Proton therapy is an effective technique for fighting cancer. To realise its full benefits, patients must be carefully assessed before each treatment and radiation plans adapted accordingly.
Researchers at the Paul Scherrer Institute propose a daily adaptive proton therapy workflow to achieve this, and highlight many more opportunities to improve efficiency and accuracy of treatment.
Read the original article: https://doi.org/10.1088/1361-6560/ac2b84
Image credit: Mircea Moria/Shutterstock
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Today we will be talking about proton therapy as a method for treating cancer. Due to the precision needed for proton therapy, patients need personalised plans every time they receive treatment. The research carried out at the Paul Scherrer Institute in Switzerland has led to the development of a proposed plan for treatment adaption in a case-by-case way.
So, what is proton therapy?
Cancers are mostly treated with surgery, chemotherapy, radiation therapy or a combination of all or some of these. Standard radiation therapy occurs in the form of x-ray radiation therapy. However, due to the nature of the x-ray’s energy distribution in the body they cannot only kill cancer cells but also potentially inflict damage to normal cells surrounding the cancer cells. This is of concern when these normal cells belong to sensitive organs like the brain, the spine, nerves or the like.
When the radiation takes place with protons, the situation is different. A beam of protons can be targeted so that the depth into human tissue at which protons deposit the peak amount of their energy, aka the Bragg peak, is precisely at the tumour. Because of this, the normal cells the proton beam passes through on its way to the tumor are much less effected and the cells beside and behind the tumor not at all, in contrast to conventional x-ray radiation.
The protons are applied by using a technique called pencil beam scanning or spot-scanning. An accelerator fires a thin beam of positively charged protons directly at a patient’s tumour leaving the surrounding tissue unaffected and healthy. The beam of protons scans through the tumour layer by layer, killing the cancerous cells which leads to tumour shrinking. The protons kill these cells by damaging their DNA mostly indirectly, and by producing free radicals within the cells which induce further DNA damage.
This pencil beam scanning technique was first developed at the Paul Scherrer Institute in the 1990s and delivered for the first time in a patient with a Gantry in 1996 and is now the standard form of proton therapy around the world. This delivery method is notably used for treating tumours in difficult locations such as the brain or the spinal cord – as any collateral damage of healthy tissues in these organs could have severe side effects. It is also incredibly important for treating child cancer patients. Since the cells in children’s tissues are more sensitive, and have healthy growth still to accomplish, normal radiation treatment can be much more hazardous for them than adults, so the precision of proton therapy is even more key.
In order to achieve the maximum level of precision, treatment should be adjusted each time it is administered. The beam of protons must be fired exactly at the tumour, so the location and size must be known before each course of treatment, in case it has changed size or position. As well as this, the protons also need the correct amount of energy to reach the appropriate depth within the body. To accomplish this, plates of varying thickness are placed in front of the beam in order to absorb the energy of the particles. So, for a shallow tumour you would need thicker plates to weaken the beam, and less plates mean a strong beam can reach deeper tissues. Alternatively, an energy selection system, or ESS, in the beam can be inserted.
Anatomical changes also affect how much energy the proton beam needs. Weight gain or loss over a longer period of time needs to be assessed but more importantly are minor changes within the body that happen daily. Even factors such as paranasal sinus or bladder or rectal filling would affect treatment for head and pelvic treatment, respectively. If left unadjusted, they might not reach the tumour and would instead be targeting healthy tissue. The adaptation of the treatment plans each time must therefore be carried out quickly and carefully to gain the clearest picture of what is happening within the patient’s body throughout the trajectory of the proton beam, allowing for utmost precision when it comes to tumour targeting.
In order to optimise treatment plans a team led by Francesca Albertini from the Center of Proton therapy at the Paul Scherrer Institute have developed an online adaptive workflow for daily adaptive proton therapy (DAPT). Treatment preparation relies first on the generation of a template plan, which includes manual contouring of the tumour. Patient CT images are then taken each time treatment is administered. These images are then used to adapt the template plan and determine correct doses for treatment, taking into account daily anatomical changes. This is carried out using the ADAPT software developed at the Paul Scherrer Institute. The software is easy to use and allows all the steps in the online workflow to be carried out safely and efficiently. It also incorporates a series of quality assessment steps which are checked before administering treatment.
This proposed workflow needs to be quick and efficient to minimise the amount of time between imaging and delivery of treatment. Steps must therefore be as simple and easy to carry out as possible. For this reason, patient imaging is done using in-room CT scans and these images are then used to adapt the treatment. This method also uses the same algorithms for the template plan as is used for generating the daily adaptive plan which increases simplicity and efficiency. This means that there are fewer changes from the conventional patient workflow making it easier to introduce into the clinic.
Pre-clinical experiments to investigate the efficiency and accuracy of this online adaptive workflow have already been undertaken. These were carried out using an anthropomorphic head and neck phantom. This mimics a patient and can be adjusted to imitate anatomical changes such as weight gain and filling of nasal cavities. The results of these experiments showed that the proposed online adaptive workflow could be safely and efficiently applied for daily adaptive proton therapy. It delivers accurate doses as confirmed by reconstructed dose calculations. The timings of the adaptive workflow were also similar to current clinical workflow timings, whilst also being more precise. Current adaptive plans used in the clinic are offline and take much longer. All of these results show that this new online adaptive workflow developed by the Paul Scherrer Institute can be safely and accurately used and is ready to be used in the clinic.
Proton therapy is an incredibly important and effective technique for fighting cancer. In order to optimise treatment, it is clear that each patient must be carefully assessed each time and radiation plans must be adapted based on the results. The daily adaptive proton therapy workflow proposed by the Paul Scherrer Institute is an excellent start on the path to achieving this. However, as Professor Damien Weber the head and chairman at the Center for Proton therapy at the Paul Scherrer Institute in Switzerland points out, there are many more exciting avenues of research, and opportunities to improve efficiency and accuracy for treatment, such as automation of the tumour contouring step. This would lead to quicker and more precise treatment, helping to treat more people with cancer and making their lives better.
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