Table 1.

SPARK organs at risk contouring guidelines

Structure name	Description
Rectum	Contour as a solid structure from recto-sigmoid junction to lower aspect of ischial tuberosities, the latter of which usually corresponds to the anorectal junction.
Bladder	Contour the whole organ as a solid structure. If CT and MR volumes disagree, use the imaging with the smaller bladder volume.
Penile bulb	Contour from MRI, if available, otherwise use CT dataset.
Prostatic urethra planning target at risk volume (PRV)	Estimate urethral position, and add 3 mm radial expansion. If IDC in situ, add 1 mm radial margin to IDC. Please note that a dedicated urethra structure (from which the Urethra_PRV is created) is desirable but not mandatory.
Neck of Femur	Contour the Left and Right NOF as solid structures to the level of the ischial tuberosity.
Remaining volume at risk (RVR)	Defined as the imaged volume within the patient, excluding any delineated OAR and the PTV. The RVR is used to identify unsuspected regions of high absorbed dose (ICRU 83).