IX Latin American IRPA Regional Congress on Radiation Protection and Safety - IRPA 2013 Rio de Janeiro, RJ, Brazil, April 15-19, 2013 SOCIEDADE BRASILEIRA DE PROTEÇÃO RADIOLÓGICA - SBPR PRECISE DOSIMETRY OF SMALL PHOTON BEAMS COLLIMATED BY AN ADD-ON DYNAMIC MICRO-MULTILEAVES COLLIMATOR. De La Fuente-Rosales L. 1, Alfonso-Laguardia R. 2, García-Yip F. 1, Ascencion Y. 1 1 Department of Radiotherapy, Instituto de Oncología y Radiobiología (INOR), Havana, Cuba liset.fuentes@infomed.sld.cu 2 Instituto Superior de Tecnologías y Ciencia Aplicadas (InSTEC), Havana, Cuba ABSTRACT Purpose: To improve the accuracy in the measurement of relative output factors (OFs) for small photon beams used in stereotactic techniques at INOR. Methods and Materials: The OFs of small photon beams defined by a dynamic micromultileaf collimator (DMLC) 3D-Line were measured with different detectors: (1) semiflex 0.125cc ionization chamber (IC), (2) pinpoint micro-ionization chamber, (3) shielded diode type P, (4) unshielded diode type E, (5) gafchromic EBT2 and (6) EBT3 films; in water or water equivalent plastic phantom, as required. Isocentric measuring setup was selected, at a depth of 5 cm with a source surface distance of 95 cm; the detector orientation was set parallel to the central axis of the beam (CAX) for chambers and diodes, and perpendicular for films. Small square fields ranging from 0.29 cm2 to 5.8 cm2 were studied; the intermediate 5.8 cm2 reference field was used for OF calculation. Relative dose factors were also computed by a Monte Carlo simulation of the same irradiation geometry, in order to compare measurement results with calculated OFs. Additionally, correction factors for all detectors relative to Monte Carlo calculated, EBT2 and EBT3 film measurements were determined. Results: The six detector types used in this work are accurate enough to determine the output factors of field sizes bigger than 2x2 cm2, i.e. the OFs showed practically no dependence with the detector type. For smaller field sizes, the 0.125 cc (semiflex) chamber underestimates the OF by more than 55% while, as expected, the rest of the detectors (with smaller active volumes) agreed in higher OF values in this region of interest. Conclusions: Significant deviations were encountered when non appropriate detectors are used for small field dosimetry. The correction factors obtained can be used for eventual reduction of errors. The use of several types of detectors is strongly recommended for OF measurements in very small fields. 1. INTRODUCTION The wide availability of treatment techniques like Stereotactic Radiotherapy (SRT) or Radiosurgery (SRS) involves an increasing use of small photon beams in the clinic. These beams can be produced in a conventional linear accelerator (linac) with the addition of a mini- or micro- multileaf collimator (mmlc), or in specialized treatment units specifically designed for stereotaxy. Care must be taken during measurements of stereotactic fields and small segments due to (a) the presence of lateral electron disequilibrium, (b) the smaller collimator aperture results in partial occlusion of the source and (c) a large detector compared to the field dimensions perturbs particle fluence in the medium and (d) the signal is affected by volume averaging effect [1]. Due to these conditions the availability of suitable detector is critical. Additionally, a change in photon and secondary electron energy spectra and as a consequence a change in beam quality may occur when decreasing field size.
Several authors described problems related to the dosimetry of small beams (Das et al.[2], Laub et al [3], Bouchard and Seuntjens [4], Sanchez Doblado et al [5], Ding et al.[6]) which are very far from reference conditions recommended in existing Codes of Practice (CoPs) or dosimetry protocols such as IAEA TRS-398 [7] and AAPM TG-51 [8]. In order to develop a standard methodology that complements the existing CoPs an international working group on reference dosimetry of small and nonstandard fields has been established by the International Atomic Energy Agency (IAEA) in cooperation with the American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This group published in 2008 a proposal of a formalism for the determination of absorbed dose to water using ionization chambers in situations different from the conventional reference conditions. The aim of this paper is to improve the accuracy in relative output factors (OFs) in small static photon beams used in stereotactic techniques at INOR based on the formalism described by Alfonso et al [9]. 2.1. Measurements 2. METHODS AND MATERIALS Measurements were performed in an Elekta Precise (Elekta Oncology Systems) dual energy linac coupled with a dynamic micro-multileaf collimator (DMLC) model L ARANCIO, manufactured by 3D-Line (now Elekta, Stockholm). The DMLC consists of 24 doublefocused leaf-pairs made of tungsten and allows dynamic arcs. Material and geometrical data of DMLC may be seen on Table 1. Six type of detectors available at INOR s were irradiated at 6MV photon beam energy, using isocentric configuration SAD= 100cm, at a depth of 5cm in a PTW/MP3 water tank or RW3 water equivalent plastic phantom. The orientation relative to the beam central axis (CAX) for each setup was chosen according the detector type. For this study square fields determined by the leaf width at isocenter from 5.8 cm2 down to 0.29 cm2 (which is the smallest field size that can be reached with the DMLC) were selected. Table 1. DMLC General Specifications Weight Leaf Material Tungsten (93%) Leaf Number Leaf Height Leaf Width at isocenter Leaf Overtravel Minimum field size at isocenter Maximum field size at isocenter Focalization 48 (24 pairs) 8 cm 2.901 mm 31.25mm 2.901mm 69.62x68.12mm Double
Leaf Maximum velocity Transmission 0.5% 2.2. Detectors Among the common requirements of external radiation therapy detectors, those used for narrow fields need to satisfy additional features like the small active volume and the tissue equivalence. The ionization chambers, which are the detector of excellence in broad beam dosimetry, are not suitable due to the presence of high dose gradients, time dose variance and non-uniform beam distributions. There is a wide variety of small field detectors in the market, but generally only few are available in the daily clinic, this justify the study of its behavior when using for absolute and relative small beam dosimetry. The detectors used in this work are summarized in Table 2. Table 2. List of detector used in this study ID. Type Manufactur Model er PFD shielded Si PTW- 60016 diode Frieburg EFD unshielded Si PTW- 60017 diode Frieburg 0.125c c IC cylindrical ion chamber PTW- Frieburg Semiflex 31010 0.015c c IC cylindrical ion chamber PTW- Frieburg PinPoint 31006 Film radiochromic film ISP EBT2 and EBT3 2.3. EBT2 and EBT3 Dose Calibration. Gafchromic film pieces of 2x3,5 cm2 were exposed to 6MV photon beam at a measurement depth of 5cm, using isocentric set-up and 10x10 cm2 field size. A Farmer type ionization chamber PTW 30013 was used to monitor the linac output and the dose delivered to the film at the same time of film irradiation. The film pieces were exposed perpendicularly to the radiation beam axis in a 30x30x21.3cm3 solid water RW3 phantom.
Figure 1: Gafchromic Films EBT2 and EBT3 Dose Calibration at the linac(left). Film Scanner with the cardboard template (right). The calibration films were scanned 21 hours after exposure in an A3 color bed scanner of 48 bits Microtek ScanMaker 9800XL. Prior to any scanning 5 blank readings were taken since this model can not warm up the lamp in preview mode. The scanner was operated in transmission mode, in RGB channels, with a resolution of 72 ppi and all the enhancements options turned off. The film pieces were scanned in landscape orientation and saved as uncompressed TIFF format. To ensure the same film position along the longitudinal axis of the scanner a cardboard frame was constructed and removed before scanning. The use of the cardboard ensures reading reproducibility and also avoids the non uniform response of the scan field, placing the ROI in the center of the scanner bed. During EBT2 and EBT3 reading, special care must be taken with the selection of ROI size and position relative to the scanner bed center. It was observed that a small change in its size leads to a different average pixel value, determining a critical variation in relative output factor estimation. Additionally, is necessary to check that there are not bad pixels inside the ROI perimeter. The ROI size for each field was selected according field dimensions. ImageJ [10] software was used for image processing. The region of interest (ROI) selected was 2x2 mm2, and only the red component was read. 2.4. Formalism for relative dosimetry The formalism proposed by the IAEA working group for relative dosimetry in machines that are not capable of reproduce the reference conditions stated by CoPs, introduced a field factor as: or (1) where: is the absorbed dose to water at a reference point in a phantom for a clinical field f clin of quality Q clin and in the absence of the chamber, the absorbed dose to water for a reference field f ref or the machine-specific reference field f msr of beam quality Q ref or Q msr, respectively in the absence of the chamber. In relative dosimetry of single static fields this field factor is usually called output factor. There are two ways of calculate this or : (a) directly as a ratio of absorbed doses to water using Monte Carlo simulations, or (b) as a ratio of detectors readings multiplied by correction factors and : or
(2) which can be measured using a suitable detector with a very small sensitive volume and an energy independent response, like radiochromic film or a liquid ion chamber, or calculated by Monte Carlo according to next equation: [ [ ] or ] (3) When correction factors are close to unity, the ratio of detector measurements is a good estimation of field factors. As in our case we used an estereotactic system attached to a linac (linac based system) we can apply the so called daisy chaining method, introducing a step between the reference field and the clinic field, an intermediate field fint, ensuring a correct measuring with an IC for field sizes larger than the intermediate field and limiting the effect of the energy dependence in small beam detectors like diodes. ( ) ( ) ( ) ( ) (4) det is the suitable detector for small field dosimetry, and; IC the ionization chamber. The correction factor can be expressed as: ( ) ( ) (5) Figure 2: Representation of the dosimetry of small static fields with reference to the so called machine-specific reference field ( from Alfonso et al formalism).
2.5. Output factor measurements In order to ensure the correct detector alignment respect to the beam central axis, before each measurement with the DMLC, beam profiles were acquired in the inplane and crossplane directions. Table 3. shows the orientation relative to the beam CAX for the five type of detectors used. In our specific case we choose 5,8x5,8 cm2 as an intermediate field (fint) which is large enough to avoid small beams conditions, and at the same time with limited energy dependence. Another advantage is that this fint can be adopted by the DMLC coupled to the linac head. Table 3. Detector orientation as respect to beam central axis. Detector type Cylindrical ion chamber Cylindrical micro ion chamber Silicon shielded diode Silicon unshielded diode Radiochromic Films Detector reference Axial axis Axial axis Axial axis Axial axis Film surface Output factors Parallel Parallel parallel parallel perpendicular 2.6. Correction Factors relative to gafchromic EBT2 and EBT3, and Monte Carlo. For all fields sizes and detectors used it was determined the ratio of the clinical and intermediate field signals ( ) ( ). A similar ratio, but in terms of dose was determined with EBT2 and EBT3 readings and with Monte Carlo results. Using the previous data, correction factors were determined following the methodology explained in section 2.3. 2.7. Monte Carlo Simulations Using the BEAMnrc code system [11] Ascención et al [12] generated a space phase of a 7x7 field from the Elekta Precise head coupled with the DMLC at 95 cm from the target. Using this data file, the particle transport in a 30x30x30 cm3 water phantom was simulated with DOSXYZ [13] code. During numerical simulation it was found that voxel size is key in the computed OFs, a voxel size of 0.25x0.25x0.2 cm3 was chosen. 3. RESULTS
3.1. Output factor measurements For field sizes between 2 2 cm2 and 5,8 5,8 cm2 the OFs measured with all detectors show an agreement within 3% relative to those measured by EBT2 and calculated by Monte Carlo. A same deviation was obtained with EBT3 films except for the 2 2 cm2 field using the semiflex IC, here the difference reached 5%. As expected, an unacceptable deviation (larger than 55%) for the smaller field size (0.29x0.29 cm2) was encountered with the semiflex IC. This difference is due to the lack of lateral electron equilibrium, the partial occlusion of the source causing overlapping penumbra. As a consequence, since the chamber is placed in a region of high dose gradient, the volume averaged reading is unreliable. Table 4. Summary of the measurement characteristics. Accelerator Elekta Precise Nominal energy 6 MV Beam quality TPR 20,10 = 0.68 Collimator Conventional MLCi (fix system 7x7 cm field), add-on mmlc 3Dline Orange (2.9 mm with leaves) Fields Square 3, 6, 9, 12, 15, 18, 21, 27, 36, 45, 60 mm (ref) Measurement 5 cm in water depth Source Surface 95 cm (SAD setup) Distance (SSD) Detectors Calculations Source of data SemiFlex 0.125 cc IC (PTW-31002), PinPoint IC (PTW-31016), Shielded diode (PTW-60008), Unshielded diode (PTW- 60012), Gafchromic EBT2, EBT3 Monte Carlo, 0.25x0.25x0.2 cm3 voxels size in water Measurements Another important result is that in most of the small fields measured with diode detectors we found that the OF in general are larger than those obtained with the EBT2, this is mainly because the material surrounding the diode detector tend to decrease lateral electronic disequilibrium. The second reason that causes an overestimation of OFs measured with diodes is that when secondary electron energy increases the electron mass collision stopping power ratio between water and silicon slightly decreases. The same tendency was also observed when comparing diodes with EBT3.
Figure 3: OFs measured with different detectors: (1) semiflex 0.125cc ionization chamber, (2) pinpoint micro-ionization chamber, (3) shielded diode type P, (4) unshielded diode type E, (5) gafchromic EBT2 and (6) EBT3 films and calculated by MC (7). When comparing the OF determined by films and diodes against MC calculation for the 0,58 0,58 cm2 field the discrepancy is bigger in the case of diodes. The reason of this can be related to water equivalence of the film material in contrast with the diode detector composition. 3.2. Correction Factors relative to gafchromic EBT2 and EBT3, and Monte Carlo. As discussed above, correction factors can be calculated by Monte Carlo or measured with a suitable detector with small sensitive volume and an energy independent response, like gafchromic films. Dose ratios using EBT2 (Fig.4, Table.5) and EBT3 (Fig.5, Table.6) films were determined for the field sizes listed in Table 4. that range from 0.29 cm2 to 5.8 cm2. Additionally, five spot cases were calculated by Monte Carlo for a redundant validation of the dose ratios (see Fig.6 and Table.7) f Figure 4: Correction factors k clin f int Qclin Q int relative to Gafchromic EBT2 film. f Table 5. Correction factors k clin f int Qclin Q int relative to Gafchromic EBT2 film.
Field Size(cm) Flex 0.125 Pin Diodo P Diodo E EBT3 P 0.29 3.18 1.37 0.92 0.91 0.85 0.58 1.52 1.12 0.94 0.99 1.01 0.87 1.14 1.04 0.96 0.99 1.00 1.16 1.06 1.03 0.97 1.00 1.02 1.45 1.03 1.01 0.98 0.99 1.02 1.74 1.01 1.01 0.98 0.99 1.01 2.03 1.00 1.00 0.99 1.00 0.98 2.61 1.00 1.00 1.00 1.00 0.99 2.9 1.00 1.00 0.99 1.00 0.98 3.77 0.98 0.98 0.98 0.98 0.97 4.64 1.00 1.01 1.00 1.00 1.00 5.8 1.00 1.00 1.00 1.00 1.00 We found that the absolute absorbed dose determination is very dependent of the time delay between exposure and reading process, the handling of the film and finally the calibration procedure followed. It is also extremely important to check the reproducibility of the whole system. The function used for calibration data fitting was a rational one recommended by Lewis et al [14]. f Figure 5: Correction factors k clin f int Qclin Q int relative to Gafchromic EBT3 film. f Table 6. Correctios factors k clin f int Qclin Q int relative to gafchromic EBT3 film. Field Size(cm) Flex 0.125 PP Diodo P Diodo E EBT2 0.29 3.74 1.62 1.09 1.08 1.18 0.58 1.51 1.11 0.93 0.98 0.99 0.87 1.15 1.04 0.96 1.00 1.00 1.16 1.05 1.01 0.95 0.98 0.98 1.45 1.00 0.99 0.96 0.97 0.98 1.74 1.00 0.99 0.97 0.98 0.99 2.03 1.03 1.03 1.01 1.02 1.02 2.61 1.01 1.01 1.01 1.01 1.01 2.90 1.01 1.01 1.01 1.01 1.02 3.77 1.01 1.01 1.01 1.01 1.03 4.64 1.00 1.01 1.00 1.00 1.00 5.80 1.00 1.00 1.00 1.00 1.00
f Figure 6: Correction factors k clin f int Qclin Q int relative to Monte Carlo. f Table 7. Correctios factors k clin f int Qclin Q int relative to Monte Carlo calculations. Field Size,cm Flex 0.125 PP Diodo P Diodo E EBT 2 EBT 3 0.58 1.52 1.12 0.94 0.99 1.00 1.01 1.16 1.07 1.04 0.98 1.00 1.01 1.02 1.74 1.02 1.02 0.99 1.01 1.01 1.02 2.9 1.00 1.00 1.00 1.00 1.00 0.99 5.8 1.00 1.00 1.00 1.00 1.00 1.00 Differences between Monte Carlo values and data measured with ion chambers in Table 7 for such small fields can be explained by the larger volume of these detectors. Other deviations may be due to geometric misalignments. 4. DISCUSSION Dosimetric data for small photon beams should be measured using detectors with small active volume, in case of using others just take into account that corrections to detector reading need to be applied. The obtained differences between MC calculated OFs and those measured with semiflex and pinpoint chambers may be related with their larger volume and small geometric misalignments, it is quite important that whichever detector we use it must be correctly aligned with the beam axis. Any slight detector offset from the CAX reduces the output factor values. Discrepancies obtained between diode measurements and MC calculations are a result of detector composition, the non-water equivalence of diode material leads to an overestimation of the output factors. 5. CONCLUSIONS This investigation contributes to the data being collected for each machine characteristics and available detectors presented in the clinic, following the methodology proposed by the IAEA working group.
The result of OF measurements for field sizes larger than 2 2 cm2 does not depend on the detector type used. When reducing the field size there is an increased in OF estimation using a wrong detector. To account for any deviation due to the handling and scanning process of the Gafchromic films Lewis et al suggested the simple two point rescaling [14], taking into account that for the same lot of EBT2 or EBT3 films the dose response can be represented by a generic function which can be adapted using one exposed reference film plus an unexposed reference film. This rescaling can improve the absolute dose estimated by the Gafchromic films and consequently the dose ratio. As expected, corrections factors obtained for EBT2, EBT3 and MC are very close to unity for field sizes larger than 2x2 cm2. As a recommendation, we consider that instead of MC simulations in water, a more realistic material composition can be used to obtain the detector response factors; this applies to diodes, EBT2 and EBT3 films. 6. REFERENCES [1] Aspradakis M., Byrne J. P., Palmans H., Conway J., Rosser K., Warrington J., Duane S., Small Field MV Photon Dosimetry, IPEM Report Number 103, UK, 2010. [2] Das, I.J., Downes, M.B., Kassaee, A. and Tochner, Z.: Choice of radiation detector in dosimetry of stereotactic radiosurgery-radiotherapy. Journal of Radiosurgery, Vol. 3, 177 185, 2000. [3] W. U. Laub and T. Wong: The volume effect of detectors in the dosimetry of small fields used in IMRT, Med. Phys. 30, 341 347, 2003. [4] Bouchard H. and Seuntjens J.: Ionization chamber-based reference dosimetry of intensity modulated radiation beams, Med. Phys., Vol. 31(9): 2454 2465, 2004. [5] Sanchez-Doblado F., Andreo P., Capote R., Leal A., Perucha M., Arrans R., Nunez L., Mainegra E., Lagares J.I., Carrasco E.: Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams. Phys. in Med. and Biol., Vol. 48, Issue 14, 2081 2099, 2003. [6] Das I.J., Ding G.X., Ahnesjö A.: Small fields: nonequilibrium radiation dosimetry, Med. Phys., Vol. 35, Issue 1, 206 215, 2008. [7] Colección De Informes Técnicos Nº 398, Determinación De La Dosis Absorbida En Radioterapia Con Haces Externos, IAEA-TRS-398, Viena, 2005. [8] Almond P., Biggs P., Coursey B. M., Hanson W. F., Saiful Huq M., Nath R., Rogers D. W. O., AAPM s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams, Med. Phys. 26(9): 1847-1879, 1999.
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