STP Specialisms | Medical Physics: The Pathway of a Radiotherapy Treatment

Hello! My name is Alick and I’m a second year Medical Physics trainee with Liverpool University Hospitals NHS FT, specialising in Radiotherapy at the Clatterbridge Cancer Centre. I initially started the STP with an undefined specialism, which meant that I chose my specialism after completing first year rotations in the different strands of Medical Physics: Radiotherapy, Imaging with Ionising Radiation, Imaging with Non-Ionising Radiation and Radiation Safety. This blog post will cover the pathway of a radiotherapy patient, from immediately after the diagnosis through to treatment. Hopefully I’ll be able to convey the important role that physics plays along the way, as well as why I chose to specialise in radiotherapy!

Planning Scan & Immobilisation

The first step following a diagnosis is to prepare for the treatment plan. This typically involves a CT scan of the patient that identifies the tumour and other notable anatomy (nearby organs at risk) so that we know where we need to irradiate and where to avoid. The key here is that the patient needs to be in the exact same position in the planning scan as they would be for the treatment itself in order to make sure that we treat the correct area (and spare healthy tissue). Therefore, we will use immobilisation devices – they aren’t as sinister as the name suggests, it just ensures that we can replicate the patient’s position during treatment. An example of this is a thermoplastic mask that is heated up and moulded to the patient’s head and neck. Many patients actually keep these masks at the end of treatment as a sort of trophy or memento!

As a trainee you will likely sit in on some ‘Mould Room’ appointments, where the immobilisation devices are made. While the planning scans are largely an ionising imaging responsibility, you are expected to understand how they work, know how to import the image data into your treatment planning system, etc. When doing treatment plans, you will also model the immobilisation device and consider its effect when calculating the dose from the treatment.

Example of thermoplastic mask.

Treatment Planning

Once we have obtained the planning image, clinicians will outline the intended treatment areas and nearby organs at risk that we want to spare from radiation. This is done so that we can model them in a treatment planning system (TPS). The clinician will prescribe how much radiation needs to be delivered to the tumour volume – the exact amount will be based on clinical trials and radiobiological data of the specific type, size and position of tumour. There will also be constraints on the radiation dose that we can give to the nearby organs at risk (OARs). This is all modelled in the TPS so that we can find the most optimal way of achieving all of these clinical goals.

The majority of radiotherapy treatments are performed using a linear accelerator (linac). Different particles and energy settings are available depending on the required dose and the type/location of the tumour. For example, more superficial tumours that are located close to the skin can be treated with electron beams due to their shorter penetration depth versus x-ray photons, thereby not irradiating as much healthy tissue beyond the tumour.

Diagram of the components of a linear accelerator and treatment couch.

The same patient position and immobilisation will be present in the treatment, allowing for the treatment plan to be created in conjunction with the planning image. Historically the planner would manually model beams, changing parameters such as beam intensity and gantry angle to conform the dose to the tumour and meet the objectives in the prescription for OARs. Modern planning techniques such as volumetric modulated arc therapy (VMAT) allow for these parameters to be changed multiple times along the beam axis as the gantry rotates around the target volume. This allows for greater conforming of dose to the target while reducing the dose to the OARs and other healthy tissue. The image below of some prostate treatment plans shows high dose areas as red and low dose areas as blue. You can really see how VMAT better conforms dose to the tumour while sparing healthy tissue.

Prostate treatment plan examples, comparing conformal radiotherapy (left) with volumetric modulated arc therapy (right).

During your training, you will learn how to do both conformal radiotherapy techniques (CRT) and more up-to-date techniques such as VMAT. You may show evidence of your planning training in the form of a presentation. Gradually trainees are given more of a responsibility in treatment planning, assisting the duty physicist with planning tasks and performing audits on historical data. Treatment planning will also make up one of the MSc university modules.

Verification Imaging

As radiotherapy treatments typically last over a period of days or weeks, it is important to make sure that the treatment plan is acceptable for this entire duration. Changes to the patient’s anatomy are common – the patient’s bladder may be more/less full than usual on a given day, or weight changes may have shifted the position of the tumour from where it was on the planning scan. Modern linacs are equipped with an onboard imaging system (e.g. cone-beam CT) to verify the position of the target prior to delivering the treatment. The couch can be adjusted to allow the plan to be correctly delivered, or if there is significant deviation from the planning scan, the plan can be adjusted. For breast and lung treatments, we can also use ‘gating’ to track the patient’s breathing and only irradiate at certain parts of the breathing cycle (and therefore when the target will be in a known position).

Trainees will extensively study the principles of verification imaging and the many different forms it comes under, as well as observing treatments where adjustments need to be made following the imaging. Also, a large of portion of the quality assurance in radiotherapy is for the imaging and gating systems on a linear accelerator, so expect to see a lot of this in your training!

Delivering the treatment

Treatment setup

So as you can already tell, a lot of moving parts go into a typical radiotherapy treatment, but we’re finally here! Once all the other stuff has been done, the patient is set up on the couch by the radiographers and the relevant immobilisation is used. The verification images are taken and the position is adjusted if necessary. Following this, the radiographers return to the control room and deliver the beam. Radiotherapy departments will have a ‘record & verify’ system that automatically tracks the delivered dose and tracks how many appointments each patient has left.

During the STP, you will regularly attend on-set, seeing the verification imaging taking place before the treatment. You will also be taught how to correct treatment plans if the verification images show changes to the patient anatomy between treatments (e.g. weight loss). A large emphasis is also placed on understanding and knowing how to use the record and verify system in the trust, including all of the IT safety considerations, confidentiality laws involved, data retention policy, etc.

Why radiotherapy?

The main thing that drew me to radiotherapy as a specialism was that it encompasses a whole host of different skills. Medical imaging, radiation physics, radiation protection and biology are all hugely important factors in the pathway as you’ve seen in this article, and because of this I found it was the most varied specialism within medical physics. One day you could be doing some quality assurance on a linac, and the next day you could be auditing some data to help develop new treatment techniques, or commissioning a new piece of equipment! It helps keep things fresh as a trainee and you can build up a wide range of knowledge.

Alick Cushing, Clatterbridge Cancer Centre/Liverpool University Hospitals NHS FT

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