The Role of Radiation Therapy

 

 

Adenocarcinomas

Radiation therapy can be used as an adjuvant to surgery or as primary palliative treatment of small bowel adenocarcinoma. Anatomic constraints and the intrinsic sensitivity of the normal small bowel to radiation limit the role of radiation as a locoregional treatment. Localized segments of the small bowel can tolerate doses of 45 to 50 Gy with conventional fractionation of 1.8 to 2.0 Gy per fraction; however, tolerance to radiation is significantly less when large volumes of bowel are in the radiation field. Because the small bowel and surrounding mesenteric lymphatics are mobile, regional treatment for malignancies of the jejunum and ileum has to include very large fields. The tolerance of the whole abdomen to radiation is less than the dose that would reliably sterilize microscopic disease. Because most of the small intestine is suspended by mesentery, surgical resection is usually adequate local therapy for adenocarcinoma of the small bowel. Given these limitations and the rarity of these tumors, the role of adjuvant radiation has not been clearly defined for adenocarcinoma of the small bowel. For lesions that involve contiguous structures, preoperative or postoperative radiation can theoretically improve resectability and local control over results that can be achieved by surgical resection alone.

 

Forty-five percent of small bowel adenocarcinomas arise in the duodenum, and for these tumors, the treatment is surgical resection. For lesions arising in the first and second portions of the duodenum, pancreaticoduodenectomy is usually required, but for more distal lesions, a segmental duodenectomy is often possible. The 5-year survival rates after surgery alone range from 20% to 60%, Johns Hopkins and Memorial Sloan-Kettering have reported 5-year survival rates of 60% and 53%, respectively, in patients who have undergone resection.

 

In duodenal cancer, regional lymphatics are often involved, and the competing risk of distant failure is modest. The spread pattern provides a rationale for preoperative or postoperative chemoradiation therapy. Although some investigations of small numbers of patients have not found adjuvant therapy to be beneficial, no study conducted has included enough patients to adequately detect even large differences in survival according to type of therapy. The role of adjuvant therapy has therefore not been defined. In pancreatic cancer, which also has a significant locoregional pattern of spread, a survival benefit has been demonstrated with postoperative adjuvant therapy.

 

The primary tumor site and regional lymphatic drainage can be safely treated with 45 to 50 Gy, a dose that reliably eliminates microscopic residual disease. Radiation portals should cover the entire duodenum and the hepatoduodenal, pancreatic, superior mesenteric, periportal, subpyloric, pyloric, and para-aortic nodal chains. Mucosal margins of at least 5 cm proximally and distally are recommended because of the potential for submucosal spread. Localized boost doses of 10 Gy at most to areas thought to be at highest risk can be given with external beam or intraoperative radiation therapy. The risk of duodenal ulceration or perforation increases sharply at total doses higher than 55 Gy. Extrapolation from treating pancreatic cancer suggests that preoperative therapy is likely to be better tolerated than postoperative therapy.

 

Palliative radiation can be beneficial in cases of unresectable or recurrent tumors. When the goal is palliation, tumor regression and symptomatic improvement can be achieved with 30 Gy, given in 10 fractions. Goals of palliative radiation include controlling blood loss, decreasing pain, and relieving obstructive symptoms.

 

 

Sarcomas

Similar principles can be applied for evaluating the role of radiation in treating sarcomas of the small bowel. Experience with sarcoma of the extremities suggests that preoperative or postoperative radiation therapy may reduce the risk for local recurrence if wide surgical margins cannot be obtained. When locally advanced disease is recognized at staging, preoperative therapy to a dose of 45 to 50 Gy is preferred.

 

 

Carcinoid Tumors

The specific indications for radiation therapy for small bowel carcinoid tumors are lacking, given the rarity of this clinical presentation. However, radiation has been shown to be beneficial in palliation of unresected disease, with perhaps less benefit for patients with carcinoid syndrome.

 

 

Choice of Radiation Techniques

The choice of radiation techniques for tumors of the small bowel depends on the tumor location and individual anatomic variations. For duodenal primary tumors, a four-field technique is usually preferred. The dose contribution from the lateral fields should be limited to 20 Gy, and appropriate shielding should be used wherever possible to limit the dose to critical adjacent structures such as the liver, kidneys, stomach, spinal cord, and uninvolved small bowel.

 

 

Summary of Radiation Therapy for Small Bowel Tumors

The role of radiation therapy in the treatment of small bowel tumors is largely undefined because of the rarity of these tumors, anatomic constraints, and the wide range of clinical presentations. Problems specific to irradiation of the small bowel include the mobility of the primary tumor and the limited radiation tolerance of uninvolved tissues. Although a role for adjuvant therapy has yet to be defined, radiation therapy remains an important modality for selected cases in the adjuvant setting and for palliation of small bowel tumors of all histologic types.