What are the dose and contouring guidelines for radiation therapy in rectal cancer treatment?

Dose and Contouring Guidelines for Radiation Therapy in Rectal Cancer

For rectal cancer treatment, radiation therapy should include the primary tumor with a 2-5 cm safety margin, presacral lymph nodes, internal iliac lymph nodes, and obturator lymph nodes, with a standard dose of 45.0-50.4 Gy delivered in 1.8-2.0 Gy fractions. 1

Radiation Dose Guidelines

Preoperative Radiotherapy

• Minimum recommended preoperative dose is 45 Gy delivered in conventional fractionation 1
• For long-course chemoradiotherapy, a total pelvic dose of 45.0-50.4 Gy over 25-28 fractions (1.8-2.0 Gy per fraction) is standard 1
• After delivering 45 Gy, an additional boost of 4-6 Gy in 2-4 fractions to the primary tumor is often recommended 1
• Short-course radiotherapy (25 Gy in 5 fractions) followed by surgery within 1 week is an alternative option for T3 rectal cancers without need for sphincter preservation 1
• For unresectable tumors, dose escalation to 54-56 Gy may be considered if technically feasible 1

Postoperative Radiotherapy

• Minimum recommended postoperative dose is 50 Gy given as external-beam irradiation 1
• Postoperative chemoradiotherapy (50 Gy, 1.8-2.0 Gy/fraction) with concurrent 5FU-based chemotherapy is no longer routinely recommended but may be used in high-risk cases if preoperative radiotherapy was not given 1

Contouring Guidelines

Clinical Target Volume (CTV) Should Include:

• The primary tumor (or tumor bed) with a safety margin of 2-5 cm 1
• The entire mesorectum, except for very early tumors [T1 sm1(-2?)] 1
• For high rectal tumors, include at least 4-5 cm distal to the tumor 1
• Presacral nodes along aa rectalis superior up to the level of S1-2 1
• Internal iliac lymph nodes up to below the bifurcation or to the level of about S1-2 1
• Obturator lymph nodes 1

Conditional Inclusions:

• Lateral nodes along aa rectalis inferior and aa obturatorii should be included for tumors below the peritoneal reflection (up to 9-12 cm from the anal verge) 1
• External iliac nodes should only be included if anterior organs (bladder, prostate, female sexual organs) are involved 1
• Fossae ischiorectalis should only be included when the levator muscles and sphincters are involved 1
• Medial inguinal nodes should only be included when the tumor grows at or below the dentate line 1
• If presacral nodes are radiologically involved, the upper border of CTV should be higher than S1-2 1

Technical Considerations

• A three- or four-field technique is recommended 1
• Three-dimensional precision radiotherapy such as 3D-CRT, VMAT, or IMRT should be used 1
• The dose to the small intestine should be limited to within 50 Gy 1
• Specific dose constraints: small bowel loops V15 <120 mL and abdominal cavity V45 <195 mL (per QUANTEC recommendations) 1
• Efforts should be made to minimize inclusion of small intestine within the radiation field by changing patient position or using other methods 1

Special Considerations by Tumor Stage

• For T1 N0 M0 tumors: Postoperative radiotherapy is indicated only if histology shows incomplete clearance, metastatic nodes, or invasion of perirectal fat 1
• For T2 N0 M0 tumors: Preoperative radiotherapy is optional 1
• For T3 and T4 tumors: Preoperative radiotherapy is standard 1
• For locally recurrent disease: Preoperative radiotherapy (50 Gy for 5-6 weeks) with concurrent chemotherapy is recommended if not previously irradiated 1

Concurrent Chemotherapy Regimens

• Capecitabine 825 mg/m² twice daily, 5 days per week during radiotherapy 1
• 5-FU continuous infusion at 225 mg/m²/day during radiotherapy 1
• Irinotecan combined with Capecitabine (doses based on UGT1A1 genotype) 1

By following these evidence-based dose and contouring guidelines, radiation oncologists can optimize local control while minimizing toxicity to surrounding normal tissues in patients with rectal cancer.