.68, 0.61 and 0.83 respectively.

Eight days (1-19) was the mean delay between the end of SCRT and the second blood count cells. 

Acute hematological toxity was observed in 75%. Fourty per cent of patients experienced G1 lymphopenia and 15.4% had G1 neutropenia. Grade 2 anemia was observed in 23.4% of patients. No patient had thrombocytopenia.

In patient who developed or worsened hematological toxicity, V8 was 84,2% (76%-97%) and Dmean was 17 Gy (14Gy-22.5Gy).The mean PTV volume was 1227 cc (950 cc -1625 cc).

Increased pelvic BM V8 and Dmean and a high PTV volume were associated with hematologic toxicity in rectal cancer patients undergoing SCRT. We suggest that  BM should be routinely considered to be an at-risk organ in rectal cancer treated with hypafractionated RT.