The use of shifted-isocentre techniques for plan evaluation

In radiotherapy treatment planning, the effect of geometric uncertainties is usually taken into account by adding margins, creating the Planning Target Volume (PTV) from the Clinical Target Volume (CTV). The use of the PTV is to ensure that the CTV receives the prescribed dose. Clinically, a plan is acceptable if the 95% isodose surface covers the PTV. For shallow tumors where the PTV extends into the build up region, PTV coverage is normally unacceptable following the above criterion due to the region of low dose near the skin surface. As well as PTV coverage, organ-at-risk (OAR) dose distribution is also used as a criterion to evaluate the plan. The use of OAR endpoints is normally applied for static DVH. An alternative planning technique is to calculate multiple subplans with different isocentres chosen using a probability distribution; in this method, a plan is acceptable if all the subplans give good coverage of the CTV and satisfy OAR endpoints. We describe the use of this method to validate the use of PTV coverage for plan evaluation for shallow tumors and to validate the use of endpoints for rectal dose evaluation in prostate cancer for IMRT and conformal planning. The results show that using PTV coverage as an indicator for plan quality in shallow tumor cases is not adequate. Single rectal DVH is not enough to evaluate the endpoints; an acceptable plan can become an unacceptable plan when patient-beam positioning uncertainties were taken account into.