Abstract Book

S37

ESTRO 37

4 Cancer Care Ontario, Cancer Care Ontario, Toronto, Canada 5 Odette Cancer Centre, Radiation Oncology, Toronto, Canada 6 Odette Cancer Centre, Radiation Physics, Toronto, Canada 7 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 8 Canadian Centre for Applied Research in Cancer Control, Health Economics, Toronto, Canada Purpose or Objective Cervical cancer remains a significant problem worldwide. Patients with locally advanced cervical cancer traditionally received concurrent external beam radiation therapy and weekly cisplatin, followed by 2D brachytherapy (2DBT). 2DBT does not allow visualization of disease extent and organs at risk (OARs), and is associated with incomplete tumour coverage and/or excessive OAR doses in some patients. MR-guided 3D brachytherapy (MRgBT) overcomes these problems and is the gold standard for treatment planning. However, MR is not widely available in some regions and CT-guided 3D brachytherapy (CTgBT) is used instead despite suboptimal target visualization. This study evaluated the cost vs. utility of best-practice MRgBT compared to CTgBT in Ontario, Canada. Material and Methods A cost-utility analysis (CUA) was conducted from the perspective of the Ontario public healthcare payer using a four-state Markov cohort model and a five-year time horizon. Patients included in the model had locally advanced cervical cancer (FIGO IB-IV) requiring concurrent chemoradiation followed by brachytherapy. Parameters for the model were obtained from published literature and reviewed by an expert panel. The CUA evaluated treatment effectiveness, expressed as quality adjusted life years (QALYs), and costs, expressed in 2016 Canadian dollars, for MRgBT and CTgBT. Results were reported as incremental cost and effectiveness ratios comparing MRgBT to CTgBT for the entire patient cohort, and for low-risk (LR, FIGO IA-IIA) and high-risk (HR, FIGO IIB-IV) patients separately. The effect of parameter uncertainty on the results was explored using deterministic and probabilistic sensitivity analyses. Results MRgBT was both more effective and less costly compared to CTgBT for the full population and the high-risk subgroup. The incremental effectiveness ratios were 0.36 and 0.44 QALYs per patient for the full population and HR subgroup, respectively. The corresponding per patient incremental cost savings were $1,373 and $2,080, respectively. For the LR subgroup, MRgBT was more effective but also more costly than CTgBT, with an incremental effectiveness ratio of 276 QALYs per patient and an incremental cost of $1,324 per patient. MRgBT remained more effective in all deterministic and probabilistic sensitivity iterations and less costly in a substantial proportion of iterations when compared to CTgBT. Conclusion Overall, MRgBT is more effective and less costly than CTgBT from the perspective of the Ontario public healthcare payer. This provides evidence to build provincial and national capacity for MRgBT, and guidance for policy-makers about future infrastructure and human resource investments to assure the availability of this treatment for all women with locally advanced cervical cancer.

Proffered Papers: PH 1: Dosimetry

OC-0077 Simplifying the design of a probe-format calorimeter for absolute clinical dosimetry J. Renaud 1 , A. Sarfehnia 1,2 , J. Seuntjens 1 1 McGill University, Medical Physics Unit, Montreal, Canada 2 University of Toronto, Department of Radiation Oncology, Toronto, Canada Purpose or Objective In this work, the implementation of an alternative thermal control system for a small-scale graphite calorimeter probe (GPC) is described. Similar in size and shape to a Farmer-type cylindrical ionization chamber, the GPC has been developed to help meet the clinical need for accurate dosimetry in non-standard fields without the need for calibration. Like all graphite calorimeters, the GPC relies on thermal control to provide a stable environment against which radiation-induced responses can be measured. To date, this has been accomplished by resistive dissipation in embedded networks of micro-thermistors, which are relatively expensive and challenging to manipulate during assembly. The purpose of study is to evaluate the feasibility of using the GPC's constituent graphite as resistive elements, thereby eliminating the need for heating thermistors, and thus vastly reducing the cost and complexity of the detector construction.

Material and Methods Based on an optimized design obtained in previous work, two prototypes capable of isothermal mode operation were constructed in-house. In isothermal mode, the entire device is subject to active thermal control and the quantity of interest is the electrical power, ΔP, necessary to maintain a stable temperature in the sensitive volume (i.e., the core) during irradiation. In one of the two prototypes, 3 and 6 thermistors with a diameter of 300 µm were embedded in the jacket and