Postoperative radiation therapy for salivary gland cancerMedical editor Jan Folkvard Evensen
Oslo University Hospital
The risk for recurrence after surgery alone varies according to localization.
Except for small tumors in special localizations, surgery must be supplemented with radiation therapy. Radiation therapy targets the area of the tumor because there is often microscopic residual disease despite "free" resection margins evaluated by light microscopy. In addition, regional lymph nodes are irradiated either for manifest regional metastases or because of the primary tumor's localization, and/or the T stage suggests high probability for microscopic disease in regional lymph nodes.
The interval between surgery and radiation therapy should be as short as possible, preferably 3-4 weeks (<5 weeks).
The target volume and dose are adapted for each patient.
- Postoperative radiation therapy is intended to remove microscopic residual disease and prevent and/or eradicate spreading to cervical lymph nodes.
|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).
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.
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.
All fields are treated at each fractionation.
- Radically operated (R0) will have 50 Gy (T1-2) or 60 Gy (T3-4)
- Non-radically operated (R1/R2) and levels with extracapsular growth will have 66 Gy
- Elective lymph nodes will have 46-50 Gy
- 5 fractionations per week
- 1 fractionation daily, Monday-Friday.
Postoperative irradiation to at least 60 Gy after a neck lymphadenectomy done if there is:
- extranodal growth in one or more lymph nodes
- tumor infiltration in lymph nodes at multiple levels
- tumor tissue in soft tissue/skin
- involved resection margins/residual tumor
- mid-linear primary tumors (contralateral neck field)
Some of the above mentioned factors may require local boosts to higher doses.
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.
The patient will have regular follow-up with the radiation therapy department.
Side effects of radiation therapy:
- Salivary gland dysfunction
- Dry mouth due to reduced saliva secretion
- Taste disturbances
- Thick mucous
- Fungal infection
It is very important to have good follow-up of the mouth and nutritional status.
- Salivary gland dysfunction
- Dry mouth
- Taste disturbances
- Tooth decay
- Periodontal disease