Dose delivery quantification enabling method for radiotherapy treatment, involves analyzing measurements to obtain information about relation between measurements in phantom and information between phantom and treatment source

(54) NAME Method to perform in vivo dosimetry in IMRT treatment (56) QUOTE PUBLICATIONS: - - (57) SUMMARY:

The present invention relates to a method of enabling quantification of dose delivery in radiotherapy treatment during patient-specific treatment of the patient utilizing measurements at predetermined time intervals with detectors positioned in the radiation beam, between the patient and the source, and converting the values ​​into corresponding measurements in a phantom.

The invention further relates to the method of obtaining said calibration factors for the detectors. Said calibration factors are obtained for each detector, field and time interval upon simultaneously irradiating the detectors and said phantom, including detectors for measuring the absorbed dose using said patient-specific treatment without patient.

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

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SUMMARY

The present invention relates to a method of enabling quantification in dose delivery in radiotherapy treatment during patient-specific treatment of the patient utilizing measurements at predetermined time intervals with detectors positioned in the radiation beam, between the patient and the source, and converting the values ​​to corresponding measurements in a phantom.

The invention further relates to the method of obtaining said calibration factors for the detectors. Said calibration factors are obtained for each detector, field and time interval upon simultaneously irradiating the detectors and said phantom, including detectors for measuring the absorbed dose using said patient-specific treatment without patient.

(Figs. 1a, 2a)

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TECHNICAL FIELD

The present invention relates to a method of calibrating detectors for use in a radiotherapy apparatus while treating a patient to verify the accuracy of the delivered dose to the patient.

BACKGROUND OF THE INVENTION

Radiotherapy has been used to treat cancer of the human body since the early 1900s. Although radiation from cancer tumors is known to be effective, the mortality of many cancer patients has remained virtually unchanged for a long time. The main reasons for this have been the inability to control the main tumor or the presence of metastases. Only by improving local control can treatment be more effective. In recent years, Treatment Planning Systems, TPS (radiotherapy system), in radiotherapy have developed a great deal and are now able to take into account the anatomy of the specific patient and plan a more optimized treatment for each individual patient, homogeneous dose to the target. and minimal doses for risk organs.

The treatment techniques to deliver this optimized treatment are more complicated than conventional treatments as each field must be modulated laterally in intensity, thereby compensating for the heterogeneity and contour of the patient, where the technique is called IMRT Intensity Modulated Radiation Therapy. The release can be done by using compensator, filters that reduce the intensity to a predetermined level in each part of the field due to the attenuation of the primary photon beam. However, when multiple fields are used (4-8), where each field requires individual compensators, this technique is time-consuming and requires a lot of work. In addition, the attenuation of the beam also causes

522 162 2 undesirable change in the spectral distribution in the beam, thereby complicating the whole process. The most common way of delivering the IMRT fields will therefore be to use MLC (Multi Leaf Collimator), a device consisting of thin blocks (blades) that can be individually positioned to block a small portion of the field, thereby forming the beam in the lateral direction to various irregular shapes. By moving the leaves during the treatment, each portion of the treated volume will be irradiated for equal time, thereby modulating the intensity over the treated area.

However, the new treatment technology requires that the patient be exactly in the expected position, which is not always easy to achieve. In addition, the requirements for proper dose delivery and thereby the quality control (KK) requirements of the treatment machine, the planning process and finally during treatment, increase. New verification and KK must be used. However, very little has been published regarding measurements during treatment, In Vivo dosimetry.

In traditional In Vivo dosimetry, measurement with a detector on the patient's skin to predict the dose within the patient is very demanding already with a fixed field (conventional therapy) due to limitations in the TPS (Treatment Planning System) in predicting the dose distribution in the area of the patient where externally generated secondary electrons contribute significantly to the delivered dose, for example, structural regions (the part where the beam enters the patient and to a depth of 5-35 mm into the patient). As a result, neither the dose on the surface of the skin nor the dose in air upstream of the patient can be correctly predicted with TPS in fixed fields and the difficulty increases with a dynamically delivered treatment. In fixed fields this is solved either by a special design of the detector, by general calibration or by a combination of the two. IIMRT treatments it is not easy to handle this either by general calibration or design due to the fact that the varying intensity of the field is patient specific. The traditional In

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Vivo dosimetry is not normally used for every fraction and thus the interference of the specially designed detectors becomes negligible. The small margins of IMRT treatments require increased dosimetry and quality control even at each fraction to minimize uncertainties and therefore the interference of the detectors used in conventional therapy becomes significant. In addition, when using IMRT, measurements must be taken at many points to verify the field topography and the lateral position of the detectors is very critical. In order to simplify the problem, it has been proposed to measure only the flux in air. However, the discrepancy from the predicted values ​​will then be difficult to assess due to a lack of understandable quantification.

Alternatively to traditional In Vivo dosimetry, it has been proposed to use imaging systems positioned downstream of the patient, film or EPID (Electronic Portal Imaging Device) where the device is calibrated to measure the dose. Such a method is discussed in “Portal dose image prediction for dosimetric treatment. radiotherapy verification I: and open beam algorithm ”by KI Pasma et al.,

Medical Physics 25 (6), pages 830-840, 1998. A comparison can then be made with calculated dose distribution using, for example, TPS (Treatment Planning System) at the position of the measuring device. An example of this is described in “In Vivo dosimetry for prostate cancer patients using an electronic portal imaging device; demonstration of internal organ exercise ”, by M. Kroonwijk et al., Radiotherapy and Oncology, 49 (2), pages 125-132, 1998. Another alternative is to calculate the dose distribution in the patient from the measured dose distribution in EPID. This is described in "Modeling the dose distribution to an EPID with collapsed cone kernel superposition", C. Vallhagen Dahlgren et al.,

Workshop in Uppsala, March 13, 2001, organized by the company MDS Nordion.

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The latter has the advantage of providing data that is easier to understand. However, measurements only downstream of the patient will always be less accurate than combined with measurements upstream of the patient and therefore will not discern if the discrepancy was caused by incorrect dose delivery of the treatment machine or due to positioning errors or changes in the patient's anatomy (the patient may lose weight etc. from original diagnosis). The latter is important not least to analyze the root of the deviation, thereby preventing it from occurring in the next treatment fraction (normally a patient receives 30 fractions before the treatment is completed).

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to separate the dose verification from the patient positioning verification during the radiotherapy treatment of a patient and to provide a method for performing the dose verification. The invention is thus a method of calibrating the detectors to be used in Vivo (in treatment) in a time-efficient and correct way to obtain high-quality, reliable dose measurements during treatment.

The object is achieved by a method characterized by claim 1. Preferred embodiments of the invention are characterized by the dependent claims.

According to one aspect of the invention, it is characterized by the steps of irradiating a phantom, measuring in said phantom, measuring with detectors (ExtDet) between the patient and the radiation source, said measurements being divided into time intervals and analyzing the measurements to obtain information regarding the relationship between the phantom measurements and between the patient and the treatment source at each time interval, which information can be used in the treatment of the patient.

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According to the invention, the ratio of the measurements can be utilized in different ways.

Since the measurements in the phantom and of the detectors are stored at specific time intervals, a proportionality between the measurements is obtained and a fluence reference can be defined. This enables the calculation of calibration factors for the detectors used in the subsequent treatment of the patient, In Vivo measurement.

The values ​​from such In Vivo measurements, after using the calibration factors, predict the dose inside a phantom as if it were in place. The quantification of a deviation in dose distribution can thereby be used to determine whether the deviation is acceptable or not. In most cases, this verification will be sufficient, providing similar results to the off-line verification.

Verification of the patient's position can then be done in a traditional way using an EPID or other methods can be used, for example, by using a diagnostic X-ray source and transmission detector in a projection outside the treatment beam. The use of diagnostic X-ray sources can have the advantage of significantly improving the image contrast and thereby the position accuracy, which is well known in the art.

After verifying major deviations in dose delivery and / or patient positioning, a second step may be to combine the two and thereby predict the dose distribution in the patient for a more accurate control of dose to the tumor, risk organs, etc.

An alternative may be a fluence verification where a reference value for each time interval is obtained for ExtDet which compares the integrated value for all time intervals with an integrated measurement in the phantom. A combination with rear projection from the EPID images or as one

522 162 6 value to the treatment planning system can provide quantitative dose data in the patient.

These and other aspects of, and advantages of, the present invention will become apparent from the following detailed description and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, reference will be made to the accompanying drawings, where

Fig. 1a schematically shows a processing machine to which a phantom is arranged, which in turn is arranged with detectors,

Fig. 1b schematically shows the arrangement of Fig. 1 but with a human body instead of the phantom;

Fig. 2a schematically shows the machine according to Fig. 1 but with a 2D detector device arranged between the machine and the phantom,

Fig. 2B schematically shows the arrangement of Fig. 2a but with a human body instead of the phantom;

Fig. 3 is a schematic view of a radiation beam, and

Fig. 4 shows off-line verification of a treatment plan.

DETAILED DESCRIPTION OF THE INVENTION A radiotherapy device utilized for the treatment of tumors with radiation is shown schematically in FIGS. The radiotherapy system is provided with conventional

522 Field-forming device (not shown), for example an MLC, to allow the lateral shape of the beam to be altered to screen off unaffected areas of the body and concentrate the beam to the tumor.

Control means (not shown) are provided for the radiotherapy system.

A table 22 is provided for a patient 20 to lie on. The table is rotatable about a vertical axis and movable horizontally and vertically to place the area to be treated on the patient in the area of ​​the beam. Furthermore, the method of the invention utilizes various detectors to measure the radiation emitted from the radiotherapy device. For example, they may include real-time surface / skin 14 sensors such as semiconductor detectors, gas detectors, scintillator detectors, etc. The detector device may be thin or include a structure to reduce the dependence of scattered radiation. It can also be designed in such a way that it is evenly measured in g / cm2 over its entire surface, thereby taking into account the different densities of the encapsulation and the detector itself at a typical beam modality.

The detectors may also be detectors for measurement between the radiation source and the fan / patient, such as imaging systems such as film or EPID. The detectors are connected to appropriate signal processing systems (not shown). The above details are well known to those skilled in the art and will not be described in detail.

The method of the present invention is intended to utilize the aforementioned equipment to enable quantification of dose delivery in radiotherapy treatment, in particular during patient-specific treatment of the patient (hereinafter referred to as In Vivo) utilizing measurements at predetermined time intervals with detectors (hereinafter referred to as ExtDet). between the patient and the source, and the conversion measurements to correspond to measurements in a phantom using the proportionality between the measurements of the detectors and the measurements in the phantom.

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The method of the present invention is further intended to preferably obtain calibration factors for ExtDet. Said calibration factors are obtained for each ExtDet per point in the defined segment of the phantom 36, Fig. 3, and said definable time interval for each field under simultaneous irradiation of the ExtDet and said phantom including detectors to measure the absorbed dose using said patient-specific treatment without patient (hereafter referred to as off-line).

An example of utilizing the method of the invention can be described by the following steps:

• An individual treatment plan for the patient is made through use of a Treatment Planning System (TPS). The anatomy of the patient is then defined using diagnostic equipment, for example, CT, Computerized Tomography, and the radiation character of the treatment device is generally defined by measurements, where both are imported into TPS. The target volume and risk organs are defined and then the optimal plan is made where criteria such as maximum dose to risk organ and minimum dose to target etc. are used. The result of the plan is information that will be used by the processing machine to define projections, beam modality, field shapes and motion of the MLC blades, etc.

• The patient-specific treatment plan, in TPS, is applied to a phantom, suitable for dose measurements, and the dose distribution within the phantom is calculated.

• Before treatment, off-line, a physical phantom, identical to that used in the calculation, is irradiated using patient-specific treatment. The dose distribution within the phantom is measured to verify the integrated dose by comparing the measurements and the plan for a complete field, subfield or fraction

522 162 9 (quality control off-line). In addition, the dose distribution for each field, at all measuring points in the fan for each time interval, is defined by the appropriate time or synchronization to the processing machine of the intensity modulated field, and stored. The above procedure is shown in Fig. 4.

• While measuring inside the fan tower, off-line, the dose is also measured using external detectors, ExtDet, on the phantom surface or at some position in the beam between the phantom and the treatment source, using the same time interval as in the phantom measurements or synchronized to them. This step can also be done by first placing detectors inside the phantom and measuring the radiation for each time interval and then placing the detectors on the phantom, reproducing the previous irradiation conditions and measuring them for each time interval. With this solution, the same detectors can be used for both measurements. The values ​​obtained for each ExtDet, for each field and time interval and interval, will then be used to calculate the calibration factors together with the dose values ​​in the phantom. Either the values ​​obtained are stored first and then the calibration factors are calculated, or they are calculated immediately.

The calibration factors are preferably calculated according to

Cal n, f, seg-n, f, p, t (i), t (i + l) ⁼ Sn, f, t (i), t (i + l) / (Dseg-n, f, p, t (i), t (i + l))&gt; where

D: Absorbed dose measured in phantom with known shape, positioning and orientation over a certain time interval

S: The integrated signal from ExtDet n: Detector element in ExtDet, 32 in Fig. 3 f: The specific field (a projection of the beam defined by a field identity)

522 162 ίο seg: A segment in the phantom described as the shaded volume of a specific detector element in ExtDet, n and in a specific projection defined by the field, f, 36 in Fig. 3 p: well-defined point in the segment

Cal: the calibration factor t (i): Time at the start of the interval, t (0) is the start time of the sequence t (i + 1): Time at the start of the interval + 1, t (T) is the end of the sequence

Dseg-nf, p, t (i), t (i + i) The dose at the point p in the phantom segment defined by the ExtDet detector element, n and the field (projection), f integrated from time t (i) to t (i + 1)

S_n, f, t (i), t (i + i) The signal from ExtDet detector element, n, in the field, f, integrated from time t (i) to t (i + 1)

Cal n, f, seg-n, f, p, t (i&gt;, t (i + 1) The calibration factor to be used with the ExtDet detector element, n, in the field, f. To convert the signal integrated from time t (i ) to t (i + 1) to obtain the dose at the point p in the phantom segment defined by the ExtDet detector element, n and the field (projection), f integrated from time t (i) to t (i + 1) • During treatment of the patient can now get the values ​​from each

ExtDet is converted to dose in the points in each segment of the phantom, as if it were in place, using the calibration factors for each time interval according to

Dseg-nf, p, t (i), t (i + 1) ~ Sn, f, t (i), t (i + 1) / Cal n, f, seg-n, f, p, t (i ), t (i + 1)

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The values ​​from all time intervals for each specific dose point in the phantom can be summed to represent the total dose at that point for each respective field of

Dseg-n, f, p ⁼ Dseg-n, f, p, t (i), t (i + 1)) - X (S_n, f, t (i), t (i + l) / Cal _n, f, seg-n, f, p, t (i), t-ötoT i-ötoT t (i + n)

The dose from all fields to each specific point can then be summed to present the total dose at all points for the complete treatment fraction (a complete treatment consists of several fractions given over several days or weeks). The total dose at each point can be directly compared to the results of the treatment plan system when applied to the phantom, similar to the off-line verification.

Deviations between the measured and calculated dose values ​​can be analyzed by using data for each time interval, thereby simplifying the analysis phase.

If the deviation is caused by incorrect movement of the leaves, the calculated dose value in the phantom may be somewhat incorrect and in such a case the exact value can be verified by using a phantom measurement simulating the movement during the incorrectly delivered treatment.

The position of ExtDet can be determined in the transverse plane of the phantom and in particular of the patient using the projection of the detectors or markings well-defined to the ExtDet by utilizing the image from an imaging device downstream of the phantom, such as EPID or radiographic film.

Incorrect positioning of the patient in the field as compared to the detector can be visualized by using markings on the detector device that illuminates the EPID image, for example articulated seeds. By using multiple projections, the positioning of the patient can be defined.

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The alternative with a fluency verification of the integrated dose in the phantom with the treatment plan and simultaneously measured reference signals with ExtDet in each time interval where S_n, f, t (i), t (i + 1) are proportional to

Dseg-n, f, p, t (i), t (i + i) make it possible to estimate the deviation in fluence for each time interval during treatment, although it is not directly convertible to the dose in the phantom.

The method of the invention can be implemented in the control and measurement system of the radiotherapy device and thereby utilize processor and storage means available there. It can, of course, be implemented in an independent unit which includes the necessary equipment such as a central processing unit CPU which performs the steps of the method according to the invention. This is done with the help of a dedicated computer program, which is stored in the program memory. It should be understood that the computer program can also be run on a universal industrial computer instead of a specially adapted computer.

The software includes computer program code elements or software code portions that cause the computer to perform the method using equations, algorithms, data and calculations previously described. Some of the program may be stored in a processor as above but also in a ROM, RAM, PROM or EPROM chip or the like. The program, in whole or in part, may also be stored on or in other suitable computer-readable media such as a magnetic disk, CD-ROM or DVD, hard disk, magneto-optical memory storage means, in volatile memory, in flash memory, such as hardware or stored on a memory card. data server.

It is to be understood that the foregoing description of the invention and the accompanying drawings are to be construed as a non-limiting example thereof and that the scope of protection is defined by the appended claims.

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