Dosimetric Optimization Of Postoperative Radiotherapy Of Rectal Carcinoma

Rectal carcinoma is a common malignant tumor, and rectal carcinoma with highincidence in Europe and the United states，new cases approximately140000, deaths about55000each year, as the third of malignant tumor mortality. According to the statistics of ourcountry in1985, the incidence of colorectal carcinoma for male fifth, sixth for female. Inrecent years, because of the improvement of living standards, rectal carcinoma incidence inChina is raising trend.The rectal carcinoma postoperative radiation modality is changed from three fieldsconventional radiation therapy to3D-CRT/IMRT along with the development of theradiation therapy technology. Much more protection of normal tissue and fatality to thetumor is needed, with the improving of patients’ requirements for life quality. In the IMRTof rectal carcinoma, the irradiation dosage of the small bowel and bladder is concerned bythe radiologist. But there is no standard program of the evaluation of small bowel for its un-fixation.Can we reduce the irradiation of small bowel by change the positioning, bladder filingstatus and radiation modality? In this study, we try to get a primal result of the idea.The goals of this study were to evaluate the effect of body position, bladder fillingstatus and radiation modality on the postoperative radiation of rectal carcinoma and tocompare the dose–volume histograms (DVHs) and determine the potential impact on acuteand long-term toxicity based on the dose to target and non-target tissues for each planningtechnique.Six patients referred for postoperative radiotherapy at the Cancer Center of China-JapanUnion Hospital were selected. The TNM stage was ⅡA or ⅡB according to the2002staging system. Each patient got the CT scan on different position and bladder filling status.Normal tissues and critical structures were contoured on axial CT slices. Normal tissuesincluded small bowel and bladder. CTV included tumor bed, total area, and soft tissue ofanterior sacral, common iliac and presacral nodal groups, ischiorectal fossa. Treatmentplanning included three fields conformal radiotherapy, three fields3D-CRT and a five-fieldIMRT. The dose–volume histograms where then compared for both treatment delivery systems.The conformal index (CI) has tiny difference between supine and prone positioning;and it similar no matter the bladder is empty or full, but it apparently different when theradiation modality shift, CI5F-IMRT>CI3F-3D-CRT>CI3F-2D, P<0.05. The homogeneity index (HI)and supine or prone independent, and HI and Bladder filling state independent, P>0.7, HIonly and irradiation technology, HI5F-IMRT=HI3F-3D-CRT>HI3F-2D. The irradiated volume ofeach organ of interest, such as small bowel and bladder, V5F-IMRT<V3F-3D-CRT=V3F-2D. Thevolume of small bowel becomes smaller when the bladder is full filled. The pronepositioning plan can reduce the irradiated volume of the bladder only when the bladder itselfis full filled.The conformal index (CI) has little relationship with the body positioning (supine orprone) and bladder filling status (empty or full), but it apparently correlates with theradiation modality, IMRT is the most conformal radiotherapy. The small bowel can get lessirradiation when the bladder is full. Although the body position hasn’t much relationshipwith the IRO irradiation volume, combine the prone positioning full bladder can decrease thedose of the bladder. On the whole, treat the patients with IMRT in prone positioningcombine with full filling bladder can reduce the normal tissue irradiated volume, which mayreduce the complication in the clinical work.