What is molecular radiotherapy?
The practice of treating patients with an administration of a radioactive drug is called molecular radiotherapy. These drugs or ‘radiotherapeutics’ may be given as a tablet, intravenously or intra-arterially. These are targeted treatments, with the drug using a particular biological mechanism to hone in on specific sites of disease. This results in the delivery of localised irradiation to target cells.
What is the aim of the treatment?
The aim of the treatment, as for all radiotherapy, is to maximise the radiation dose that is delivered to the sites we which to treat while minimising the radiation dose delivered to those we don’t. The desired effect is to destroy or palliate the target sites and minimise toxicity due to exposure of non-target tissue.
Advantages of MRT
Molecular radiotherapy has a unique advantage over systemic therapeutics as the radiation can be imaged. This allows us to measure directly where the drug goes to, where it is taken up, and how long it stays in one place. From this we can calculate the radiation dose that is delivered throughout the body.
Patients treated with radiotherapeutics can be imaged to allow us to see where the radiation has gone to. Dosimetry takes this one step further, by actually calculating not just the exact amount of radiation in different parts of the body, but the radiation dose that is delivered.
Because people are different, the same amount of radiation administered to two people may have very different distributions within the body. The radiation doses delivered to healthy organs, such as the bone marrow or the kidneys, can vary by a factor of 10. The radiation delivered to tumours can be up to 100 times greater in one patient that in another.
They can even be very different to different tumours within the same patient. This requires very careful planning on the amount of radiation necessary to deliver a successful treatment. Personalisation of these treatment can therefore only be achieved with dosimetry.
Rules and Regulations
Regulatory authorities often require a dosimetry study to be undertaken before a radiopharmaceutical is bought to market.
A European directive brought into force in February 2017 mandates dosimetry both in advance of treatment to predict the radiation doses that will be delivered, and following a therapy to verify that the radiation doses were delivered as planned.
“exposures of target volumes shall be individually planned and their delivery appropriately verified”
In the UK, this directive has been implemented into legislation via The Ionising Radiation (Medical Exposure) Regulations 2017
“doses to non-target volumes and tissues must be as low as reasonably practicable and consistent with the intended radiotherapeutic purpose of the exposure.”
The International Commission on Radiological Protection (ICRP) also makes the strong recommendation for dosimetry to inform treatments.
“Individual absorbed dose estimates should be performed for treatment planning and for post administration verification of doses”
Patient-specific dosimetry is growing in line with the new products that are emerging and offer many opportunities to understand and improve the treatments. The RMDC is unique in offering a wide breadth of practical and advisory services.
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