Uncertainties of cumulative dose assessment for prostate IMRT
Intensity-modulated radiation therapy (IMRT) is capable of providing conformal doses to the target volume while minimizing doses to nearby normal tissue for radiotherapy of localized prostate cancer (1). However, the clinical gains of prostate IMRT may be limited by geometric uncertainties of a patient. The prostate movement between fractions, i.e., inter-fractional motion, is associated with patient setup uncertainty as well as internal organ displacement between fractions. The inter-fractional motion contributes to the deviation in the delivered doses to the target volume and organs at risk (OARs) (2). Depending on the filling state of the rectum and bladder, the prostate can move by more than 1 cm (3,4). Image guidance before each fraction can reduce these errors and deliver a precise prescription dose to the target volume while sparing doses to OARs. On the other hand, intra-fractional motion refers to the external patient movement as well as internal organ motion during the actual treatment. The major intra-fractional motion of the prostate gland is generally due to changes of gas in the rectum and surrounding digestive track (5). This temporal behavior of prostate motion can vary rapidly and substantially, which could result in large discrepancies between planned doses and actually delivered doses to a patient.