What are the details of radiotherapy planning for gastric cancer, including indication, contouring of Gross Tumor Volume (GTV), Clinical Target Volume (CTV), Planning Target Volume (PTV), Organs At Risk (OAR), dose prescription, and dose constraints?

Comprehensive Radiotherapy Planning for Gastric Cancer

Radiotherapy (RT) is an integral part of treatment for gastric cancer, with specific planning details required to maximize tumor control while minimizing toxicity to surrounding organs. 1

Indications for Radiotherapy

• RT can be delivered in preoperative, postoperative, or palliative settings based on disease stage and patient factors 1
• Postoperative chemoradiotherapy based on the Macdonald protocol or perioperative chemotherapy based on the MAGIC approach are both acceptable standards of care 1
• RT should be considered for all patients undergoing gastric surgery with curative intent unless contraindicated by poor performance status 1

Patient Positioning and Simulation

• All patients should be simulated and treated in the supine position 1
• CT simulation with 3-dimensional treatment planning is strongly recommended 1
• Intravenous and/or oral contrast should be used during CT simulation to aid target localization 1
• Immobilization devices are strongly recommended to ensure reproducibility of daily setup 1, 2
• Patients should avoid heavy meals for 3 hours before simulation and treatment 1
• Standardized meals prior to treatment planning and each treatment can help reduce gastric volume variation 1

Target Volume Delineation

Gross Tumor Volume (GTV)

• For preoperative RT: Primary tumor and involved lymph nodes identified on diagnostic imaging (EUS, UGI, EGD, CT scans) 1
• For postoperative RT: Tumor bed, anastomosis or stumps identified using pretreatment diagnostic studies and surgical clips 1

Clinical Target Volume (CTV)

• Includes GTV plus areas at risk for microscopic disease 1
• Target volume should encompass tumor bed and regional nodes, extending 2 cm beyond proximal and distal margins of resection 1
• Regional node coverage varies based on primary tumor location:

Proximal One-Third/Cardia/GE Junction Tumors:

• Include 3-5 cm margin of distal esophagus, medial left hemidiaphragm, and adjacent pancreatic body 1
• Nodal areas: paraesophageal, perigastric, suprapancreatic, and celiac lymph nodes 1
• Include paracardial and para-esophageal lymph node beds 1
• Exclude pancreaticoduodenal and splenic nodal beds 1

Middle One-Third/Body Tumors:

• Include body of pancreas 1
• Nodal areas: perigastric, suprapancreatic, celiac, splenic hilar, porta hepatic, and pancreaticoduodenal lymph nodes 1

Distal One-Third/Antrum/Pylorus Tumors:

• Include head of pancreas 1
• Exclude splenic nodal beds 1
• Nodal areas should be tailored based on tumor location, T and N stage 1

Planning Target Volume (PTV)

• Add margin to CTV to account for setup uncertainties and organ motion 1
• Consider respiratory motion using 4D-CT imaging 1
• Strategies to incorporate internal organ motion should be used for treatment planning individualization 1

Organs at Risk (OAR) and Dose Constraints

• Critical organs to protect: liver, kidneys, spinal cord, heart (especially left ventricle), and lungs 1
• More conservative dose parameters than those in the original Macdonald study are now recommended 1
• Kidney constraints: Use renal perfusion scans to refine beam geometry based on risk and organ function 1
• Lung DVH parameters should be considered predictors of pulmonary complications 1

Radiation Technique and Dose Prescription

• The recommended dose range is 45-50.4 Gy delivered in fractions of 1.8 Gy per day 1
• 3D conformal RT has generally superseded the techniques described in the original Macdonald study 1
• Although AP/PA fields can be used, a 4-field technique (AP/PA and opposed laterals) can spare the spinal cord with improved dose homogeneity 1
• Intensity-modulated radiation therapy (IMRT) provides further benefit with lower doses to normal structures 1, 3, 4
• IMRT can reduce median kidney dose by >50% compared to conventional 3D planning 4
• For patients with high-risk features, consider more conformal techniques to reduce toxicity 3, 4

Supportive Care During Treatment

• Close patient monitoring and aggressive supportive care are essential during RT 1
• Prophylactic antiemetics should be given when appropriate 1
• Antacid and antidiarrheal medications may be prescribed when needed 1
• Consider enteral and/or parenteral nutrition if caloric intake is inadequate 1
• Oral and/or intravenous hydration is often necessary throughout chemoradiation 1
• Feeding jejunostomies may be placed if clinically indicated 1
• Monitor levels of vitamin B12, iron, and calcium in postoperative patients 1

Special Considerations

• For proximal tumors where the esophagogastric anastomosis is above the carina, radiation may cause excessive lung and cardiac toxicities 1
• For patients with borderline renal function, radiation may increase risk of chronic renal impairment; consider chemotherapy alone in these cases 1
• Respiratory motion and gastric volume variation should be accounted for in treatment planning 1
• A multidisciplinary team approach is essential for optimal treatment planning 1