Measurement and Comparison of Output Factors Using Two Detectors for NOVAC7 IntraOperative Radiotherapy Accelerator

The aim of this work was to evaluate and compare the performance of comparatively new synthetic PTW 60019 microDiamond with PTW 60017 Diode E detector in measuring the output factors (OF) of IntraOperative Radiation Therapy (IORT) electron beams. For a given electron beam, OFs are defined as the ratio of the dose for any applicator size at the depth of maximum to that for a reference applicator. IORT is an innovative treatment technique that delivers a large single fraction of radiation dose to the tumor bed during surgery. The electron beams considered in this study were generated by the mobile NOVAC7 system. This device produces high-dose-per-pulse electron beams with four different energies in the range from 3 MeV to 9 MeV. We performed measurements for two higher energies, namely 7MeV and 9 MeV. The beam collimation was performed through Perspex (PMMA) cylindrical applicators with different diameters. The accurate dose delivery of IORT tightly depends on the precision of measured dose by reference applicator and the output factors of clinical applicators. The output factors were measured using microDiamond and Diode E detectors. The microDiamond detector performance was compared with a Diode E detector. Determined output factors of two detectors were in good agreement. The maximum deviations of output factors for microDiamond were found 2.74%, and 2.17% for 7 MeV and 9 MeV, respectively with respect to the PTW Diode E. The microDiamond detector was shown to exhibit excellent properties for output factor measurements and could be considered as a suitable tool for electron beam dosimetry.


Introduction
IntraOperative radiation Therapy (IORT) is a technique that delivers a high single fraction of radiation dose to the tumor during the surgical procedure. IORT can be combined with external beam radiation therapy (EBRT) or used as a single radiation dose [1] [2]. Recently, some types of dedicated accelerators including Mobetron, NOVAC and LIAC are introduced for IORT. NOVAC and LIAC are able to produce a high dose per pulse electron beam and possible to apply the whole dose of radiation during the surgery in the operating room. The NOVAC7 accelerator system is different from the conventional linacs firstly, because it is a mobile accelerator. Moreover, the NOVAC7 is equipped with a "hard-docking" collimation system with a design considerably different from those currently used for IORT. The characteristics of the NOVAC7 beams and those of the other IORT beams are then expected to be appreciably different from each other. Most important feature of this mobile dedicated radiotherapy device is the production of a very high dose per pulse electron beam. There are many advantages of IORT modality: sharp dose falloff, ideal dose distribution, short treatment time, high radiobiological effectiveness and normal tissue protection [3]. On the contrary, the high dose rate used by mobile accelerators can affect the accuracy of the dosimetric systems utilized for accelerator commissioning and quality assurance measurements.
The irradiation procedure in IORT is usually determined based on tabulated data which describe the dose distribution according to the energy, field size and depth. It is a duty of a medical physicist to find such beam parameters that the dose distribution fits best with prescribed dose. The dose distribution from electron beams is difficult to predict due to complex dependence on the beam energy and complicated trajectory of particles affected by scattering foils, collimating elements, such as applicator inserts and patient body. The dose per Monitor Unit (D/MU) can be calculated by the relative output factor (OF).
Ionization chambers are the most frequently used equipment in dosimetry of radiation therapy [4]. The dosimetric characterization of IORT beams is a non-trivial and time-consuming procedure, often requiring the use of different detectors. Ionization chambers can also be used to perform the dosimetric measurements at dose per pulse, but charge collection efficiency must be determined taking into account the free electron effects on the charge transport in the air cavity [5]. In order to overcome inaccuracy with ionization chambers measurements of IORT fields, several different correction methods have been proposed in recent years. So, it may be convenient to use less angular, energy and correction factor dependent such as p-type diode, synthetic diamond, Fricke gel dosimetry, electron paramagnetic resonance with Alanine. In recent, due to high spatial resolution, solid state detectors are used in IORT electron beams for dose profile measurements and relative dosimetry [6]. Recently PTW Freiburg, Germany introduced a synthetic single crystal diamond based PTW 60019 micro-Diamond detector with the development in Rome University "Tor Vergata" [7].

IORT Accelerator
Irradiation were performed using the 3D movable NOVAC7 (Hitesys SpA, Aprilia (LT) Italy 1997) which was installed and commissioning few years ago in the ASST Papa Giovanni XXIII Hospital in Bergamo, Italy are shown in Figure 1.
The NOVAC7 is a mobile accelerator with the dimension of 232 cm in length and 194 cm in height, specifically designed to perform intraoperative radiotherapy.
This device accelerates electrons to four different nominal energies in the range from 3 MeV to 9 MeV. These energies are denoted with the codes A_ (3 MeV), Figure 1. A photograph of the 3D moveable NOVAC7 system for IntraOperative radiation Therapy (IORT).
B_ (5 MeV), C_ (7 MeV) and D_ (9 MeV) by the manufacturer. For output measuring only two higher energies C_ (7 MeV) and D_ (9 MeV) were used. At the lowest energies A and B, the PDD curve around -Zmax is rather sharp and the uncertainty on the detector positioning at -Zmax has a pronounced effect on the detector response. This effect is much less important at the higher energies C and D.
And also, we cannot measure the output for less than 6 MeV energy using micro-Diamond as the range of use energy of this detector for electron 6 -25 MeV [13].
The beam outputs were collimated through cylindrical applicators made of Perspex (PMMA) that fastened to the radiating head. In this work, applicators with inner diameters of 100 mm, 80 mm, 70 mm, 60 mm, 50 mm and 40 mm were used. The distance from source-to-surface (SSD) was 80 cm, except for the applicator with the diameter of 100 mm for which the SSD was 100 cm. For the dosimetric characterization of the beams percentage depth dose curves (PDD) were measured in a water phantom by means of a PTW 60017 Diode E detector. The dose per pulse at the depth of maximum dose was determined using alanine dosimeters.

Dosimetry Systems and Electrometers
The outputs measurements were performed for two energies and different ap- can only be measured though after sufficient pre-irradiation to fill the traps.
Contrariwise, the diode E is an unshielded p-type silicon diode detector and is well established for dose distribution measurement in small and large fields electron beam. The characteristics and basic differences between the two detectors are shown in Table 1.
The microDiamond detector was positioned with its axis parallel to the beam direction as recommended by the manufacturer. The reference point was assumed to be positioned on the central axis of the device, 1 mm below the detector surface. The diode E detector was irradiated with its main axis parallel to the beam axis and its effective point of measurement was assumed 0.6 mm below the top surface following the manufacturer's. A PTW Unidos E Universal Dosemeter was used for the relative output factors determination.

Dosimetric Measurements
Measurements were performed in an MP3-XS IORT PTW Motorized water M. R. Islam et al.

Beam Characteristics
The

Output Factor
Output factors were determined only for two highest energies codes C_ (7 MeV) and D_ (9 MeV). The output factor values for different combinations of electron energy/applicator size determined by PTW 60019 microDiamond and PTW 60017 Diode E detectors are shown in Figure 3 and Figure 4 respectively. From the above figures, it is shown that by increasing the field size at the same energy for both detectors, output factors are decreasing. By increasing the field size, the energy fluence within the radiation field decreases. Therefore, the absorbed dose and output factor decreases. On the other hand, by increasing the electron energy at the same field size, output factor increases. Increment of beam energy can increase the energy fluence. This increase in energy fluence would increment the output factor. Regarding output factor measurements, the micro-Diamond results were in good agreement with the diode E detector. But for this detector the results indicate a slight lower response than Diode E.
The mean relative difference between the output factors measured by two detectors at different energies and applicators size are listed in Table 3.
It is shown in Table 3, there is a good correlation between the results of output factors for both detectors. The maximum difference between the results of microDiamond and Diode detectors was equal to 2.74% and 2.17% in case of 40 mm applicator for 7 MeV and 9 MeV respectively.

Conclusions
One of the main aims of our study was to investigate and compare the dosimetric characteristics of a synthetic microDiamond detector with a solid state detector for electron beam. The synthetic PTW 60019 microDiamond detector was shown to exhibit excellent properties for output factors measurements in comparison with well-established PTW 60017 Diode E detector in small fields of electron beam. Indeed, for this detector, the results indicate a slight lower-response with dose per pulse. Differences in output factors measured by the microDiamond and the PTW 60017 Diode E detector were found 2.17%, with the largest differences in high-energy/small field conditions.
The microDiamond dosimetric characteristic OF, makes this dosimeter suitable for precise dosimetry in Intra Operative Radiation Therapy with high-dose-per-pulse electron beams. Compared to the ionization chamber, the microDiamond does not require any correction for influence quantities, charge recombination and stopping power variation with water depth. Based on the results and other features like allowing excellent spatial resolution for very precise beam, it may be concluded that the PTW 60019 microDiamond could be considered as an accurate tool for output factors as well as relative dosimetry measurement of intraoperative electron beam.