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| Cancer radiotherapy |
Notions of brachytherapy | |
Brachytherapy consists in the use of radioactive sources to deliver radiotherapy inside the tumour.
The great difference compared to external radiotherapy is that the photon irradiation begins its path through the tumour where its activity very quickly decreases before irradiating adjacent healthy tissues.
The apparition of easy to handle radioelements (such as Iridium 192 or Caesium 137) and the post-loading technique enabled the rapid development of brachytherapy. First of all, hollow catheters or vectors (without any radioactivity) are placed within the tumour under local or general anaesthesia. When their correct position is checked and when dosimetry confirms a regular dose distribution, the catheters are then charged with radioactive elements.
The major advantage of brachytherapy is the possibility to deliver high doses in small volumes with, at least in theory, no parasitic dose to the surrounding organs. This requires extensive experience in order for the radiotherapist to correctly insert the catheters as well as accurate dosimetry. The initial 'surgical' nature of this irradiation requires detailed patient information and explanation.
Originally described by B. Pierquin and then improved by a number of radiotherapists, this system allows the irradiation of tumours of varying shapes and volumes. It consists in inserting parallel tubes. The radiotherapist chooses the number, the position and the space between the catheters so that the treated volume corresponds to the tumour volume.
In simple terms, if the volume to be treated is small, few catheters separated within small spaces will be inserted; if the volume is significant, many catheters will be inserted with a slightly larger space between them.
Homogeneous irradiation is difficult to obtain: the dose is higher in the immediate vicinity of the sources and lower at a distance.
The lines should therefore be parallel, on the same surface, equidistant and each source should have an identical irradiation rate (or kerma flow).
The irradiation duration is calculated according to the source activity, the desired dose and the risk of overdosing.
low rate brachytherapy
This is the standard brachytherapy with an irradiation duration of 1 to 5 days, with a low dose rate (30 to 100 cGy/hour). The dose at the contact of the source is high but quickly decreases (within a few mm) in inverse proportion to the square of the distance, thus allowing excellent protection of healthy surrounding tissues.
Patients should remain bedridden for a few days in lead-walled room; the use of source storage devices (curietrons) enables the improved protection of hospital personal when administering the necessary hygiene care during such prolonged treatment.
high rate brachytherapy
This technique uses sources with elevated radioactivity thus reducing the irradiation time and, consequently, patient immobilisation. The session is performed within a blockhouse quite similar to those used in standard radiotherapy and using the same radioprotection measures.
Patients are 'charged' during a short period of time, and for certain tumours, they can return home (for instance in irradiation of the vaginal cuff after radical hysterectomy for corpus uteri cancer) or to a normal hospital room, between the irradiation sessions, with the non-radioactive implants still within the tumour.
pulsed brachytherapy
This technique uses a very active point source which moves inside the catheters and stops for various periods of time at different places, according to the dose calculation. The longer the source stays in a position, the higher the irradiation will be around this position. The dose can therefore be completely adapted to the tumour (as in conformational radiotherapy). Pulse brachytherapy is a low rate brachytherapy.
This technique necessitates very sophisticated programs to calculate the movement of the source within each catheter and a special device to drive the source into its various positions.
permanent implants
This technique has been developed for prostate cancer and uses 125Iodine . The application time is infinite since the small implants are inserted and then left inside the prostate. Dose calculation depends on the initial dose and on the half life of the radioactive element. With 125Iodine, 50% of the initial dose is delivered over the first 60 days, 75% over 120 days and 87% after 180 days.
Brachytherapy is often only part of the treatment.
Most often, it is associated with surgery (for of the majority of gynaecological tumours), with external radiotherapy (in gynaecology or for prostate cancer) and more recently with chemotherapy (potentialisation of radiotherapy).
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