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Radiation Dose - How much is safe?

The proper dose of radiation depends on the type of cancer (lymphomas are sensitive to low doses and sarcomas require high doses) and the size of the cancer (large tumors require a higher dose.) Other factors such as how much surgery has been performed or if chemotherapy is being given will affect the required tumor dose.

How much is safe will depend on which normal structures are in the way of the beam (the lens of the eye is very sensitive but muscle tissue tolerates a very high dose) and what percent of the organ is being hit (if most of the organ is involved then the dose must be kept lower.) Radiation given in one dose is more damaging than if it is divided into smaller fractions (referred to as fractionated radiation.) The following tables below are from Ruben (Perez, Brady Textbook) and is similar to data from other sources (see here.)

see recent review on spinal cord tolerance here
 
  • Table 1- Tolerance dose to an organ based on giving the dose in one fraction or fractionated, using TD5/5 (5% chance of injury showing up over the next 5 years) and TD 50/5 (50% chance of an injury)
  • Table 2 - Tolerance dose if entire organ is treated with a single dose and the expected complication
  • Table 3 - Tolerance dose if fractionated and whole or partial organ is hit
  • Table 4- Tolerance dose based on the percent of the organ hit by the radiation and here
  • Dose constraints for normal tissues are now well defined in IMRT protocols (go here)
                                                     Parameters of Therapy: Tolerance Doses (TD5/5–TD50/5)
Single Dose (Gy)x Fractionated Dose (Gy)
Ovary 2–6 Testes 1–2
Bone marrow 2–10 Ovary 6–10
Testes 2–10 Eye (lens) 6–12
Eye (lens) 2–10 Kidney 20–30
Mucosa 5–20 Thyroid 20–40
Gastrointestinal 5–10 Lung 23–28
Lung 7–10 Skin 30–40
Colorectal 10–20 Liver 35–40
Kidney 10–20 Bone marrow 40–50
Vasculoconnective tissue 10–20 Heart 43–50
Liver 15–20 Gastrointestinal 50–55
Skin 15–20 Vasculoconnective tissue 50–60
Peripheral nerve 15–20 Spinal cord 50–60
Spinal cord 15–20 Brain 55–70
Brain 15–25 Peripheral nerve 65–77
Heart 18–20 Mucosa 65–77
Bone and cartilage >30 Bone and cartilage >70
Muscle >70 Muscle >70
From Rubin P. Law and order of radiation sensitivity: absolute versus relative. In: Vaeth JM, Meyer JL, eds. Frontiers of radiation therapy and oncology. Basel: Karger; 1989:7–40, with permission.

 
                                                      Normal Tissue Tolerance to Therapeutic Irradiation
Organ TD5/5 Volume TD50/5 Volume Selected End Point
1/3 2/3 3/3 1/3 2/3 3/3
Kidney 50 30 23 — 40 28 Clinical nephritis
Brain 60 50 45 75 65 60 Necrosis, infarction
Brainstem 60 53 50 — — 65 Necrosis, infarction
Spinal cord 5 cm: 50 10 cm: 50 20 cm: 47 5 cm: 70 10 cm: 70 20 cm:— Myelitis, necrosis
Lung 45 30 17.5 65 40 24.5 Pneumonitis
Heart 60 45 40 70 55 50 Pericarditis
Esophagus 60 58 55 72 70 68 Clinical stricture/perforation
Stomach 60 55 50 70 67 65 Ulceration, perforation
Small intestine 50 — 40 60 — 55 Obstruction, perforation/fistula
Colon 55 — 45 65 — 55 Obstruction, perforation/ulceration/fistula
Rectum Volume: 100 cm3 60 Volume: 100 cm3 80 Severe proctitis/necrosis/fistula
Liver 50 35 30 55 45 40 Liver failure
TD, tolerance dose.
From Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeutic irradiation. Int J Radiat Oncol Biol Phys 1991;21:109–122, with permission.

 

 

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