Impact of VMAT-IMRT compared to 3D conformal radiotherapy on anal sphincter dose distribution in neoadjuvant chemoradiation of rectal cancer

Abstract

BackgroundNeoadjuvant radio- or chemoradiation (nIRT) therapy is the standard treatment for loco-regional advanced rectal cancer patients of the lower or middle third. Currently, intensity modulated radiation therapy (IMRT) is not the recommended radiation technique even though IMRT has advantages compared to 3D-radiation regarding dose sparing to organs at risk like small bowel and urinary bladder. So far, the benefit of IMRT concerning the anal sphincter complex is not examined. With this study we intended to evaluate the dose distribution on the anal sphincters of rectal cancer patients treated with IMRT in comparison with 3D-techniques.MethodsWe selected 16 patients for the IMRT-group and 16 patients for the 3D-group with rectal cancer of the middle third who were treated in our institute. All patients received 45 Gy in a chemoradiation protocol. Patients in both groups were matched regarding stage, primary tumor distance to the anal verge and size of the tumor. We delineated the internal and external anal sphincters, the addition of both sphincters and the levator ani muscle in all patients. Subsequently, we evaluated and compared dose parameters of the different sphincters in both groups and analysed the configuration of the isodoses in the area of the caudal radiation field, respectively.ResultsMost of the relevant dose parameters of the caudal sphincters (Dmean, Dmedian, V10–V40) were significantly reduced in the IMRT-group compared to the 3D-group. Accordingly, the isodoses at the caudal edge of the target volume in the IMRT group demonstrated a steep dose fall. The levator ani muscle always was included into the planned target volumes and received the full dose in both groups.ConclusionsThe modern VMAT-IMRT can significantly reduce the dose to the anal sphincters for rectal cancer patients of the middle third who were treated with conventional chemoradiation therapy.

Dosimetric comparison of organs at risk in 5 different radiotherapy plans in patients with preoperatively irradiated rectal cancer

“…[ 7 – 9 ] Comparison of 3-dimensional conformal radiation therapy (3D-CRT) and IMRT showed better sparing of OARs, dosimetric results, and target coverage in favor of IMRT; therefore, IMRT has become the standard radiation technique in pelvis located cancers. [ 10 – 12 ]…”

Standard (8 weeks) vs long (12 weeks) timing to minimally-invasive surgery after NeoAdjuvant Chemoradiotherapy for rectal cancer: a multicenter randomized controlled parallel group trial (TiMiSNAR)

“…Although results from comparative randomized clinical trial are not available yet, IMRT is usually associated with less dose to organ at risk, such as urinary bladder, small bowel and anal sphincters (in selected cases). This is translated into better clinical outcomes, in terms of gastrointestinal toxicity, genitourinary toxicity and skin side effects [16–20].…”

Radiation-Induced Pelvic Colorectal Obstruction After Neoadjuvant Chemoradiotherapy and Sphincter-Saving Surgery for Rectal Cancer: Is IMRT Better?

“…A previous meta-analysis of 859 patients from six studies showed that IMRT was associated with lower incidences of ≥ grade 3 acute overall gastrointestinal toxicity, diarrhea, and proctitis compared with 3DCRT [8]. Compared with conventional chemoradiotherapy, IMRT can also signi cantly reduce the dose distribution to the anal sphincters for patients with rectal cancer [13]. Whether IMRT can decrease the dose distribution to proximal resection margins and radiation injuries need further studies.…”