ESTRO meets Asia 2024 Congress Report

Immobilisation devices are the fundamentals of daily successful radiation treatment. Globally, proton therapy has been an emerging treatment modality for the past decade. Accurate and reproducible patient positioning is paramount in proton therapy due to the unique nature of this particle therapy. The effectiveness of proton therapy is highly dependent on tissue position and its thickness and density along the beam path. High-quality imaging and a stable position go in tandem to ensure the best possible proton beam delivery.

Having good immobilisation will secure the patient’s position and determine the target water-equivalent thickness (WET) depth along the beam’s path. Any variation will affect the beam deposition, and this will result in an imprecise dose to the target volume or overdosing of neighbouring critical organs. To achieve optimal therapeutic efficacy, dedicated proton immobilisations are used to reproduce patient position and lessen motion during treatment.

Associate Professor Caroline Wright of Monash University, Melbourne, Australia, delivered an engaging pitch session, in which various valuable aspects of immobilisation were discussed. Prof Wright described ‘Comfort’ as the key to stable immobilisation and patient positioning. Conversely, Professor Helen McNair of The Royal Marsden NHS Foundation Trust in London, UK, touched on the importance of good training for radiation therapists (RTTs) in effective immobilisation. Indeed, pre-treatment RTTs must be competent, knowledgeable and able to utilise advanced immobilisation gadgets to fix patients’ positions comfortably. Moreover, clear communication with patients during the CT simulation procedure and good management of their expectations will help to alleviate anxiety along with unintended movement during simulation.

The fabrication of customised immobilisation devices requires highly trained RTTs. They need to comprehend the complexity of beam arrangements and select the right devices for daily positioning. RTTs are strongly encouraged to huddle  challenging proton cases with the physicist, dosimetrist and radiation oncologist. This will help them to strategise how best to use the immobilisation devices to stabilise the patient and possibly to reduce systematic errors that arise from pre-treatment.

The brain/head and neck regions present considerable challenges due to the target volumes, critical anatomical structures and inhomogeneities. The edge effects of the baseplate thickness, table or cushion may increase range uncertainty. Hence, the immobilisations that is used in proton therapy is solid, narrow and thin. The utilisation of mouth-bite as an internal immobilisation in proton therapy is not well developed as it has been found that patients commonly move their tongues involuntarily. As for the thorax-abdominal region, motion management is complex. Hence, additional advanced techniques such as real-time gated proton therapy with fiducial markers, compression belts, or surface-guided radiation therapy are utilised to capture the moving targets most effectively and reduce intra-fraction motion. With regard to prostate proton therapy, besides external contour, internal tissue motion can cause significant impacts on the proton beam’s depth-dose distribution. Thus, rectal balloon and rectal spacer are explored to minimise internal motion and to reduce the dose to the rectum.

It is impossible to eliminate all errors since they are highly patient-dependent. Despite the use of advanced gadgets to immobilise patients on the proton 6DOF robotic couch, patient comfort and communication must be prioritised to achieve successful treatment outcomes. Another effective way to secure proton beam precision is through the use of daily cone beam CT to correct random and systematic set-up errors. A well-designed, simulated proton patient is comfortable, stable and reproducible. Moving forward, immobilisation will continue to evolve to optimise patients’ comfort while maintaining high accuracy in proton therapy.
 

Sin Sze Yarn

Senior Principal Radiation Therapist

Advanced Practice Radiation Therapist (Head and Neck)

Division of Radiation Oncology, National Cancer Centre Singapore

Email: sin.sze.yarn@nccs.com.sg

LinkedIn: Sze Yarn Sin