This retrospective study included patients with LARC (UICC, “Union Internationale Contre le Cancer” stage III) who were treated with preoperative chemoradiotherapy and SIB to at least one LPLN. All patients had magnetic resonance imaging (MRI) of the pelvis and computed tomography of the chest and abdomen as staging procedures.

Lateral pelvic lymph nodes were considered affected if they met a least two of the following criteria: round shape, short axis diameter of more than 7 mm, and mixed signal intensity on MRI (analogous to Ogura et al. 2019, Kim et al. 2008, and Li et al. [2, 11, 12]).

Radiotherapy was planned as intensity-modulated radiotherapy (IMRT) on a linear accelerator with daily cone-beam computed tomography (CT)-based image guidance in all cases. IMRT planning was performed on CT scans with 3 mm slice thickness. Patients were positioned in supine position as per our institutional standard. Target volume definition for the primary tumor and the local lymphatics was according to international guidelines [13]. For small bowel and the bladder, a maximum point dose of 56 and 62 Gy, respectively, was accepted. All patients received chemoradiotherapy at the Department of Radiation Oncology of the University Hospital Tübingen, Germany. The primary tumor, the mesorectum, and lateral pelvic lymph node stations received 50.4 Gy in 28 fractions, and positive LPLNs received a median dose of 60.2 Gy (interquartile range 60.2–61.6 Gy). A case example with positive LPLNs and the treatment plan is shown in Fig. 1a, b. Concomitant chemotherapy was applied as 5‑FU alone or combined with oxaliplatin, as previously reported [14]. Consolidative chemotherapy with FOLFOX or CAPOX was considered if organ preservation was intended or to improve disease-free survival, after the publication of two randomized phase III studies proved the benefit of total neoadjuvant therapy (TNT) [15, 16].

a Example of a female patient who presented with two round-shaped lateral pelvic lymph nodes (white arrow; b). The lymph nodes (in green; corresponding planning target volume [PTV] in black) were treated with 61.6 Gy in 28 fractions; the red line shows the 95% isodose relative to the 61.6 Gy volume. The pink line indicates the 95% isodose relative to the 50.4 Gy PTV in blue prescribed to the mesorectum and elective nodal volumes. Bowel loops are segmented in orange. c The patient achieved a clinical complete response at all tumor sites (primary, mesorectal lymph nodes, and lateral pelvic lymph nodes) and was managed nonoperatively

Total mesorectal excision was performed according to standard techniques. Abdominoperineal extirpation was indicated if the distal end of the tumor was within 5 mm of the dentate line after radiochemotherapy. Patients were considered for nonoperative management when a clinical complete response (cCR) was seen after chemoradiotherapy. A cCR was defined as the absence of any residual finding beyond a white scar, telangiectasis, or a small ulcer on endoscopy. In this scenario, MRI was used to evaluate previously suspicious lymph nodes. Lymph nodes were considered negative if they were smaller than 5 mm in the short axis.

Total mesorectal excision quality was evaluated according the MERCURY grading system [17], and postoperative complications were evaluated according to Clavien–Dindo [18].

Continuous data are reported with the median and interquartile range (IQR). Toxicity was assessed according to the Common Terminology of Adverse Events (CTCAE). Time-to-event data were calculated according to Kaplan–Meier. Overall survival (OS) was the time from primary diagnosis until death. Disease-free survival (DFS) was calculated from primary diagnosis until death or any recurrence. For local control (LC), progression of the dose-escalated lesion on an imaging study was considered an event. Median follow-up was determined according to the inverse Kaplan–Meier method. For group comparisons, the log-rank test was used. Statistical analyses were conducted in SPSS (SPSS 26, IBM, Armonk, NY, USA) and R (version 4.0.3; R Foundation, Vienna, Austria).

The study was approved by the ethics committee of the Medical Faculty of Tübingen, Germany (approval ID: 733/2015B01).