‘As low as possible, reasonably achievable’- the motto of radiation - Dr. Manoj Wakde
Radiation therapy is one of the three main modalities used to kill cancer and it intrigued me when I recently had the chance to visit and shadow doctors at the Apollo Proton Cancer Centre in Chennai witnessing the revolutionizing proton therapy technology in the field of cancer care which is making patients’ lives easier and pain-free.
Being the first proton center in India, The Apollo Proton Cancer Centre has grown to become one of the fastest-growing cancer centers in the world. This specialized hospital has provided care to billions of cancer patients from over 145 countries in the world.
During this shadowing, I met Dr. Manoj Wakde, and Dr. Ashok Reddy who are both radiation therapists, and Dr. Dayananda Sharma Shamurailatpam, a medical physicist. I took this as an opportunity to learn more about this technology and how it’s helping cancer patients by conducting interviews regarding proton therapy.
1. How does radiation therapy work?
Radiation therapy aims to kill cancer cells using energy. The energy, carried by radiation, is deposited on DNA strands to break them, eventually destroying the cells. Although, radiation can lead to 3 possible outcomes:
⦁ Both strands of DNA are broken
⦁ One strand is broken so the strand can be repaired and the cell can be viable again
⦁ Repaired but mutated- it can become a cancer cell
‘This is why radiation is a double-edged sword- it stops cancer but can also cause it so must be used judiciously.’ – Dr Dayananda
Tumors absorb energy per unit mass, which is why physicians have to then quantify the amount of energy required for the treatment of the tumor with minimal damage to other cells, keeping in mind the maximum dose of energy that can damage cancer cells without going to other healthy organs. To achieve this, it is vital to perform a risk-benefit optimization so that a curable dose of radiation is provided and the risk of cancer spread or damage to other cells can be avoided.
What makes Proton therapy so unique?
Proton therapy is based on the principle of Bragg peak (loss of energy when radiation passes through an object). It differentiates proton therapy from other forms of treatment by giving protons the stopping power. Protons are charged particles that have a mass unlike gamma or alpha rays. This means that after a certain point, their velocity will become zero- the protons will stop traveling- and since they are charged they will drop most of their radiation in the place they stop at. Therefore it reduces the dose of radiation emitted to the peripheral cells near the tumor.
High-energy X-rays, which are another form of radiation, deposit a lot of energy in their pathway toward the tumor and once reaching it, give out their maximum energy, which never falls to zero. Therefore the dose emitted never ends and passes through the body, affecting critical organs behind the tumor.
On the other hand, the proton beam therapy has a very low entry dose. Once reaching the tumor it then quickly reaches its maximum emitted dose at the tumor site and then falls to zero. Due to this proton therapy does not emit any radiation to the surrounding healthy cells.
How is the treatment planned?
Apollos’ proton machine generates energy up to 26.2 million electron volts which can travel up to 32 cm into the body! However, sometimes the tumor is closer than 32 cm which is why less amount of energy sufficient to reach the tumor needs to be generated.
The proton beam released from the machine has a sharp 3 mm diameter called a pencil beam. The tumor in 3 dimensions is considered as made of multiple 3 mm slices. At each sitting, radiation of different energies is given to different slices to reach the tumor, spot by spot.
The process of planning proton treatment is formed in a systematic order. First doctors prepare a mask for the area of treatment using what is called an immobilization device. For example, it can be a head and neck mask, brain mask, etc. They do a CT simulator scan to localize the area of the tumor. This CT scan then goes to the treatment planning systems where doctors figure out how to treat the tumor and its surrounding areas. The medical physicians plan the route of treatment and plan the doses of energy given. After all of this is completed the doctors then look into the plan and a quality assurance check is done. Once the go-ahead is given, the patient starts to receive the dose during the sessions.
Medical physicians have dedicated computers with software designed for planning proton therapy. Using the CT scan and the algorithms of their computer software, they can create a 3-dimensional volume/figure of the patient. From that, they measure the volume of the patient, the subvolume of the tumor, and other critical organs surrounding it. They also use a specific algorithm that tracks the path of the proton beam and the dose of radiation emitted in the pathway. Further, they decide the direction and technique of the radiation to be used, for example, 3-dimensional conformal radiation therapy, conventional radiation therapy, or intensity-modulated radiation therapy.
They also check the dose of energy given to any surrounding critical organs and sit with the radiation oncologist to see whether these doses might affect the healthy organs.
I had mentioned earlier that the doctors carry out a quality assurance check. But how does this check happen? Well after creating a computerized-automated plan, to test whether it meets the criteria of achieving more than a 95% accuracy and similarity rate with the computer plan a ‘phantom’ is used. This phantom is essentially a water tank to test the plan. Since the human body is mainly made of water, to perform the quality assurance test they must use a medium that closely mimics the body. They use a detector to see the dose deposition at different depths and identify depth dose characteristics.
How are the protons generated for the therapy?
Protons exist in the nucleus of every atom. One of these atoms is hydrogen, which consists of only one proton in its nucleus and is efficient for protons to be generated through electrolysis or ionization from hydrogen gas. The extracted protons are accelerated in a cyclotron at the speed of light to generate energy. Then, huge magnets are used to deflect any negative charges around positively charged protons to steer the proton beam to move directly in a straight pathway to the treatment rooms from the cyclotron. The proton therapy machine called the gantry then rotates around the patient and uses magnets to focus the beam on the area of the tumor 1 2. This machinery is imported from Belgium- IPA company. It’s a huge gantry behind the room.
How is the patient prepared for the therapy?
Proton therapy works for all solid tumors but not for blood cancers. It doesn’t require much preparation for the patient, however, if there is a protocol for the specific tumor it has to be followed. For example, prostate cancer patients should have an empty rectum and full bladder so that the dose goes to the bladder. Food habits remain mostly the same, only a few should be changed like the prohibition of oily or spicy foods, and packaged food as well. Patients may have a bit of nausea as the side effect of proton therapy which is the bare minimum as compared to other therapies.
It is vital to remain still while getting the radiation so that the radiation goes to the tumor and no other areas of the body. To do this immobilisation devices are created but to completely ensure stability there is also Surface-guided radiotherapy and provision RT that tracks the patient. If the patient moves even a mm the alarm goes off. If they move out of the limits the beam will be stopped. There are also CCTV cameras to monitor the patient.
How long does each treatment session last?
The timings of the treatment depend on the tumor, from 10-15 minutes it can go up to 30-45 minutes as well. A patient can have a minimum of 5 and a maximum of 35 sittings of the therapy. Each patient goes through counseling and vital checks before each session to know that the patient is feeling fine.
Citations:
Dheera Vandini Mehndiratta
About the author: Dheera is a grade 11 student in the Shri Ram School Moulsari and is interested in biochemistry. She loves dance, science and playing the piano. Recently she interned at the Apollo Proton Cancer Centre in Chennai and was fascinated by the Proton therapy, the form of radiation therapy used in the centre, and has written a science journalism article on this form of therapy.