Acute Toxicity From Short-Course Radiation Therapy for Rectal Cancer: A Single-Institution Experience.

Purpose: Short-course radiation therapy (SCRT) is an important neoadjuvant treatment of locally advanced rectal cancer. Although SCRT offers logistical advantages and possibly reduced toxicity compared to long-course chemoradiation, emerging data have raised concerns about long-term outcomes and underrecognized adverse effects. This study examines acute toxicities after SCRT, including a poorly understood syndrome, acute lumbosacral plexopathy (ALSP). Methods and Materials: Patients with locally advanced rectal cancer treated between 2016 and 2022 with neoadjuvant SCRT (25 Gy/5 fractions) were retrospectively analyzed. Acute toxicity was assessed up to 30 days after SCRT completion. ALSP was defined as pelvic or buttock pain with or without radiation down the lower extremities. Patients without reported ALSP were matched to cases based on N stage, tumor location, treatment position, and treatment modality, and the lumbosacral plexus and psoas muscles were contoured for matched patients. Dosimetric and clinicodemographic variables were compared for patients with and without toxicity using the Mann-Whitney U test and χ Results: Among 71 patients identified, the median age was 56 years, and approximately half were female. The 30-day incidence of any-grade toxicity was 75%; one-third of first toxicities occurred more than 10 days after the start of SCRT. Notably, 10 (14%) patients developed ALSP, presenting at a median of 2.5 days after treatment initiation and resolving within 1 week. Most were treated with nonsteroidal anti-inflammatory drugs or acetaminophen, but 2 required additional medications. ALSP was not associated with planning target volume dose, small bowel dose, lumbosacral plexus dose, or muscle composition parameters. Conclusions: Three-quarters of patients treated with SCRT develop any acute toxicity, and ALSP was identified in 14% of patients. All cases of ALSP were transient, without late sequelae. These findings highlight the importance of toxicity surveillance beyond the treatment window. Future work should identify predictors of ALSP and potential mitigation strategies.