Pelvis contouring

The anus is the last part of the bowel; its sphincter muscles control the passing of stool.

• The NRG anal cancer template describes the anal canal as running from the anal verge to the anorectal junction, with the internal and external sphincter muscles around it.
• Gay 2012's AnoRectum combines the anus and rectum, starting at the anal verge marked with a radiopaque marker at simulation.
• In the NRG vulva template the rectum starts 2 cm above the anal verge, so the anus covers the segment below it.

Contour the anus separately from the rectum when your protocol limits them separately, and check that the two contours meet without gaps or overlap.

The bladder stores urine until it is passed.

• Gay 2012: from the base to the dome.
• The NRG anal cancer template: contour the outer bladder wall and fill it in as a solid structure.
• Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): the bladder wall and all urine, ending at the base of the prostate. Timmerman also defines a suprapubic bladder wall structure — the front, lower wall resting on the pubic bone, from the prostate up 2–3 cm.

Bladder filling changes the volume a lot, so follow your protocol's filling instructions.

The external genitalia are the outer sex organs.

The NRG anal cancer template requires a "Genitals" structure — male or female external genitalia — and notes that consistent repositioning of male genitalia at simulation and treatment matters. It gives no further boundaries. Detailed boundaries from an open published source: verified source required; contour as your protocol describes.

The femoral head is the ball of the hip joint, carrying the body's weight from the pelvis to the leg.

Sources draw quite different structures:

• Gay 2012 (proximal femur): from the top of the ball of the femur down to the lowest level of the ischial tuberosities, including the trochanters. A bone threshold auto-contour helps, but edit it afterwards.
• The NRG anal cancer template (femoral head): the femoral head excluding the femoral neck, with the bottom border at the lesser trochanter.
• Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): contour the right and left separately.

Because the volumes differ, a limit only makes sense on the structure it was written for.

Bone marrow makes the body's blood cells; in adults, much of the active marrow lies in the pelvic bones.

The NRG anal cancer template contours BoneMarrow from the outer bone contours of the sacrum, iliac crests, acetabula and proximal femurs as one solid, continuous structure, stopping at the bottom at the level of the ischial tuberosities.

Definitions vary between protocols, and a limit applies only to the structure it was written for, so match your contour to your protocol.

The penile bulb is the base of the corpus spongiosum, the erectile tissue around the urethra at the root of the penis.

Gay 2012: the part of the bulbous spongiosum immediately below the urogenital diaphragm; do not extend it forward into the shaft. It is best seen on MRI (bright on T2) or on CT with contrast in the urethra; on CT it sits behind the urethra and is round.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): start at the bottom of the pelvic diaphragm (urethral sphincter) and extend down and forward up to 3 cm.

The rectum stores stool before it leaves the body.

Gay 2012 gives two versions:

• Rectum (GU): from the lowest level of the ischial tuberosities up to where the rectum loses its round shape on axial images and joins the sigmoid in front.
• AnoRectum (GYN): the same top, but starting at the anal verge marked at simulation, so it includes the anus.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): one structure including the rectal wall and all contents; its Table 9 instruction reads "start contouring 10 cm superior to PTV then inferior to anal sphincter." Make sure your rectum and anus contours match the definitions behind your limits.

The urethra carries urine from the bladder out of the body; its first part runs through the prostate.

In Richardson 2021 the prostatic urethra was contoured inside the prostate CTV from the bladder neck to the prostate apex, on the sagittal view with a 2 mm brush. Large TURP defects inside the CTV were included. A further 3 mm expansion made a cylindrical urethra PRV.

The study found a thin (0.9 mm), 3D T2-weighted MRI cut variation between observers markedly compared with planning CT or a standard 3 mm T2 scan. The urethra can still need some estimation, so use the method your protocol describes.