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Primary radiation therapy for salivary gland cancer


Medical editor Jan Folkvard Evensen
Oncologist
Oslo University Hospital

General

Radiation therapy targeted towards the primary tumor. Regional lymph nodes are also irradiated for regional metastases.

Depending on the localization of the primary tumor and/or T stage, regional lymph nodes may have to be irradiated due to high probability of microscopic disease.

This therapy is given primarily to patients with inoperable tumors, or preoperatively to render tumor more accessible for surgery.

The target volume is adapted individually.

Indication

  • Salivary gland cancer

Goal

  • Eliminate tumor
  • Reduce tumor volume
  • Remove or limit extent of regional metastases

Definitions

Target volume

 

Definitions of target volumes according to ICRU (International Commission on Radiation Units and Measurements)
CTV (Clinical Target Volume) Tissue volume that contains a GTV and/or subclinical microscopic malignant disease, which has to be eliminated.
ITV (Internal Target Volume) Volume encompassing the CTV and IM. (ITV = CTV + IM)
PTV (Planning Target Volume) Geometrical concept. Defined to select appropriate beam sizes and beam arrangements, taking into consideration the net effect of all the possible geometrical variations and inaccuracies in order to ensure that the prescribed dose is actually absorbed in the CTV. Its size and shape depend on the CTV but also on the treatment technique used, to compensate for the effects of organ and patient movement, and inaccuracies in beam and patient setup.
OAR (Organ at Risk) Normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose.
PRV (Planning Organ at Risk Volume) Includes margin around the OAR to compensate for changes in shape and internal movement and for set-up variation.
TV (Treated Volume) Volume enclosed by an isodose surface.
IV (Irradiated Volume) The volume that receives a dose that is significant in relation to normal tissue tolerance.
CI (Conformity Index) Relationship between TV and PTV (TV/PTV).

Preparation

Before the first session of radiation treatment, a customized plastic mask is made for the head/neck of the patient to immobilize the area to be treated.

This is followed by a CT examination while in the mask to mark tumor tissue and organs at risk.

In the head/neck region, there are many organs at risk with limited tolerance for radiation such as the:

  • spinal cord
  • parotid gland
  • optic tracts
  • brain stem
  • internal ear

A uniform dose distribution over the target volume is ideal with complete avoidance of critical organs. In practice, however, this is impossible to achieve. There will always be a compromise between what is possible and desired.

Preparation for simulation which involves modeling and drawing of radiation fields takes about one week. When this is completed, the patient is ready to start radiation treatment.

Naxogin®

Naxogin® is a drug that mimics the effect of oxygen.

Cells are three times more radiosensitive in the presence of oxygen (oxygen effect). Because of inadequate blood supply, squamous epithelial carcinomas over a few millimeters lack oxygen. A result of this is that cells in certain areas can survive radiation therapy and be a source for persistent disease or recurrence. 

Radiation therapy is therefore more effective on tumor with adequate oxygen supply. Naxogin is now routinely used for radiotherapy of laryngeal cancer in Denmark and Norway.

The emetic effect of the drug, however, is a disadvantage.


Implementation

Dose and fractionation

Patients are treated with photons, but sometimes in combination with electrons .

The dose rate should be between 0.5 and 5 Gy per minute. A homogeneous dose should always be striven for, possibly with use of a compensation dose.

It is important to maintain the planned treatment schedule.

Standard treatment 

All fields are treated at each fractionation.

  • 46 Gy to union of all ITV (=CTV)
  • 70 Gy to union of all GTV (concomitant boost, 6 fx/week)
  • 6 fractionations per week
  • 1 fraction daily, Monday-Friday
  • The 6th fractionation is given additionally on one of the week's first 5 days, but always with a 6 hour interval.

Unforeseeable interruptions

Not more than one extra fractionation per week is the goal, and missed sessions should be given within one week. This is done by giving an extra fractionation on the weekend, or the same day as the planned fractionation with ≥ 6 hours interval.


Follow-up

The patient will have regular follow-up with the radiation therapy department.

Side effects of radiation therapy:

Acute

  • Mucositis
  • Salivary gland dysfunction
  • Dry mouth due to reduced saliva secretion
  • Taste disturbances
  • Pain
  • Thick mucous
  • Fungal infection

It is very important to have good follow-up of the mouth and nutritional status.

Late

  • Salivary gland dysfunction
  • Dry mouth
  • Taste disturbances
  • Tooth decay
  • Periodontal disease
  • Osteoradionecrosis
  • Trismus

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