Thermal optimization of Brachytherapy: predicted therapeutic benefit of simultaneous IHT and HDR-BT

Introduction: It is known that hyperthermia (HT) is a potent sensitizer to radiotherapy (RT). The sensitization effect of HT is highest when applied simultaneously with RT. In order to have a therapeutic benefit, the RT and/or HT doses should be focused in the target tissue (1). This can be achieved combining high-dose-rate brachytherapy (HDR-BT) and interstitial HT (IHT) . The recent advances in thermoradiobiological modeling (2) offer new opportunities in quantitatively evaluating the sensitizing effect of HT on RT.

Objectives: In this study we investigate whether simultaneous application of IHT with dual electrode capacitive coupled applicators and HDR-BT, so called Thermobrachytherapy (TBT), in prostate cancer leads to a therapeutic benefit over a plan based on HDR-BT only, when the temperature is optimized on radiotherapeutic dose criteria.

Materials & methods: For the dosimetric comparison we used the equivalent physical dose (EQDphys), as calculated with the temperature dependent linear quadratic model (2), assuming the thermoradiobiological parameters α37/β37=3, α44/α37=0.77, β44/β37=1.79 (3) for both target and normal tissues. We included patients that have been treated with HDR-BT monotherapy consisting of two fractions of 13.5 Gy each for prostate cancer and assumed that the implanted catheters were TBT applicators. We simulated the temperature distribution of each of the two electrodes of the applicators, and optimized the applied power based on radiotherapeutic dose criteria when combined with 85% of the clinically planned physical dose distribution (Fig. 1). We compared the EQDphys of the TBT plan with the physical dose of the HDR-BT only plan based on the clinically defined constraints and objectives.

Results: On a total of 10 patients, the TBT plan achieved the same target objective as the HDR-BT only plan, while significantly reducing the EQDphys delivered to the urethra, rectum, and bladder (Fig. 2).

Conclusion: Our calculations indicate a therapeutic benefit of simultaneous HDR-BT and IHT in prostate cancer treatment by means of organ at risk sparing, when the temperature is adapted based on radiotherapeutic dose criteria. There is still room for further optimization of the combined TBT dose to maximize the therapeutic benefit.

References:


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