PMC5723493_01
Beam's-eye view (BEV) of field #7 at the abdominal isocenter in the collimator-rotated plan. By rotating the collimator angle to make the PTV more "diagonal" in the BEV, the abdominal beams could encompass more of the pelvic PTV, and the pelvic beams could encompass more of the periaortic PTV.
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By rotating the collimator angle and making the PTVs more "diagonal" in the BEV, the abdominal beams could encompass more of the intrapelvic nodes (Fig. 1), and the pelvic beams could encompass more of the periaortic nodes.
The 7 gantry angles overlaid on top of the pelvic isodose distribution for (a) the original IMRT, gantry 0 plan, (b) the 3D (gantry 0) plan, and (c) test 5 IMRT plan with opposite gantry angles. The bladder (purple) and the sigmoid colon (gray) are at the center and are deeply embedded in the intrapelvic nodes (red). Both IMRT plans tailored the prescription isodose (yellow) around the bladder and sigmoid colon, excluding these two organs from the highest dose. The 3D plan, as expected, delivered a convex uniform dose for the whole area, and the doses to bladder and sigmoid colon were as high as those of the PTVs.
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Test 5 was a plan with all gantry angles inverted 180  and opposite to the original. The seven gantry angles are depicted in Fig. 2.