Towards treatment planning and treatment of deep-seated solid tumors by electrochemotherapy
© Miklavcic et al; licensee BioMed Central Ltd. 2010
Received: 3 November 2009
Accepted: 23 February 2010
Published: 23 February 2010
Electrochemotherapy treats tumors by combining specific chemotherapeutic drugs with an intracellular target and electric pulses, which increases drug uptake into the tumor cells. Electrochemotherapy has been successfully used for treatment of easily accessible superficial tumor nodules. In this paper, we present the first case of deep-seated tumor electrochemotherapy based on numerical treatment planning.
The aim of our study was to treat a melanoma metastasis in the thigh of a patient. Treatment planning for electrode positioning and electrical pulse parameters was performed for two different electrode configurations: one with four and another with five long needle electrodes. During the procedure, the four electrode treatment plan was adopted and the patient was treated accordingly by electrochemotherapy with bleomycin. The response to treatment was clinically and radiographically evaluated. Due to a partial response of the treated tumor, the metastasis was surgically removed after 2 months and pathological analysis was performed.
A partial response of the tumor to electrochemotherapy was obtained. Histologically, the metastasis showed partial necrosis due to electrochemotherapy, estimated to represent 40-50% of the tumor. Based on the data obtained, we re-evaluated the electrical treatment parameters in order to correlate the treatment plan with the clinical response. Electrode positions in the numerical model were updated according to the actual positions during treatment. We compared the maximum value of the measured electric current with the current predicted by the model and good agreement was obtained. Finally, tumor coverage with an electric field above the reversible threshold was recalculated and determined to be approximately 94%. Therefore, according to the calculations, a small volume of tumor cells remained viable after electrochemotherapy, and these were sufficient for tumor regrowth.
In this, the first reported clinical case, deep-seated melanoma metastasis in the thigh of the patient was treated by electrochemotherapy, according to a treatment plan obtained by numerical modeling and optimization. Although only a partial response was obtained, the presented work demonstrates that treatment of deep-seated tumor nodules by electrochemotherapy is feasible and sets the ground for numerical treatment planning-based electrochemotherapy.
Electrochemotherapy is a type of tumor treatment that combines the use of specific chemotherapeutic drugs which have an intracellular target and low membrane permeability, with application of electric pulses to the tumors to increase drug uptake into cells. It provides good local tumor control when the two modalities combined are optimized; i.e. drug choice, distribution and concentration in the tumors, in addition to adequate electric pulse parameter selection and pulse delivery leading to cell membrane electroporation of the tumor tissue [1–4].
The drug used in electrochemotherapy needs to be adequately distributed in the tumor and present at a sufficient concentration. For treatment of small subcutaneous tumor nodules, such as melanoma metastases, intratumoral bleomycin or cisplatin administration is recommended, whereas for treatment of bigger tumor nodules, intravenous injection of bleomycin is used. Drug doses needed for treatment are provided in the published Standard Operating Procedures . For optimal drug distribution within the tumor after intratumoral injection, only a few minutes are needed between the drug injection and electroporation of tumors. After intravenous bleomycin injection, at least 8 minutes are needed for the drug to be in a pharmacological peak in the tumor and the drug remains at a sufficient concentration for at least another 20 minutes .
The second prerequisite for successful electrochemotherapy is that the whole tumor mass is exposed to a sufficiently high electric field. This can be achieved by appropriate selection and placement of electrodes, and application of electric pulses of adequate amplitude. The distribution of the electric field after application of electric pulses by plate or needle electrodes has already been extensively elaborated for small tumor nodules . These settings and electrodes provide efficient treatment of superficial tumor nodules up to 3 cm in diameter in a single electrochemotherapy session and have even been used to treat bone cancer [7–9]. However, to enable treatment of deep-seated tumors, a design of long needle electrodes and in particular their positioning with respect to the tumor is needed. If solid tumors of 3-4 cm diameter are located deep in the body, choosing electrical parameters that would result in a good clinical response and that would have no or minimal damage to normal tissue is of the outmost importance, especially in cases where tumors are located close to vital organs. Numerical modeling in treatment planning is the proposed approach that also allows verification of the electrical parameters based on the clinical response to electrochemotherapy .
In order to also develop electrochemotherapy for treatment of deep-seated tumors, the aim of our study was to treat a deep-seated melanoma metastasis in the thigh of a patient by custom-made long needle electrodes. Based on treatment planning, the electric pulse parameters and positioning of electrodes were determined. The patient was treated accordingly by electrochemotherapy with bleomycin. The response to treatment was clinically, radiographically and histologically evaluated. Due to a partial response of the treated tumor (reduction in size by more than 50%), the metastasis was surgically removed after 2 months and pathological analysis was performed. Based on the data obtained, we re-evaluated the electrical treatment parameters in order to correlate the treatment plan with the clinical response of the electrochemotherapy-treated metastasis.
Materials and methods
Clinical data of the patient
Numerical treatment planning
Based on the PET-CT radiographs, the position of the melanoma metastasis was determined. Its size was 20 × 14 mm in diameter and it was located in the right thigh, 20 mm under the skin. The location was estimated to be on or minimally invasive into thigh muscle fascia.
where σ and φ are tissue electric conductivity and electric potential, respectively. The boundary conditions used in our calculations were a constant potential on the surface of the electrodes and electrical insulation on all outer boundaries of the model. For tumors, the reversible electroporation threshold was considered to be 400 V/cm, for adipose tissue 100 V/cm, for muscle tissue 200 V/cm and 80 V/cm, in the perpendicular direction to muscle fibers and in the parallel direction, respectively. The irreversible threshold was set to 900 V/cm for all tissues.
In treatment planning, a numerical model of electroporation was used that did not take into account changes in tissue conductivity during electroporation. After the treatment, in order to compare the measured electric currents during electric pulse delivery and the currents predicted by the model, a model that took into account the changes in tissue conductivity was used. The conductivity dependencies on the electric field σ(E) of all tissues were approximated by a step function .
where F is fitness, VTrev is the tumor volume subjected to the local electric field above the reversible threshold and VHirrev is the volume of healthy tissue subjected to a local electric field above the irreversible threshold. The weights in the fitness function were chosen to reflect the importance of the individual parameters for efficient ECT. Namely, VTrev is crucial for efficient ECT; therefore its weight is larger (100) than the weight of VHirrev. The algorithm was stopped after 500 iterations - this number of iterations gave good solutions in previous studies [15, 16] and the quality of the treatment plan was compared to previously set treatment plan requirements - when a sufficiently high quality treatment plan was achieved, i.e. the whole tumor was covered with a sufficiently high electric field and very little of the surrounding tissue was affected by the field. The optimization took 5 h on a Windows XP, 3.0 GHz, 2 GB RAM desktop computer.
Electrodes, pulse generator and pulse parameters
Four custom-made electrodes made of stainless steel, 1.8 mm in diameter with sharpened tips, insulated except for the upper 4 cm, were used. An additional 1.2 mm in diameter electrode was considered for direct insertion into the center of the tumor. The electrodes were connected to independently controlled generator outputs of the Cliniporator Vitae (IGEA, Carpi, Italy). The Cliniporator Vitae device is a pulse generator with 6 independently controlled and electrically insulated outputs each providing up to 3000 V, max current 50 A, delivering 8 rectangular electrical pulses (rise time 1 μs) of 100 μs duration at a pulse repetition frequency of 4 Hz . The current and voltage are measured and logged with a precision better than 3%, which allows for pulse delivery control and post-treatment evaluation. Pulses were delivered 10-12 minutes after i.v. bolus injection of bleomycin.
Optimized treatment plan
The results of optimization were electrode positions and minimum voltages for each electrode pair. Electrode positions outside the tumor were similar in both configurations, i.e. very close to the tumor surface; however the fifth electrode inside the tumor significantly reduced the required voltage to achieve efficient membrane electroporation of cells in the whole tumor, thereby also reducing damage to healthy tissue. The maximum voltages required were 3000 V and 2500 V in the four and five electrode configuration, respectively. The volume of irreversibly permeabilized healthy tissue according to the treatment plan was 13.8 cm3 (of that 11.5 cm3 adipose tissue and 2.33 cm3 muscle tissue) in the four electrode configuration and 12.3 cm3 (of that 10.4 cm3 adipose tissue and 1.88 cm3 muscle tissue) in the five electrode configuration. Depth of insertion was a few millimeters deeper than the tumor, slightly penetrating the muscle tissue.
Treatment and response to the treatment
The treatment was performed in general anesthesia and due to the myorelaxant given to the patient, only minor muscle contractions were observed. No other side-effects were noticed. The patient reported no discomfort after the treatment, and left the hospital after 2 days. The response to treatment with electrochemotherapy was followed ultrasonographically at regular time intervals (Figure 1). The first post-operative ultrasound showed a substantial decrease in the tumor volume (more than 50%), while the second showed a regrowth of tumor tissue.
Numerical model validation
Agreement between maximum measured electrical currents and currents calculated in the numerical model
Measured current [A]
Calculated current [A]
We present here the first study of deep-seated tumor electrochemotherapy based on numerical treatment planning. Namely, electrochemotherapy was, until now only used for superficial and accessible tumor nodules, with an approximately 80% objective response rate [1, 7, 18–22]. In this, the first reported clinical case, a deep-seated melanoma metastasis in the thigh of the patient was treated by insertion of long needle electrodes around the tumor. A new electric pulse generator was used that provides higher voltages and currents and has six independently controlled insulated outputs, which thus allow for treatment of deeper-seated tumors by delivering electric pulses between different pairs of electrodes.
The electric conductivity values taken into account in the numerical model have been obtained from measurements done mostly on large animal tissues, which are not necessarily valid for human tissue as well . Furthermore, these values are not valid for electroporated tissue, as electroporation increases tissue conductivity. In one of our previous studies, we measured the conductivity of rat muscle and liver tissue and the conductivity increased between 3.2-3.8 times for both tissue types . Also, comparing the measured and calculated electric current for rabbit liver resulted in an estimated conductivity increase of 3.6 . In this study, an increase in conductivity of 3.5 as a result of electroporation was used for all tissues, a choice which seems to be at least partly validated by the agreement between the measured and calculated electric current in our study (Table 1).
In our post-treatment model, tissue electroporation thresholds for muscle and tumor were taken from previous studies in which these thresholds were determined by comparing in vivo measurements and numerical modeling of electroporation of different tissues [14, 23, 24]. In all these studies, the assumption was made that the values of the electric field, at which a change in tissue properties occur, coincides with the electroporation thresholds. This assumption was already considered both theoretically and practically in previous studies and can be considered as justified [25–27].
Two different electroporation models were used in our present study; the first which took into account changes in tissue properties and the second which did not, a simplification that made the calculation much faster. Both models predicted similar electroporation volumes (results not shown), while only the first model could predict the electric current density. As such, the second model was used for optimization-based treatment planning and the first one for validation by comparing the measured and calculated electric currents.
According to the model used after the treatment, inaccuracies in positioning of the electrodes are most likely responsible for the inadequate electroporation of the entire tumor volume, although possible deviation from the assigned electrical conductivities and/or deviations from the assigned electroporation threshold for the tissues cannot be disregarded. Nevertheless, good agreement between the predicted and the measured delivered currents implies that the conductivity values chosen were very close to the real values. Under the assumption that the positioning of the electrodes was responsible for the inadequate tumor electroporation, calculations show that only a small percent of the tumor was not successfully electroporated (app. 6%); however this was enough for the tumor to survive and start growing again. While normally these relatively small errors in electrode positioning (Figure 4) would not lead to an unsuccessful treatment, the proximity of three different tissues with very different conductivities made the treatment very sensitive to electrode positioning. Adipose tissue that surrounded the tumor had by far the lowest conductivity, which meant that according to the voltage divider principle the electric field was largest in the adipose tissue around the tumor and not in the tumor .
Electrochemotherapy of a deep-seated tumor was performed in a patient with the aim to verify the treatment approach, and the use of treatment planning in optimizing the positioning of the electrodes and electrical parameters. Although the configuration of five electrodes was recognized as the best in treatment planning, it was not possible to execute it due to "floating" of the tumor in the adipose tissue. The four-electrode position was thus used and at follow-up of the tumor growth, significant tumor reduction was observed (Figure 1). For effective treatment, however, all viable tumor cells have to be destroyed in order to prevent tumor regrowth. Therefore, as calculated in this tumor model, even a small percentage of remaining viable tumor tissue (6%) after electrochemotherapy was enough for tumor regrowth. Nevertheless, this clinical case demonstrates that treatment of deep-seated tumor nodules by electrochemotherapy is feasible and that optimization of the treatment approach by tumor numerical modeling is of significant help.
This research was supported by the Slovenian Research Agency under various grants. We would also like to thank IGEA, Carpi (MO), Italy for providing us with the electrodes and Cliniporator VITAE. Written consent for publication was obtained from the patient.
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