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Stereotactic radiosurgery (SRS) is commonly deployed in cancer centres worldwide for the treatment of single and metastatic tumours in the brain, exploiting multiple narrow beams from different directions to deliver conformal, high-dose radiation to the disease target in one or a few fractions while minimizing collateral damage to surrounding healthy tissue and organs-at-risk (OARs).

Yet despite the at-scale clinical adoption of stereotactic treatment systems, the precision targeting inherent to SRS remains a non-trivial operational and QA challenge for the radiation oncology team, necessitating a high degree of accuracy in target localization and dose delivery – not least when it comes to focusing “high-payload” radiation onto metastatic small lesions and having it fall off as quickly as possible. Put simply, small errors in the SRS treatment machine or workflow can amplify the clinical complications, resulting in significant under-treatment of portions of the tumour volume and/or overdosing of adjacent healthy tissues.

The Stereotactic End-to-End Verification (STEEV) phantom is designed to minimize the scope for such errors by providing high-fidelity patient simulation during SRS commissioning and pre-treatment patient QA checks. Developed by CIRS, a US manufacturer of tissue-equivalent phantoms and simulators for medical imaging, radiation therapy and procedural training, STEEV gives medical physicists the QA tools they need to check all of the key workflow steps through SRS treatment planning, management and delivery — from diagnostic imaging with CT, MR and PET to QA of the treatment plan versus delivered dose.

In terms of specifics, STEEV’s anthropomorphic exterior allows for the use of multiple positioning and fixation devices, reflecting the diversity of SRS clinical implementations. Internal details – such as cortical and trabecular bone, brain, spinal cord, teeth, sinuses and trachea – provide realistic clinical simulation to evaluate the effects of complex intra- and extra-cranial anatomies. At the same time, a range of geometric and tissue-equivalent target inserts underpin comprehensive image QA, geometric machine QA and treatment planning system (TPS) QA – all of which ensures increased confidence in the SRS treatment system when it comes to targeting accuracy and dose distribution accuracy.

Strategically, CIRS has also positioned STEEV as a core QA platform for the independent dosimetry auditing of new and established SRS treatment centres. A case study in this regard is the central role played by STEEV within a UK benchmarking exercise to regulate the provision of cranial SRS services, with 30 participating centres audited in an end-to-end test that incorporated local clinical procedures for immobilization devices, CT scanning, target contouring, treatment planning and treatment delivery.

The independent SRS audit was run by a team of medical physicists and biomedical engineers from several prominent UK cancer centres in collaboration with the National Physical Laboratory (NPL), the UK’s National Metrology Institute. The lead investigator was Catharine Clark, now honorary professor of translational radiotherapy physics at University College London, while out in the field the QA assessment of participating SRS centres was undertaken by Clark’s PhD student Alexis Dimitriadis (now a medical physicist at the International Atomic Energy Agency in Vienna).

Ahead of the on-site QA work, Clark and her colleagues first customized the STEEV phantom to contain a single irregularly shaped target (approximately 8 cm3) located 10 mm anterior to the brainstem. Their phantom design allows the introduction of interchangeable cuboid inserts – to image and irradiate – in the centre of the brain and the insertion of radiation detectors through two parallel cylindrical access cavities. “STEEV is a versatile solution that is readily adapted to suit a clinic’s local best practices,” Clark explains. “In this respect, we worked closely with the product engineering team at CIRS, as well as colleagues in the NPL workshop, to design the phantom inserts exactly to our specifications.”

With the phantom optimized, the end-to-end SRS QA at each clinic saw EBT-XD Gafchromic films and alanine pellets used to measure absolute dose – inside both the planning target volume (PTV) and the brainstem – followed by comparison versus TPS predicted dose distributions. In summary, the PTV alanine measurements from gantry-based linacs showed a median percentage difference to the TPS of 0.65%, while CyberKnife systems registered a median difference of 2.3%, with Gamma Knife showing the smallest median difference of 0.3%. Similar trends were observed with alanine measurements in the OAR, showing median differences of 1.1%, 2.0% and 0.4% for gantry-based linacs, CyberKnife and Gamma Knife systems respectively. All of the SRS platforms showed comparable gamma passing rates between axial and sagittal films.

Although logistically challenging, independent assessment of SRS dosimetric delivery is an invaluable exercise. That’s especially so – as per the UK study – when the evaluation is mapped across a multiplatform, multicentre SRS setting in which each clinic has its own unique planning processes and priorities. As such, the UK audit, which was conducted in 2016-17, quantified what is achievable with intracranial SRS systems, yielding a gold-standard data set that has subsequently been used to set tolerances for clinical trials as well as future external SRS audits. What’s more, the variations in clinical approach observed between the participating centres provided a starting point for greater convergence and standardization in terms of what constitutes SRS best practice.

“The national SRS audit has been extended to include multiple brain metastases, which will be used to credential centres to join clinical trials,” explains Clark. “Following the publication of our results, several international centres have also requested and undertaken this audit, which we have carried out remotely.”

Phantom profile: CT image of STEEV with thermoluminescent dosimetry insert. (Courtesy: CIRS)

When used in conjunction with multimodality imaging inserts, STEEV provides an end-to-end QA solution that’s applicable across a range of SRS treatment systems. Product highlights include:

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