External Radiation TherapyMedical editor Anne-Birigtte Jacobsen MD
External radiation therapy is used in combination with chemotherapy for curative purposes. It can also be used in combination with endocavitary therapy for curative and palliative purposes, or alone as palliative treatment.
It is normal to group squamous cell carcinomas and adenocarcinomas as one with regards to radiation therapy. Both are moderately sensitive to radiation. Radiation therapy serves both curative and palliative treatment purposes.
For curative radiation therapy, it is important to uphold the planned treatment duration.
- Eliminate tumor - cure
- Reduce tumor volume – palliation
- Relieve tumor symptoms - palliation
The tumor, including enlarged lymph nodes, in the ipsilateral hilus and mediastinum are defined as the target volume (GTV). Outside of this, 1–5 cm margins are placed to include microscopic spreading, tumor movement during respiration, and variations in tuning (CTV and PTV). The volume of the risk organs in the radiation field such as medulla, heart, and lungs must be minimalized with regarding maximal tolerable dosage.
|Definitions of target volume according to ICRU
(International Commission on Radiation Units and Measurements)
|GTV (Gross Tumor Volume)
||Palpable or visible/demonstrative area of malignant growth.
CTV (Clinical Target Volume)
Tissue volume containing GTV and/or subclinical microscopic malignant disease.
|ITV (Internal Target Volume)
Volume containing CTV plus inner margin taking into account inner movement and changes in CTV.
|PTV (Planning Target Volume)
||Geometric volume containing ITV with set-up margin taking into account patient movement, variation in patient positioning, and field modeling.
Target volume and radiation technique
The radiation area is set based on the CT images.
- The patient is not restrained, but stabilized on a wingboard with their arms up next to their head.
- A CT for image-taking in a dedicated CT machine is performed and preferably with a contrast agent.
- The localization of the tumor and risk organs are drawn directly in on the images and create a 3-dimensional radiation volume. The risk organs in the thorax are the spinal cord and lungs. Many patients with this diagnosis also have chronic obstructive pulmonary disease (COPD).
- The radiation therapists create a suggestion for the radiation field which must be adjusted and approved by both the medical physicist and treating doctor. Ideally, a uniform dosage distribution over the target volume is desired and no radiation to the critical organs. In reality, this is impossible to achieve. The dosage distribution is therefore a compromise between what is possible and what is desired.
- For the CT-taking, it is very important the patient is well medicated for pain and is able to lie still on their back. A premedication is administered or an extra dose of opiates if necessary.
- The patient lies on a flat bench with only a thin mattress.
Preparation for simulation including tuning and tracing of fields takes about 1 week. After this, the patient is ready to start radiation therapy.
The patient must go to the simulator for tracing of the field about 1 week after the CT is taken. The field shape is drawn with a maker on the patient's skin. For simulation, it is also important the patient is well medicated for pain and is able to lie still. The simulator tuning often lasts for 1 hour. The simulator is an X-ray apparatus with set-up and dimensions like a radiation apparatus. This allows for the possibility of taking control images of the field set-up. These will be used for comparing the shade images of the radiated fields obtained on the radiation apparatus.
After the simulation, the final treatment plan is transferred to the radiation machines, and the treatment is initiated usually after 1-2 days.
Curative radiation treatment
The dosage speed is 0.5–5 Gy per minute. A homogeneous dose is striven for, possibly using compensation. All fields (most often 3) are treated with each fraction.
Conventional fractioning combined with chemotherapy
50 Gy is distributed into 25 fractions over 5 weeks combined with 3–4 CiFu courses (cisplatin, 5-FU). Risk areas obtain 46 Gy over 23 fractions.
Hyperfractionated treatment is used with the intention of curing the disease (radiation only).
The treatment is administered twice daily with 35 Gy distributed over 20 fractions over 2 weeks toward ITV. After a pause in treatment of 2 weeks, 28 Gy is distributed over 14 fractions over 9 days toward GTV. This gives a total dosage of 64 Gy.
Two weeks later, the treatment is concluded with an endocavitary dosage of 8 Gy. The total dosage is then 72 Gy over 7½ weeks, which is a curative dosage for squamous cell carcinomas. The treatment is well tolerated. The disadvantage is that microscopic cancer cells are not treated; It is considered too toxic to combine this radiation treatment with chemotherapy.
Palliative radiation therapy
Radiation therapy is administered in dosages of 30–36 Gy over 10–12 fractions daily. When an external dosage of 30 Gy is given, it is often supplemented with an endocavitary treatment of 8 Gy after a 1 week break.
The patient is checked with endoscopy for the first time 6 weeks after concluded treatment. Later checks are individual and occur at the patient's local hospital.
Side effects which can occur after external radiation treatment are:
problems swallowing due to soreness and pain
reduced lung capacity
leukopenia – induced by chemotherapy
thrombocytopenia – induced by chemotherapy
fibrosis in the esophagus - delayed effect
fistulas - usually due to infiltration of the tumor to respiratory passage – delayed effect