Proton Therapy Cancer Treatment In Singapore: Singapore Institute Of Advanced Medicine Holdings

Dr Looi Wen Shen

Dr Looi Wen Shen graduated from NUS YLL-SOM and completed his specialist training at NCCS. Prior to joining SAM, he was a Consultant at NCCS and a Clinical Assistant Professor at Duke-NUS Medical School. He also practised at the University of Florida Proton Therapy Institute, USA, where he completed formal subspecialty fellowship training in proton therapy and paediatric radiotherapy. He has also received further training at the Taipei Medical University Hospital Proton Centre for regionally endemic diseases such as liver cancer and nasopharyngeal cancer.

There have been vast developments in the delivery of conventional X- rays. The introduction of intensity-modulated radiation therapy (IMRT) and stereotactic ablative radiotherapy (SABR) have changed the landscape. However, they all still have the same drawback of penetrating the patient completely, leaving an exit dose. This results in more dose splash within the patient. Proton therapy fixes this drawback as the beam can stop within the patient due to its biological properties. This reduces the radiotherapy dose to normal issues and organs. Aside from the more obvious reductions in side effects, this also reduces the risk of second malignancies from radiation exposure.

An example of a situation where proton therapy is beneficial is shown on the left. This child required craniospinal irradiation, i.e. radiotherapy to the whole brain and spine. Using conventional X-rays results in a dose splash to all the organs anterior to the treatment target. Proton therapy allows radiotherapy to be delivered while sparing these organs. This patient will have a lower dose to the oral cavity, thyroid, heart, and abdominal contents. This translates into a lower risk of side effects in these organs. In females, a lower dose to the ovaries can help to preserve fertility. Proton therapy is particularly suitable for children with cancer for a few reasons. Firstly, they are not only growing but also developing. Radiotherapy can affect this growth and development, so minimising exposure to radiotherapy is paramount. Secondly, their bodies are relatively smaller than adults. Therefore for a fixed tumour size, a larger proportion of normal issues receives unwanted radiotherapy. Thirdly, childhood cancer survivors have many years ahead of them to develop late-onset side effects.

The Varian ProBeam™ has a cyclotron-powered full 360-degree gantry with a gantry-mounted imaging system that allows precise delivery of pencil beam scanned proton therapy. SAM has a contract with Varian Medical Systems that guarantees an operational up-time of 95% or more. SAM’s ProBeam™ system is the only proton therapy system in Singapore which can eventually be upgraded to deliver clinically applicable FLASH proton therapy.

SAM’s proton therapy system has some unique advantages over other systems. The full gantry has an important advantage over the half-gantry design seen in some systems such as the IBA Proteus®ONE. While half-gantry systems often require a repositioning of the patient to treat the contralateral side midway through each session, full-gantry systems are able to deliver the full fraction each session without having to reposition the patient. This allows faster delivery of proton therapy that may reduce patient movement and also facilitates techniques such as breath-holding, which is important in proton therapy for liver cancer. The gantry-mounted imaging has an important advantage over the wall-mounted imaging design seen in some systems such as the Hitachi PROBEAT™. Wall-mounted imaging systems prohibit imaging from certain angles and can affect the verification of treatment delivery.

The Varian Halcyon™ system is a unique jawless, double-stack MLC, ring gantry linear accelerator that uses flattening-filter-free beams to deliver x-ray radiotherapy. These unique design features offer the advantage of very fast delivery. From a treatment perspective, this may reduce patient movement and also facilitate techniques such as breath-holding. This integrates seamlessly with the SDX® voluntary breath-hold system, which is important in radiotherapy for breast cancer as it can reduce cardiac doses and subsequent heart disease.