What tolerance do you have for moving a patient (pelvis) back to the reference capture taken after the initial CBCT, without needing a further CBCT to confirm?
If your tolerance is over 0.5cm what’s your evidence to confirm this as safe practice?
Thanks
Nick
Hi Nick
May be there is literature to confirm what kind of movement is too much etc. We have 5 mm RTD tolerance in pelvis .
I can give an example, which may make clear why it cannot be anyway wider. In pelvis CBCT matches, we have cases where we have to leave residual errors to the surrounding targets after matching exactly to the main target. For example, if the match is in prostate cases exactly in markers, there is always some kind of residual error to the lymph nodes. Lets think that the matching is done to the markers and the residual matching error to the lymph nodes is 7 mm. We decide to treat the patient. Now, the patient moves 5 mm during the treatment based on SGRT. We take images. The error to the markers is 5 mm and to the lymph nodes may then be 1.2 cm, which neither are acceptable, definitely not in the lymph node area at least, even if there is more space for displacements.
My point is that we cannot think that the error is zero everywhere in PTV at the time we reset the reference surface and begin to treat the patient with 0 RTD and this should always be taken into account.