Essential dose constraints, fractionation schemes, and curated external resources for radiation oncology practice and board preparation.
Educational Reference Only
These constraints and regimens are for educational reference. Institutional protocols, patient-specific factors, and clinical judgment should always guide treatment decisions. Constraints vary by clinical context and may differ from values listed here. Always consult current NCCN, ASTRO, and institutional guidelines.
Dose constraints define the maximum acceptable radiation dose to organs at risk (OARs). These values are derived from clinical data correlating dose to probability of complications. Serial organs (like the spinal cord) have strict maximum dose limits because damage to any segment impairs function. Parallel organs (like the lungs) have volume-based constraints because they can tolerate damage to a portion of the organ.
Key sources: QUANTEC (2010) for conventional fractionation, HyTEC (2021+) for SBRT/hypofractionation. Institutional protocols may differ.
Head & Neck
| Structure | Type | Conventional Constraint | SBRT Constraint | Toxicity Endpoint |
|---|---|---|---|---|
| Spinal Cord | serial | Max <45-50 Gy | 1fx: <14 Gy; 3fx: <21 Gy; 5fx: <25-30 Gy | Myelopathy |
| Brainstem | serial | Max <54 Gy | 1fx: <15 Gy; 3fx: <23 Gy | Brainstem necrosis |
| Parotid Glands | parallel | Mean <20-26 Gy (spare at least one) | — | Xerostomia |
| Optic Nerves / Chiasm | serial | Max <54-55 Gy | 1fx: <8-10 Gy | Optic neuropathy |
| Cochlea | serial | Mean <45 Gy (aim <35 Gy) | — | Sensorineural hearing loss |
| Pharyngeal Constrictors | parallel | Mean <50-55 Gy | — | Dysphagia / aspiration |
| Mandible | serial | Max <70 Gy | — | Osteoradionecrosis |
| Brachial Plexus | serial | Max <60-66 Gy | 3fx: <24 Gy | Plexopathy |
| Larynx (uninvolved) | parallel | Mean <40-45 Gy | — | Laryngeal edema / dysfunction |
Thorax
| Structure | Type | Conventional Constraint | SBRT Constraint | Toxicity Endpoint |
|---|---|---|---|---|
| Lungs (Combined) | parallel | V20 <30-35%, Mean <20 Gy | V20 <10%, Mean <8 Gy | Radiation pneumonitis |
| Heart | parallel | Mean <26 Gy, V25 <10% | Max <30 Gy in 3fx | Cardiac toxicity |
| Esophagus | serial | Mean <34 Gy, V60 <17% | Max <27 Gy in 3fx | Esophagitis / stricture |
| Proximal Bronchial Tree | serial | Max <80 Gy | 3fx: <30 Gy; 5fx: <40 Gy | Bronchial stenosis / fistula |
| Chest Wall / Ribs | serial | V30 <30 cc | 3fx: <30 Gy to 30 cc | Rib fracture / pain |
| Great Vessels | serial | Max <75 Gy | 3fx: <39 Gy | Vascular injury |
Abdomen & Pelvis
| Structure | Type | Conventional Constraint | SBRT Constraint | Toxicity Endpoint |
|---|---|---|---|---|
| Liver | parallel | Mean <30-32 Gy (whole liver) | >=700 cc <15 Gy | RILD |
| Kidneys (combined) | parallel | Mean <18 Gy, V20 <32% each | 200 cc <17.5 Gy in 3fx | Nephropathy |
| Small Bowel | serial | Max <50-52 Gy, V45 <195 cc | Max <25-30 Gy in 3-5fx | Perforation / obstruction |
| Stomach | serial | Max <54 Gy | Max <22.5 Gy in 3fx | Ulceration / perforation |
| Duodenum | serial | Max <54 Gy | Max <22.5 Gy in 3fx | Ulceration / perforation |
| Rectum | parallel | V70 <20%, V65 <35%, V60 <50% | — | Proctitis / bleeding |
| Bladder | parallel | V70 <35%, V65 <50% | — | Cystitis / hematuria |
| Femoral Heads | serial | V50 <5%, Max <52 Gy | — | Avascular necrosis |
| Penile Bulb | parallel | Mean <52 Gy, D90 <50 Gy | — | Erectile dysfunction |
| Cauda Equina | serial | Max <45-50 Gy | 3fx: <21 Gy; 5fx: <30 Gy | Cauda equina syndrome |
Breast
| Structure | Type | Conventional Constraint | SBRT Constraint | Toxicity Endpoint |
|---|---|---|---|---|
| Heart (left-sided) | parallel | Mean <4 Gy, V25 <5% | — | Cardiac events (long-term) |
| LAD Coronary | serial | Mean <10 Gy | — | Coronary artery disease |
| Contralateral Breast | parallel | Mean <3-5 Gy | — | Secondary malignancy |
| Ipsilateral Lung | parallel | V20 <15-20%, Mean <10 Gy | — | Radiation pneumonitis |
| Thyroid | parallel | Mean minimize (SCF plans) | — | Hypothyroidism |