Session Item

In the era of growing interest for Total Neoadjuvant Treatment (TNT) in rectal cancer several studies show improved oncologic outcomes by intensifying chemotherapy regimens before surgery. However not every patient is fit for such a strategy of chemo-intensification. Another element of the treatment of rectal cancer with potential to be intensified is radiotherapy. We report on a new approach with dose intensification through a simultaneous integrated boost (SIB) in short-course preoperative radiotherapy for rectal cancer patients (SCRT). 

Between February 2014 and June 2019, 41 patients with rectal adenocarcinoma (27 male, 14 female; median age 68 years), were treated with SIB-SCRT (fig1). Inclusion criteria for SIB were advanced age, severe comorbidity precluding surgery, or tumor high-risk features requiring prompt systemic therapy. The planning target volume (PTV_pelv) consisted of the mesorectum and lymphatic areas at risk (CTV_pelv+ 5mm) and received 25 Gy over 5 consecutive days, with a SIB between 27.5-35 Gy on PTV_boost (GTV_boost+5mm) according to the extent of the disease and proximity to OARs. A 4 arc 10FFF VMAT plan (Eclipse™) was used (fig1). PTV coverage was defined as the volume receiving the prescribed dose. OAR dose constraints were derived from published hypofractionation schemes:  D_5cc<19.5Gy, D_max 35Gy for small bowel (SB); D_15cc<18.3Gy for bladder (B) and D_20cc<25Gy for sigmoid (S), with D_max 38Gy for both; D_10cc<30Gy for femoral heads (FH); and D_5cc<30Gy, D_max 32Gy for cauda equina (CE). Pre-treament QA (ArcCheck®) was performed for all patients using gamma (3%/3mm) passing rates >90%. Patients were set-up with CBCT imaging for all fractions.

At a median follow-up of 38 months (mean 46±36), 23% of patients died of systemic disease progression. Treatment was well-tolerated by all patients. Acute and late RTOG Gastrointestinal (GI) score of grade 3 occurred in 14% and 0% of the cases (no grade 4), and genitourinary (GU) grade 2 in 5% and 4% (no grade3), respectively. Mean CTV was 418±199cc and 57±46cc for the pelvis and the boost, respectively. Dose coverage was 98±3% (PTV_pelv) and 98±6% (PTV_boost). SB and B sparing was the main reason for reduced PTV coverage. Dose tolerances were met for all OAR with a few exceptions for the B, S and SB (see fig 2), in order to preserve the PTV_pelv standard coverage level (25Gy). Dose to FH was exceeded for one patient due to encompassed bone metastasis within the treatment field .  QA results were fulfilled for all patients (99±1%), showing that tighter gamma passing rate criteria can be applied.

Dose intensification in short-course RT for rectal cancer is a new, feasible and safe approach to be considered in selected rectal cancer patients. Whether SIB-SCRT can achieve higher rates of clinical or pathological responses compared to the standard long-course chemoradiotherapy needs to be evaluated prospectively in randomized clinical trials.