Physical examination is an important part of the staging and planning process and should include detailed assessment of the characteristics of the primary tumor (size, anal sphincter competence, invasion of adjacent structures) as well as an assessment of inguinal lymph nodes.

Lymph nodes in the inguinal region that are suspicious but borderline should be biopsied (nearly 50 % of suspicious nodes are related to reactive hyperplasia).

Standard imaging includes CT or MRI of the pelvis to assess the primary tumor and status of the regional lymph nodes. CT of the chest and abdomen completes the metastatic workup.

These tumors can be well visualized on PET, so PET/CT is becoming a standard part of staging and recommended for planning to help delineate extent of gross disease (Fig. 1).

Areas of low uptake on PET should not supersede physical exam findings or abnormalities seen on CT.

The patient can be simulated in the supine position in a body mold or other immobilization device to ensure reproducibility. Prone position with the use of a belly board can be used to allow for anterior displacement of the bowel, but setup reproducibility is more variable and using bolus or additional electron fields to supplement dose to the inguinal regions would not be possible. A radiopaque marker should be placed on the anus.

CT simulation using ≤3 mm thickness with IV contrast should be performed to delineate the pelvic blood vessels and gross tumor volume (GTV). If PET/CT is available, a PET/CT fusion should be obtained to aid in target volume delineation. MRI may also be useful.

Bladder filling/emptying should be considered. A full bladder may keep bowel from migrating into the pelvis. An empty bladder may be more reproducible.

We recommend daily orthogonal kilovoltage images and weekly cone beam CT scans (to assess soft tissue) to verify alignment during treatment. Cone beam CTs may be done more frequently if there is significant variation in bladder and/or rectal filling.

Conventional radiation therapy for anal canal cancers is complex due to the need to irradiate the groins as well as the pelvis. The “thunderbird” technique was the most common method used to treat anal cancer. An example of the thunderbird technique compared to an IMRT plan is shown in Fig. 2. In the thunderbird technique, a wide AP field is used to encompass the inguinal lymph nodes. The PA field is narrowed to exclude the inguinal lymph nodes so that the femoral heads are also excluded. An additional boost field must therefore be used to supplement dose to the inguinal regions. This can be accomplished with either photons (with a skin match or a deep match) or electrons. A more detailed description of these thunderbird technique variations is described by Gilroy et al. (2004). Figure 2 shows a standard dose distribution of a photon/electron thunderbird technique.

RTOG 0529 (Kachnic et al. 2013) has established the feasibility of IMRT in a multi-institution setting and demonstrated lower rates of grade 2 or higher dermatologic toxicity and lower rates of grade 3 or higher gastrointestinal or genitourinary toxicity when compared to historical controls in the RTOG 98-11 (Ajani et al. 2008) trial.

The primary gross tumor volume (GTV-P) is defined as all gross disease on physical examination and imaging.

The nodal GTV (GTV-N) includes all nodes that are ≥1.5 cm, PET positive, or biopsy proven. In the absence of biopsy, any clinically or radiographically suspicious lymph nodes should be included in the GTV-N.

The high-risk clinical target volume (CTV-HR) should include the entire mesorectum, the bilateral internal iliac nodes inferior to the inferior-most level of the sacroiliac joint, and the inguinal or external iliac lymphatics if the inguinal nodes are involved. Additional details are given in Table 2.

The low-risk CTV (CTV-LR) should include the uninvolved inguinal, external iliac, and internal iliac nodes superior to the inferiormost level of the sacroiliac joint.

Figure 3 shows a case example of a T2N0 anal cancer.

Figure 4 shows a case example of a T3N3 anal cancer.

Detailed contouring atlases available include the RTOG anorectal contouring atlas (Myerson et al. 2009) and the Australasian GI Trials Group Atlas (Ng et al. 2012).

The RTOG anorectal contouring atlas (Myerson et al. 2009) describes 3 CTV regions that should be included for all patients with anal canal cancer. CTV-A includes the perirectal, presacral, and internal iliac regions. CTV-B includes the external iliac nodes. CTV-C includes the inguinal region. Table 3 provides a more detailed description of these regions.