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External radiation therapy of thyroid cancer


Medical editor Anne-Birgitte Jacobsen MD
Oncologist
Oslo University Hospital

General

Radiation therapy can be given postoperatively, in combination with radioactive iodine therapy, or alone, and is central in the treatment of anaplastic thyroid carcinomas.

Radiation therapy is effective for symptom-causing metastases which do not take up iodine. The dose per fraction and total dose depends on the expected lifetime of the patient and the localization of the metastasis. Fractions vary between 8 Gy x 1 to 2 Gy x 35.

Indications

  • Undifferentiated (anaplastic) thyroid carcinomas
  • Papillary and follicular thyroid carcinoma in
    • stage IV - with macroscopic tumors with symptoms or life-threatening disease progression after surgery or treatment with radioactive iodine (131I)
    • locoregional recurrence - inoperability or nonradical surgery

Goal

  • Cure the disease 
  • Reduce the volume of the tumor
  • Remove or limit the extent of metastases

Definitions

Target volume

Target volume definitions according to ICRU
(International Commission on Radiation Units and Measurements)
GTV (Gross tumor volume) Palpable or visible/identifiable 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 and inner margin taking into account inner movement and revisions of CTV. 

PTV (Planning Target Volume)

Geometric volume containing ITV and one set-up margin taking into account patient movement, variation in patient positioning, and field modeling.

OAR (Organ-at-Risk) Normal tissue in which radiation sensitivity may influence planning and/or dose.

PRV (Planning organ-at-risk volume)

Geometric volume containing risk volume with set-up margin.
TV (Treated Volume) The volume within an iso-dose surface considered sufficient based on intent of treatment.
IV (Irradiated Volume) The volume to receive a significant dose relative to normal tissue tolerance.
CI (Conformity Index) The relationship between the planning target volume and the treated volume (PTV/TV).

Preparation

The target volume and dosage are adapted individually.

Ideally, the dosage distribution is uniform in the target volume and no radiation will reach any critical organs. This is realistically impossible to achieve, therefore, a compromise must be made between what is possible and what is the goal.  

Preparation for radiation therapy includes:

  • individual customization of a plastic mask to immobilize the head/neck
  • repeat CT
  • tracing of the tumor and risk volume on each section
  • preparation of dosage plan
  • approval of dosage plan
  • tracing for field limits

This will take about one week after which radiation therapy can begin.

Risk organs

In the region around the thyroid gland, there are risk organs with limited radiation tolerance such as:

  • medulla spinalis
  • brain stem
  • parotid gland

Implementation

All fields should be treated with each fraction. It is very important that the planned time of treatment is carried out.

For unforseen discontinuation of the treatment, no more than one extra fraction can be given per week, and a skipped treatment should be given within one week. 

Anaplastic thyroid carcinoma

  • 60 Gy to GTV   / ITV  
  • 10 fractions per week (2x daily weekdays), fraction dosage 1.8 Gy  
  • The radiation treatment is often given together with weekly doxorubicin 20 mg

Postoperative treatment of papillary and follicular thyroid carcinomas T4 tumors

  • 50-60 Gy to GTV, possibily 50 Gy to ITV
  • 5 fractions per week, fraction dosage 2 Gy

Locoregional recurrence and inoperable papillary and follicular thyroid carcinomas

  • 60-70 Gy tol GTV, possibly 50 Gy to ITV
  • 5 fractions per week, fraction dosage of 2 Gy

Follow-up

The patient must have regular follow-up at least once per week at the radiation clinic.

Side effects of radiation therapy

Acute

  • Mucositis
  • Salivary gland dysfunction 
  • Dry mouth due to reduced saliva production
  • Taste disturbances
  • Painful swallowing
  • Pain
  • Thick mucus
  • Thrush 

Regular follow-up of the oral cavity is very important with fields reaching higher up.

Delayed

  • Salivary gland dysfunction 
  • Dry mouth
  • Taste disturbances
  • Cavities
  • Periodontal disease
  • Osteo (radio) necrosis
  • Trismus

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