The AMC-8 system is capable of 3-D SAR control and its SAR distribution is more favourable than for the 2-D AMC-4 system. This result promises improvement in clinical tumour temperatures.

In regional hyperthermia, optimization techniques are valuable in order to obtain amplitude/phase settings for the applicators to achieve maximal tumour heating without toxicity to normal tissue. We implemented a temperature-based optimization technique and maximized tumour temperature with constraints on normal tissue temperature to prevent hot spots. E-field distributions are the primary input for the optimization method. Due to computer limitations we are restricted to a resolution of 1 x 1 x 1 cm3 for E-field calculations, too low for reliable treatment planning. A major problem is the fact that hot spots at low-resolution (LR) do not always correspond to hot spots at high-resolution (HR), and vice versa. Thus, HR temperature-based optimization is necessary for adequate treatment planning and satisfactory results cannot be obtained with LR strategies. To obtain HR power density (PD) distributions from LR E-field calculations, a quasi-static zooming technique has been developed earlier at the UMC Utrecht. However, quasi-static zooming does not preserve phase information and therefore it does not provide the HR E-field information required for direct HR optimization. We combined quasi-static zooming with the optimization method to obtain a millimetre resolution temperature-based optimization strategy. First we performed a LR (1 cm) optimization and used the obtained settings to calculate the HR (2 mm) PD and corresponding HR temperature distribution. Next, we performed a HR optimization using an estimation of the new HR temperature distribution based on previous calculations. This estimation is based on the assumption that the HR and LR temperature distributions, though strongly different, respond in a similar way to amplitude/phase steering. To verify the newly obtained settings, we calculate the corresponding HR temperature distribution. This method was applied to several clinical situations and found to work very well. Deviations of this estimation method for the AMC-4 system were typically smaller than 0.2 degrees C in the volume of interest, which is accurate enough for treatment planning purposes.

The Effective Field Sizes (EFS) and the Effective Heating Depths (EDH) of Contact Flexible Microstrip Applicators (CFMA), tuned at a frequency of 434 MHz, were determined on two fat/muscle phantoms. One phantom was box-shaped with a flat top layer of 1 cm thick artificial fat and the other one a tube of which the cross-section was elliptically shaped (25 x 36 cm) having a fat equivalent shell of 1 cm thick. For the muscle material a 6 g/l saline (NaCl) solution was used. On the flat rectangular phantom, the effective field size at 1 cm depth in saline was measured to be 4.7 x 13.5 cm, 17.5 x 17.7 cm and 12.5 x 14.0 cm for the 1H-applicator, the 3H-applicator and the 5H-applicator, respectively. For the 3H-and the 5H-applicator, the Specific absorption rate (SAR) distribution at 1 cm depth showed a single maximum of SAR for a thin bolus, which split into two separate "hot spots' for a thicker bolus. The Effective Heating Depths in the phantom with a flat surface were of the order of that of a plane wave (approximately 1.4 cm), whereas a larger EHD of 2.4 cm was achieved below the 3H-applicator bent to fit the elliptical phantom. Due to the large effective field size and the "flatness' of the SAR distribution, the applicators 3H and 5H are suitable to adequately treat large superficial tumours.

In hyperthermia treatments performed with a radio-frequency phased array, the main issue to apply the excitation amplitudes and phases of the applicators for which tumour heating is optimal, i.e. the maximal therapeutic gain without unwanted side effects. Due to the complex interaction of the radiated EM-field and the patient's tissues, it is very difficult to find these optimal excitation (amplitude and phase) parameters by intuition. Calculation of the EM-field distribution within the patient can aid in finding the optimal excitation setting. However, this remains a difficult task because of the degrees of freedom available (2n - 1, with n the number of applicators in the array) and because a large temperature elevation may occur at healthy tissue sites resulting in unwanted side effects, e.g. pain or healthy tissue damage. Therefore, determining the excitation amplitudes and phases yielding optimal tumour heating can be done effectively only by application of a computerized optimization procedure. Optimization of the temperature distribution in the patient requires detailed knowledge of the thermal tissue parameters. Techniques for determining these properties are not commonly available and the use of averaged values for parameters like the tissue perfusion is expected to introduce large errors for individual patient treatment planning. As a consequence, the SAR distribution, being proportional to the temperature increase at treatment start, is more often selected for optimization. The 'optimized' excitation amplitudes and phases are found by maximization of a certain SAR ratio. Several propositions for this SAR ratio have been reported in the literature, e.g. the ratio of the SAR at the tumour site and the SAR at sites where unwanted side effects may occur. However, the definition of these ratios does not constrain the SAR value at these tissue locations to a safe value. In this paper, a tool for the optimization of the SAR distribution including the specification of constraints is presented. The tool focuses on the definition of the average SAR as a function of the excitation amplitudes and phases in a volume of arbitrary size (e.g. the tumour volume or the whole patient volume). These functions can be applied in either customized or commercially available optimization routines and they enable the definition of constraints for the average SAR in a certain volume. The described tool is illustrated for a patient case, showing the flexibility and easy application of the tool.

Background Online adaptive radiotherapy has the potential to reduce toxicity for patients treated for rectal cancer because smaller planning target volumes (PTV) margins around the entire clinical target volume (CTV) are required. The aim of this study is to describe the first clinical experience of a Conebeam CT (CBCT)-based online adaptive workflow for rectal cancer, evaluating timing of different steps in the workflow, plan quality, target coverage and patient compliance. Methods Twelve consecutive patients eligible for 5 × 5 Gy pre-operative radiotherapy were treated on a ring-based linear accelerator with a multidisciplinary team present at the treatment machine for each fraction. The accelerator is operated using an integrated software platform for both treatment planning and delivery. In all directions for all CTVs a PTV margin of 5 mm was used, except for the cranial/caudal borders of the total CTV where a margin of 8 mm was applied. A reference plan was generated based on a single planning CT. After aligning the patient the online adaptive procedure started with acquisition of a CBCT. The planning CT scan was registered to the CBCT using deformable registration and a synthetic CT scan was generated. With the support of artificial intelligence, structure guided deformation and the synthetic CT scan contours were adapted by the system to match the anatomy on the CBCT. If necessary, these contours were adjusted before a new plan was generated. A second and third CBCT were acquired to validate the new plan with respect to CTV coverage just before and after treatment delivery, respectively. Treatment was delivered using volumetric modulated arc treatment (VMAT). All steps in this process were defined and timed. Results On average the timeslot needed at the treatment machine was 34 min. The process of acquiring a CBCT, evaluating and adjusting the contours, creating the new plan and verifying the CTV on the CBCT scan took on average 20 min. Including delivery and post treatment verification this was 26 min. Manual adjustments of the target volumes were necessary in 50% of fractions. Plan quality, target coverage and patient compliance were excellent. Conclusions First clinical experience with CBCT-based online adaptive radiotherapy shows it is feasible for rectal cancer. Trial registration Medical Research Involving Human Subjects Act (WMO) does not apply to this study and was retrospectively approved by the Medical Ethics review Committee of the Academic Medical Center (W21_087 # 21.097; Amsterdam University Medical Centers, Location Academic Medical Center, Amsterdam, The Netherlands).