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

Hendrik Dapper,Daniel Habermehl,Jan C. Peeken,Stefan Münch,Kai Borm,Stephanie E. Combs,Iván Rodríguez

doi:10.1186/s13014-018-1187-7

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.

Highlights

Neoadjuvant radio- or chemoradiation therapy is the standard treatment for loco-regional advanced rectal cancer patients of the lower or middle third
We evaluated the effect of intensity modulated radiotherapy versus conventional 3 dimensional (3D) irradiation on the dose distribution of the anal sphincters to estimate the risk of anal incontinence
Patient selection and radiation techniques Between 2008 and 2016 a total of 154 rectal cancer patients were treated with neoadjuvant radiation therapy in our institution, 106 with intensity modulated radiation therapy (IMRT) (either RapidArc (88) or TOMO (28)) and 48 with 3D-RT

Summary

Introduction

Neoadjuvant radio- or chemoradiation (nIRT) therapy is the standard treatment for loco-regional advanced rectal cancer patients of the lower or middle third. Due to improved dose sparing of organs at risk (OAR), intensity modulated radiation therapy (IMRT) has become the sole or equivalent standard in tumors of the pelvic which are treated with (neo)adjuvant, definitive or palliative radiation [12, 13] IMRT is usually associated with less dose to the rectum, small bowel and the urinary bladder [14,15,16,17,18] This translated into better clinical outcome measured by grade 2 and ≥3 acute gastrointestinal toxicity, genitourinary toxicity and skin side effects [19, 20]. Despite these advantages of IMRT, due to marginal misses in the first experiences of IMRT in rectal cancer, up to now, 3D-RT is still the recommended technical standard of treatment, even though IMRT is acceptable for special cases that do not meet dose constraints with 3D-RT [21]

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