Comparison of conventional and three-dimensional conformal CT planning techniques for preoperative chemoradiotherapy for locally advanced rectal cancer

Abstract

Objectives:We assessed the impact of three-dimensional (3D) conformal planning vs conventional planning of preoperative chemoradiotherapy (CRT) for locally advanced rectal cancer (LARC) on small bowel and bladder sparing and in optimising coverage of tumour target volume. Methods: Conformal and conventional plans were created for 50 consecutive patients. The conformal plan delineated a gross tumour volume (GTV), a clinical target volume (CTV) 1 to cover potential subclinical disease spread, a CTV2 to outline the mesorectum and lymph node areas at risk, and a planning target volume (PTV) to cover set-up error and organ movement. The conventional plan was created using digitally reconstructed radiographs (DRRs). Patients were treated with a dose of 45 Gy in 25 fractions with concurrent chemotherapy over 5 weeks. Dose-volume histograms (DVHs) were created and compared for GTV, PTV, small bowel and bladder. The GTV was covered by the conventional plan in all patients. Results: Significant differences were shown for median PTV coverage with conformal planning compared with conventional planning: 99.2% vs 94.2% (range 95.9-100% vs 75.5-100%); p,0.05. The median volume of irradiated small bowel was significantly lower for CT plans at all DVH levels. Median bladder doses did not differ significantly. Conclusion: 3D conformal CT planning is superior to conventional planning in terms of coverage of the tumour volume. It significantly reduces the volume of small bowel irradiated with no decrease in the rate of R0 resection compared with published data, and at the present time should be considered as the standard of care for rectal cancer planning. Chemoradiotherapy (CRT) followed by total mesorectal excision is the standard for care when MRI staging demonstrates threatened surgical margins in locally advanced rectal cancer (LARC) [1,2]. Radiotherapy planning for rectal cancer uses conventional orthogonal simulation with standardised radiation fields based on patterns of loco-regional relapse in relation to pelvic bony anatomy [3]. Three-field conventional orthogonal planning is considered an acceptable technique for planning preoperative CRT and major trials evaluating long-course chemoradiation for rectal cancer have permitted the use of conventional planning within their protocols [4,5]. In recent years, the treatment of rectal cancer has improved through advances in the planning and delivery of radiotherapy as well as improved preoperative imaging with MRI, the development of surgical techniques using total mesorectal excision (TME) and more accurate histopathological reporting [6]. Radiotherapy planning must ensure all clinically and radiologically identifiable disease is encompassed while still minimising the dose to the surrounding organs at risk, particularly the small bowel and bladder. Potential areas of microscopic spread and the appropriate pelvic lymph nodes should also be treated.Preoperative MRI has improved the knowledge of pelvic anatomy and identification of pelvic lymph nodes at risk according to tumour l...

The Use of DRR Designed Fields Is Unnecessarily Toxic and Geographically Inadequate, Analysis of Drr Designed Fields in Preoperative Radiotherapy in Rectal Cancer

“…In our study the most evident result was the big volumetric difference between DRR arm and contouring arm, were DRR volume was nearly double that of contouring with the mean value of 2886 cm3 for DRR vs 1405 cm3.for PTV contouring. Our results are little bit larger than other similar studies were 2D PTV volume were usually larger but not that much generous [9].…”

Is Two-Dimensional Field Definition Sufficient for Pelvic Node Coverage in Rectal Cancer Compared to Technical Three-Dimensional Definition?

“…The under-dosage was, in most cases, located in the anterior and inferior part of the target volume. 3D TP provided a significantly lower irradiation of the small bowel without any difference on the bladder 23 .…”

“…However, enrollment was limited to patients treated preoperatively, and all were positioned on a false table-top device resulting in displacement of the small bowel from the pelvis, as verified with CT scan 10 . The bladder was not considered since prior reports did not identify a significant difference in bladder irradiation with the two tech- niques 22,23 . Lastly, tumor location was not specified.…”

Radiation induced toxicity in rectal cancer patients

“…The lim it ing fac tors for deliv er ing tumoricidal doses of ra dio ther apy ac cord ing to rec om men da tions are pre scribed (Fig ure 1. ) 34 In or der to avoid oc cur rence of se vere com pli ca tions at sur round ing nor mal tis sue, it is nec es sary to fol low dose con straints of or gans at risk (blad der, small bowel and fem o ral heads). Med i cal phys i cist cal cu lates dose dis tri bu tions and fields ar range ment, based on pre scrip tions done by ra di a tion oncologist.…”