Stereotactic Radiosurgery (SRS) and Stereotactic Radiation Therapy (SRT) applications for the management of intracranial lesions are rapidly adopted by the Radiation Therapy (RT) community resulting in safer treatments and a better quality of life for patients than ever before.
The clinical efficacy and superiority of SRS/SRT against conventionally fractionated radiotherapy, such as Whole Brain RT, for the treatment of brain malignant primary and metastatic lesions, benign tumors, and functional disorders, have been proven by the scientific community over the last two decades. The main advantage of SRS is the accurate delivery of high radiation doses with sub-millimeter precision, in a single or a hypofractionated treatment scheme. Compared with conventional RT, the delineated target volume in SRS is smaller, the conformity of the dose distribution is higher, and the dose gradient from the periphery of the target to the surrounding healthy tissue is steeper. To fulfill these characteristics, contemporary SRS treatment plans are designed and delivered through the combination of sophisticated software with high precision hardware components for the main treatment system. Careful, integration between different sub-systems and the main system is required to further increase treatment accuracy and efficacy. However, the integration of these systems in delivering customized treatment for each patient increases the complexity of the whole procedure and consequently the possibility of unintended radiation exposure to healthy tissue. The consequences of errors when delivering high-dose fraction/s of radiation – just millimeters beyond their intended site – can have a far-reaching physiological impact compromising tumor control probability and normal tissue complication probability treatment objectives.
The key differences between conventional RT and SRS, as well as the high level of complexity in SRS increase the requirements for accuracy and precision in each treatment, highlighting the need for a Patient-Specific Quality Assurance (PSQA) protocol in the clinic. According to international guidelines and recommendations, a PSQA procedure in SRS should include the verification of patient setup and immobilization, an independent check of the treatment plan including all the individual treatment delivery parameters, absolute and relative dose measurements, and a dry-run of the treatment delivery. The aforementioned checks of a patient’s SRS treatment can be performed within the framework of End-to-End testing of each treatment plan. Through this procedure which acts as a treatment simulation, the clinical team can assess the overall treatment process accuracy from start to finish, including the patient’s immobilization, imaging, treatment planning, setup, image guidance, and treatment delivery. Therefore all links in the treatment chain are holistically tested and verified. Each link of this End-to-End testing should be implemented by the clinical team member who clinically performs it, so the procedure simulates the treatment realistically.
The PSQA program of each clinic should determine the appropriate methodology and equipment for each SRS modality in order to conduct the treatment verification accurately for each patient. The equipment that is commonly used for PSQA includes a large variety of dosimetry detectors, phantoms and/or devices that simultaneously act as both phantoms and detectors. The most common detectors that are suitable for dosimetry measurements in SRS are small-volume ionization chambers, dosimetry diodes, microdiamond detectors, plastic scintillators, radiochromic films, polymer gels, electronic portal imaging devices calibrated for dose response, and 2D or 3D detector arrays. The vast majority of these detectors are placed within a phantom that represents the patient’s head to perform dose measurements. Phantoms for SRS applications are either of a generic standard shape such as spheres and cylinders or are anthropomorphic.
Within the framework of the described equipment available for PSQA in SRS, the pre-treatment imaging of the phantom for planning purposes is performed once and the structure sets of each patient are relocated to match the detector’s plane or point resulting in selective structures’ dosimetry instead of the actual plan evaluation. Then, the patient’s treatment plan is recalculated on the phantom geometry and the measured dose is reconstructed within the patient’s anatomy resulting in artificially determined in vivo dose distribution acting more as a plan-specific rather than a true patient-specific approach. As mentioned above, the dose gradients in SRS are too steep and the target sizes too small for the detector’s resolution, especially in 2D or 3D detector arrays. In the case of electronic portal imaging devices, the treatment plan is verified without the presence of any phantom and the measured portal images are combined with a back-projection algorithm to predict the dose in the patient’s Computed Tomography (CT) dataset.
The current status of PSQA in SRS doesn’t take into account the effect of the actual patient’s anatomy and lacks patient-specific 3D dosimetry. A promising solution that addresses these challenges is the combination of patient-specific phantoms with polymer gel 3D dosimetry. The rapid advances in 3D-printing technology permit swift creation of an actual patient facsimile with bone- and tissue-mimicking materials using the CT data of the real patient. Moreover, the usage of a polymer gel as a brain equivalent material, in terms of interaction with radiation, and as a 3D dosimeter, provide an evaluation of every patient’s targets without any compromise. This methodology can be adopted in an End-to-End PSQA framework for each patient where the patient’s immobilization equipment and setup/monitoring methodologies are utilized.
About the author
Emmanouil Zoros is a Medical Physicist – Product Manager at Athens-based RTsafe. The company develops a suite of quality assurance products and services that enable the evaluation and verification of the entire radiotherapy process ahead of patient treatment. Uniquely, RTsafe has invented a patient-specific QA process combining an anatomically-faithful model of each patient’s head and brain tissue which, with associated evaluation services, contributes to minimising risks associated with RT interventions. Emmanouil has a Diploma in Mathematics and Physics from the National Technical University of Athens, an M.Sc., and a Ph.D. in Medical Physics from the National and Kapodistrian University of Athens.