April 11, 2013

Today, we will be discussing colon and anal cancers.

Colorectal cancer is the most common gastrointestinal (GI) malignancy and has the best prognosis since esophagus and pancreas are usually diagnosed with advanced-staged disease.  Adenocarcinoma.  Risk increases with age, 90% occurring over 50 years of age.  It is the second leading cause of cancer death in the U.S. (third if you separate men and women).  Risk: diet high in animal fat and low in fiber.

Polyps are considered a precursor to the development of malignancy.  Virtually all patients with the hereditary condition Familial Adenomatous Polyposis (FAP) (studding of the entire bowel wall by thousands of polyps), if left untreated, develop colon cancer.  Three staging systems:  Dukes, Astler-Coller (MAC), AJCC (TNM).  TNM is most common.  Level of invasion into and through bowel is important for prognosis.

• Treatment techniques for colorectal cancer

1.      Surgery is treatment of choice.

a.       Low anterior resection (LAR) – rectum.

b.      Abdominoperineal resection (APR) – rectum.

c.       Colon resections vary by location.

2.      Radiation is most common as an adjuvant therapy for rectal cancer:  pre- or post-op in conjunction with chemotherapy due to high failure rate of surgery alone with nodal involvement.

3.      Cancer of the large bowel more commonly affects the rectum or distal colon.

4.      Presentations for rectal cancer:  rectal bleeding (hematochezia), change in bowel habits, diarrhea vs constipation, change in stool caliber, Tenesmus (spasms of the rectum accompanied by a desire to empty the bowel).

As needed, parts of the colon can be removed to get rid of the cancer by cutting out the cancer portion and suturing the ileum to the leftover colon.

APR:  Lower third of the rectum.  An anterior incision is made into the abdominal wall to construct a colostomy.  Then the perineal incision is made to resect the rectum, anus, and draining lymphatics, pulling the entire en bloc specimen out through the perineal opening.

• Field design

1.      Patients with rectal cancer are at a high risk of local recurrence; fields are typically designed to encompass the primary tumor volume and pelvic lymph nodes, shrinking the field to the primary target volume to a higher dose.

2.      Anatomic boundaries depend on surgical procedure preformed, relating to the areas most at risk for recurrence.

3.      Malignancies of the large bowel usually spread via direct extension, lymphatics, and hematogenous spread.  Direct extension of the tumor is typically in a radial fashion, penetrating into the bowel wall rather than longitudinally.

4.      For most patients with rectal cancer recurrence, it occurs in the posterior aspect of the pelvis – including metastasis to internal iliac and presacral lymph nodes (which aren’t included in a standard surgical resection and need to be included in the radiation portals).

5.      3-field (PA and opposed laterals wedged) is used to give homogenous dose to the tumor bed while sparing anterior structures.

6.      4-field (AP/PA) and opposed laterals may be used when the anterior structures (ex. prostate or vagina) are at risk for involvement or are involved.

7.      The most common type of distant metastasis is blood-borne spread to the liver, lung is second.

• Dosing

1.      For irradiation of the pelvis the dose-limiting structures are the small bowel and kidneys.

2.      How might you reduce small bowel dose?  Through patient positioning, full bladder, multiple-shaped fields, and dosimetric weighting.

3.      Dose is more easily reduced pre-op than post-op. Why might that be?  Dose is more easily reduced pre-op than post-op because the resection removes the rectum and peritoneum, allowing the small bowel to fall lower into the pelvis.

4.      The more superiorly the field extends, the more precautions are necessary to avoid small bowel injury and complications.

5.      Common dose to the large volume (tumor plus regional nodes) is 4500cGy with reduced field (primary tumor bed) to 5000-5500cGy in 6-6.5 weeks.

6.      Note:  doses in excess of 50Gy aren’t possible unless the small bowel is entirely excluded from the field.

• Simulation & positioning

1.      Use of rectal tube.

2.      Barium contrast injected into rectum.

3.      Oral contrast is given 45 min prior to scan to localize the small bowel.

4.      In female patients, a tampon soaked with iodinated contrast is inserted into the vagina.

5.      BBs are placed on the anal verge to reference the perineal surface on films.

6.      Lateral treatment portals and prone positioning with full bladder distention allows the small bowel to be excluded from the treatment volume.

• Acute side effects

1.      Worse when chemotherapy is concurrent

2.      Diarrhea, abdominal cramps & bloating.

3.      Proctitis – What is this?  Inflammation of the rectum/anus.

4.      Bloody or mucus discharge.

5.      Dysuria – What is this?  Painful urination.

6.      Leukopenia and thrombocytopenia.

7.      Moist desquamation (in patients who have perineum treated).

• Chronic side effects

1.      Less common, but more serious.

2.      Persistent diarrhea.

3.      Increased bowel movement frequency.

4.      Proctitis.

5.      Urinary incontinence.

6.      Bladder atrophy.

7.      Most common long-term complication is damage to small bowel leading to enteritis, adhesions, and obstruction.

• Anal cancer

1.      Squamous cell carcinoma. 

2.      Occur more often in women. 

3.      Median age 60. 

4.      There has been an increase in men younger than 45 attributed to male homosexuality and anal intercourse. 

5.      Risk factors: genital warts, genital infections, HPV, anal intercourse in men or women before age 30, and immunosuppression.

• Treatment techniques

1.      Standard treatment is a combination of radiation and chemo, with AP resection the most common surgical procedure for local recurrence

2.      Technique is a four-field or AP/PA with electron fields to inguinal nodes and including a boost to the tumor bed with perineal electron field

3.      What does it mean when we say AP/PA fields?  Anteroposterior/Posteroanterior fields.

• Dosing

1.      Radiation alone: 6000-6500cGy with field reduction after 4500cGy to reduce small bowel toxicity

2.      With combined modalities: 3060-4500cGy followed by shrinking field boost of 1440-2440cGy to the primary tumor.

3.      A higher total dose is advocated for T3/T4 disease.

4.      Dose-limiting structures: femoral heads and necks, genitalia/perineum, small bowel, and bladder.

5.      What is the TD 5/5 for the small bowel? Femoral head?  4500cGy for small bowel and 5200-5500cGy for femoral head.

• Side effects

1.      Moist desquamation.

2.      Bone marrow suppression – RTs are responsible for monitoring patient blood counts and reporting low counts

3.      What is a low WBC?  The normal WBC count range is 3.0-10.5 for adults meaning anything less than 3.0 is low.

Reflections:
One of the risk factors for colon cancer is eating animal fat and not eating enough fiber.  Personally, I think it is eating meat in place of eating fruits and vegetables.  Our bodies are not very good at metabolizing meat products.  We are much better at metabolizing vegetables and fruits.  I don't know remember where I saw or heard it, but a carnivores digestive system is not like ours.  They eat the meat and poop it out within three hours, and that apparently is the reason they can eat raw meat and be healthy.  The meat doesn't have enough time to spoil inside their bodies making them sick.  If we eat raw meat, it is going to be in our bodies long after the meat has spoiled and made us very sick.  We cook our meats so that we can enjoy them, but really we are ruining the meat's nutrition properties.  If we want to reduce the colon cancer maybe we should eat less meat including fish, chicken, and turkey.