The study compares two patient setups (Standard Prone vs. Genital Drop Technique) with identical radiation treatment technique and parameters. After the two setups have been planned, the treating faculty reviews the setups and will choose the setup he feels that will give the patient improved dosimetry to the local regions and hopefully reduce toxicity and improve treatment tolerability.

This is a study of two different setups when treating a patient with standard of care radiation doses. The study compares two patient setups (Standard Prone vs. Genital Drop Technique) with identical radiation treatment technique and parameters. After the two setups have been planned, the treating faculty reviews the setups and will choose the setup he feels that will give the patient improved dosimetry to the local regions and hopefully reduce toxicity and improve treatment tolerability.

Patients will not be randomized or stratified by demographic or disease risk assessment, but treatment will be tailored to stage specific current standards of care.

An extra (non standard of care) CT is required to analyze the data.

Inclusion Criteria:

• Histologically proven anal cancer (squamous, basaloid, or cloacogenic carcinoma) or distal rectal cancer (adenocarcinoma variants) that traverse the dentate line and are eligible for definitive chemoradiation or preoperative radiation therapy, respectively.
• Any histological grade.
• Age: >18 years.
• Gender: Male patients with average external genitalia.
• Race: Any.
• Language: English speaking only.
• Performance status: Good (Karnofsky Performance Status of >60%). (ECOG 0-2).
• UTMB patients
• Stages: Early local disease to locoregionally advanced disease per the 2002 American Joint Committee on Cancer (AJCC) Staging System.
• Anal Cancer: Stages I-IIIB (see Section 11).
• Distal Rectal Cancers that pass the dentate line and into the anal canal only: Stages IIA-IIIC (see Section 11).

Exclusion Criteria

• Histological variants other than those listed above in 4.a.
• Patients with cystocele or abnormal scrotal edema will be excluded. (Although the Genital Drop Technique may specifically benefit this group of patients, however this study is to be performed with average patient anatomy).
• 2002 American Joint Committee on Cancer (AJCC) Staging System. (Anal Cancer: Stage IV.; Distal Rectal Cancer: Stage IV, and tumor 2-node 0-metastasis 0 (T1-T2N0M0) patients).
• Performance status: Poor (Karnofsky Performance Status of <60%). (ECOG 3-4).
• Non-English speaking patients.
• Prior pelvic radiation.
• Prior pelvic malignancy.
• Anal cancer/rectal cancer surgery, except for biopsy at study site.
• Patient's mental condition and social support is such that he can neither understand the nature of the protocol nor comply with its requirements.

Male anal cancer and distal rectal cancer patients 18 years or older.