Background and purpose: Current IGRT protocols only correct for organ motion through a 3D translational movement of the treatment couch. The aim of this study was to quantify the relative importance of rotational vs. translational corrections in bladder and prostate IGRT.
Materials and methods: The data available consisted of a set of 9 bladder cancer patients each having a planning CT scan and between 3 and 8 repeat CT scans throughout their treatment course, with the bladder and prostate (for 5 of the 6 male patients) outlined on all scans. An algorithm was written to determine both the optimum translation and rotation angles required to align the repeat CTVs with their planning CTV. Angles considered were those possible through couch roll, rotation and tilt. The optimum shifts and angles were determined as those that minimised the volume of the repeat scan CTV lying outside the volume of the planning CTV. Two different situations were investigated: 1) 3D translation only (3 degrees of freedom (DoF)) and 2) rotation after applying the optimum 3D translation (6 DoF). Those repeat scans where rotation provided the greatest increase in CTV coverage were further investigated by determining the effect of rotation on the size of the treatment margins required and the volume of the resulting PTV.
Results: For the bladder, the overall average volume percentage (across scans and patients) of the repeat CTV included in the planning scan CTV was increased from 85.7% without IGRT to 89.5 and 90.1% with 3 DoF and 6 DoF, respectively. The corresponding results for the prostate were 79.4, 86.9 and 87.5%. The resulting decrease in treatment margins required was determined for the 3 bladder and 3 prostate situations where including rotation had the largest impact. In 2 of the 6 situations the resulting PTV volume was reduced by approximately 20% when using an isotropic margin, but this reduction was considerably less when the margins were individually optimised.
Conclusion: When treating either the bladder or prostate alone, translational IGRT correction was by far the most important action necessary to ensure alignment of the repeat CTV with the planning CTV.