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| Radiothérapie des cancers |
Recent technological progress | |
Radiotherapists (and the industry) have endeavoured over the last 20 years to improve radiotherapy techniques, in particular to increase the dose on the tumour volume whilst decreasing the toxicity to healthy tissue, which represents the treatment’s collateral damage.
Four major intentions of such progress:
improved anatomical data acquisition,
improved beam definition,
improved calculation and optimisation of distributed dose,
improved control during treatment execution.
Great progress has been made to improve the precision of radiotherapy.
This improvement has been made possible thanks to the development of personalised systems for each and every patient:
better treatment supports adapted to every patient to avoid movement during irradiation (for instance avoiding different pelvic curvatures when lying on the couch),
thermoformed personal fixations for cervical, lumbar or pelvic regions.
where necessary, stereotaxic fixation (for brain tumours)
These fixation systems are used during the simulation phase (under scanner or RMI) allowing a precise positioning for dose calculation.
The rigorous application of such devices avoids many reproducibility errors due to changes in patient position between fractions.
Imaging techniques have undergone vast progress allowing the fusion of scanner or MRI slices. The use of Pet-Scan allows the differentiation between bronchial tumours and their pulmonary consequences (retractile condensation).
Image fusion is now daily practice.
The definition of the volume to be irradiated is the responsibility of the radiotherapist who now has the necessary tools to display the target volume and the volume of organs at risk on the computer screen.
3D reconstruction increases the accuracy for target drawing but also for organs at risk.
However, as we previously explained, spontaneous patient movement and repositioning difficulties constitute a risk of parasitic irradiation. The latter has already been dealt with above. Spontaneous movements, however, are difficult to correct.
One of most important patient movements is due to breathing: hence the idea to synchronise irradiation with respiratory movements using respiratory gating. Two techniques are available:
the position of the organs to be irradiated is located when the patient blocks his/her breathing for a short period of time (either in forced inspiration or forced expiration) and the irradiation occurs only during these periods,
the position of the organs is located through a reference point (on the skin, in the organ) that a machine linked to the accelerator is capable of recognising and the irradiation follows the patient's natural movements.
Other movements are difficult to identify (for instance, prostate position varies according to bladder vacuity, the presence of gas in the rectum, but also with breathing and the varying pelvic curvatures for each radiotherapy session).
To obtain regular good quality beams satisfying the radiotherapist’s needs is the fruit of close collaboration between the medical physicist and the radiotherapist and of a rigorous quality control program. Great progress has been made in this field:
Medical physicists have very powerful computers at their disposal containing all the anatomical data, all the beam penetration data and the physical beam data concerning accelerators within the radiotherapy unit. They can therefore calculate the dose received on each part of the patient’s body.
3D isodose curves are now regularly plotted allowing the medical physicist to propose more and more precise ballistic improvements.
A simple way to improve ballistics is to increase the field number during treatment. Hence, parasitic doses on organs at risk are reduced. Nowadays, this is simple to achieve since accelerators are automatically driven by computers in accordance with to dosimetry calculations.
Instead of having to lay heavy masks, necessitating complex manipulation and re-positioning for each field, modern accelerators are equipped with a lead multiple leaf device, situated in the accelerator head. Each leaf can be moved with millimetre precision and the multitude of leaves allows the construction of very complex volumes, with volume variation during irradiation time (intensity modulation).
The 'beam’s eye view' technique enables the precise calculation of the dose received by each body volume. It also allows permanent irradiation control via 'digitally reconstructed radiographs' (portal imaging).
Further precision can be added by modifying the energy of the incident beam thus altering its penetration power. Intensity can also be modulated by modifying the duration of irradiation to each tumour zone.
Up to very recently, all dosimetry calculations were made in 2D. The patient was considered as a cylinder and calculations were made for each slice.
The use of 3D techniques (and possibly 4D techniques if the duration of irradiation is taken into account) enables considerably improved accuracy in the prediction and calculation of the dose actually received by critical organs ('room view technique'). The pictures are fascinating but the genuine benefit they offer the physicist is debatable.
Progress in simulation (due to progress in computer calculation speed) allows physicians to modify certain beam characteristics and to study the effects of such modifications.
Inverse calculation is an extrapolation of the medical physicist’s work. The computer receives the original data: such volume should receive such dose, however such other volume (organ at risk) should receive a smaller dose. The morphology of the irradiated zone and protected zones is drawn. The optimisation limit is set (in order for it to be maintained during treatment to avoid an excessive number of fields, excessive irradiation duration or varying patient positioning).
The computer then calculates a determined number of loops and optimises the physicist’s first proposition. This inverse calculation should, in theory, strongly reduce parasitic doses to organs at risk and allow an increase in the dose to the tumour.
By multiplying the number of fields (which can be directly set between the computer and the accelerator) and by modifying the irradiation volume, this inverse calculation can define very complex irradiation volumes (see an example for cervical nodes with dosimetry in sagittal and coronal views.
These techniques are used for complex irradiation with a curative intent or when second or further irradiation treatment becomes necessary.
A further set of technical modifications during irradiation have also resulted in considerable progress.
The setting up of a computer network within the radiotherapy unit allows accelerator control via programs which take into account the various calculations explained above. This offers improved security levels for patient treatment:
checking of patient position through more elaborated techniques than the laser beam alone,
checking of accelerator arm position,
checking of the positions of each of the collimator leaves,
more accurate checking of irradiation duration for the various fields,
continuous recording of all treatment components.
Accelerators produce beams which cross the patient and can be assessed either by radiographic images or by using numerical systems called portal imaging.
The images obtained during treatment can therefore be compared with those obtained during treatment preparation ('digitally reconstructed radiographs'). Such controls have revealed minor variations in patient positioning that were previously unknown.
The electronic measurement of radiation beam intensity observed after crossing the patient enables the calculation of a quasi 'in vivo' dosimetry using scanographic calculations. Such calculations are much easier to achieve than those obtained using a small dosimeter placed on the patient's skin (see page on total body irradiation).
All of these techniques reinforce the security of irradiation treatment and increase its technical possibilities. The only limitation is due to the beam itself which will always cross the patient before and after the tumour and will therefore deliver a parasitic dose, even if we endeavour to reduce it to a minimum.
Hence the relevance of combining these classical radiotherapy techniques with other techniques either using different beams (protons, neutrons, ions), perisurgical irradiation or brachytherapy which are all methods capable of concentrating supplementary irradiation within a small volume.
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