Re-defining rectal volume and DVH for analysis of rectal morbidity risk after radiotherapy for early prostate cancer.

Improved prostate cancer cure rates have been attributed to higher radiotherapy dose prescriptions delivered more safely by modern conformal/intensity-modulated radiotherapy (IMRT) methods. As the dose becomes more concentrated conformally on the prostate, the volume of the rectum "at risk" for damage becomes smaller and more focal on the anterior rectal wall between the upper and lower axial limits of the planning target volume (PTV). The rectal dose-volume histogram (DVH) traditionally studies the whole volume of the rectum, and such definition for "avoidance" planning presupposes that rectal tolerance depends on "whole organ" radiation tolerance (as might, for example, lung or kidney). However, rectal morbidity with modern prostate radiotherapy is determined by anterior rectal wall tolerance between the superior and inferior limits of the PTV; this, we argue, is not dependent on whole organ tolerance. Recent published studies attempting to improve rectal DVH definition have studied the rectal wall only and concluded that rectal wall DVH is more relevant than whole rectum. In this manuscript, it is first demonstrated that a large and more relevant difference exists when comparing whole rectal DVH to "PTV limits" rectal DVH. Secondly, when considering "PTV limits" rectal DVH, the wall vs whole perimeter comparison differs little. Furthermore, by adopting a "PTV limits" DVH, the inferior right quartile of the DVH accurately reflects the dose distribution to the most vulnerable section of the anterior rectal wall. With improving IMRT technologies, scrutiny of this part of the rectal DVH will most accurately predict rectal sparing - reflected in this manuscript by the less precipitous decline of the TomoTherapy DVH vs the three-dimensional conformal DVH towards the maximum dose point received by the rectum.