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Monte Carlo dosimetric evaluation of high energy vs low energy photon beams in low density tissues
Radiotherapy and Oncology 79 (2006) 131–138 www.thegreenjournal.com
Dosimetry
Monte Carlo dosimetric evaluation of high energy vs low energy photon beams in low density tissues Miltiadis F. Tsiakalos*, Sotirios Stathakis, George A. Plataniotis, Constantin Kappas, Kiki Theodorou Medical Physics Department, Medical School, University of Thessalia, Larissa, Hellas, Greece
Abstract Background and purpose: Low megavoltage photon beams are often the treatment choice in radiotherapy when low density heterogeneities are involved, because higher energies show some undesirable dosimetric effects. This work is aimed at investigating the effects of different energy selection for low density tissues. Patients and methods: BEAMnrc was used to simulate simple treatment set-ups in a simple and a CT reconstructed lung phantom and an air-channel phantom. The dose distribution of 6, 15 and 20 MV photon beams was studied using single, AP/PA and three-field arrangements. Results: Our results showed no significant changes in the penumbra width in lung when a pair of opposed fields were used. The underdosage at the anterior/posterior tumor edge caused by the dose build-up at the lung-tumor interface reached 7% for a 5!5 cm AP/PA set-up. Shrinkage of the 90% isodose volume was noticed for the same set-up, which could be rectified by adding a lateral field. For the CT reconstructed phantom, the AP/PA set-up offered better tumor coverage when lower energies were used but for the three field set-up, higher energies resulted to better sparing of the lung tissue. For the air-channel set-up, adding an opposed field reduced the penumbra width. Using higher energies resulted in a 7% cold spot around the air-tissue interface for a 5!5 cm field. Conclusions: The choice of energy for treatment in the low density areas is not a straightforward decision but depends on a number of parameters such as the beam set-up and the dosimetric criteria. Updated calculation algorithms should be used in order to be confident for the choice of energy of treatment q 2006 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 79 (2006) 131–138. Keywords: Radiotherapy; Heterogeneities; Energy choice
For most cancer centers, the choice of the energy for the treatment of lung tumors is the low energy photon beam of 6 MV, while higher energy photon beams are used for the treatment of deeper located tumors in lung [7,28]. Lateral scattered electrons out of the beam path and the second build-up effect in low density/tissue interfaces, are known to result in a reduction of dose near the beam edge and at the interface, especially for higher energy photon beams [3,7,13,14,20,30]. The magnitude of the above phenomena depends on the beam energy and field size and have been previously investigated [6]. It has also been shown that although most of the treatment planning systems (TPS) fail to accurately predict the dose distribution under such conditions [1,7,11,13,14,16,22,29,30], Monte Carlo calculations give very good results [8,19,23]. Effects of lateral electronic equilibrium loss have been reported for single fields, and it has been shown that there is a significant reduction in dose and an increase in the
penumbra width when low density materials are present for energies above 10 MV [7,15,21,28]. Based on the findings of the above investigators the Radiation Therapy Oncology Group (RTOG) 91–05 has proposed the use of energies in the range of 4–12 MV [21]. In the case, where two opposed fields (AP/PA) are the preferable choice of treatment for a lung tumor and the patient anterior–posterior separation is greater than 20 cm, it has been reported that the dose delivered close to the chest wall with 6 MV photon beams may exceed the dose to the prescription point at midline by 10% or more [9]. This high value of dose at small depths can cause skin reactions and even parenchymal lesions in the underlying lung. Increasing the photon beam energy to 18 MV the resulting central axis dose will be more uniform while there will be significant reduction in dose to the superficial structures [28]. Controversy about the choice of energy continues as various investigators supported that addition of an opposed
0167-8140/$ - see front matter q 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2006.02.012
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or multiple fields may reduce the magnitude of underdosage caused by penumbra enlargement [2,12,27,28] In this work, we will compare the dose distributions in a phantom consisting of lung equivalent material, a phantom reconstructed from CT images of a lung patient and an airchannel phantom, by means of Monte Carlo for 6, 15 and 20 MV beam energies, using different beam geometries. The Quality Index (QI) for the above beams is 0.667, 0.755 and 0.794, respectively.
Materials and methods A phantom simulating a lung tumor (Fig. 1a) and a realistic anthropomorphic phantom reconstructed using CT slices from a patient were used to study the dose distribution in lung. The resulting distributions are given through the profiles and the central axis dose distribution. Dose Volume Histograms (DVH) were calculated for the CT based phantom case in order to help evaluate the different techniques and isodose distributions. Finally, an air-channel phantom was used to investigate the dose distribution close to the airtissue boundaries. BEAM [24] code was used to model a Varian Clinac 2100 C/ D linear accelerator and phase space data for 6, 15 and 20 MV were created. The percent depth dose (PDD) and profiles of measured data in different depths were used for verification of each one of the beam energies. The agreement between measured and calculated data was better than 1%.
DOSXYZnrc The phantom simulating a tumor in the lung, was created using the DOSXYZnrc [17] graphical user interface. The dimension of the block shaped phantom was 14!14! 14 cm with 2 cm wall made of water like material (rZ1 g/ cm3). Inside the phantom, another block of 10!10!10 cm thick lung equivalent material (rZ0.3 g/cm3) was created that contained a 5!5!5 cm water like material (Fig. 1a). The voxel size for the phantom was 0.3 cm in all
directions. The dimensions were chosen in order to be able to compare the results with the work of previous investigators that used similar geometry [4,7,15,28]. The isocenter was always located at the center of the tumor. The resulting dose distributions had statistical uncertainty less than 2% in all cases. A full set of DICOM CT slices of a patient with lung tumor (average lung density 0.28 g/cm3). were used to build a realistic phantom that was inserted in DOSXYZnrc to calculate the dose distribution using the 7!7 cm2 field. The voxel size of the reconstructed phantom was 0.3!0.3! 0.4 cm. The MC simulation for the CT based phantom was performed for the same energies and beam set-up as the ones used for the cubic lung phantom. Finally, an air-channel (densityZ0.0012 g/cm3) phantom was also created that consisted of a 3!3!14 cm air-channel inside a 14!14!14 cm water phantom. The air-channel runs completely across the phantom (Fig. 1b), and a 5! 5 cm2 field was used at 6, 15 and 20 MV. The voxel size for this phantom was 0.3 cm for all directions.
Simulation set-up For the lung phantom we used a 5!5 cm2 field incident from the top for our first simulation. This set-up was used to compare the penumbra, the fringe [7] (distance between the 50 and 90% relative doses) and the second build-up effect of a single field when a low density material is surrounding a water-like medium and it was repeated for all three photon beam energies. Next, two opposed fields (AP/PA) of equal weight were used in order to examine how the penumbra/fringe and the build-up is affected by the presence of an opposed field. The choice of the 5!5 cm2 field was made in order to examine small fields, where the problem could be more pronounced. The field was designed to be adjacent at the tumor edge in order to initially investigate the magnitude of the penumbra/fringe enlargement in relation to the tumor edge. This enlargement provides an indication
Fig. 1. Phantom representing (a) 5!5!5 cm tumor inside lung (b) Air-channel phantom.
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about the optimal margin we should use clinically in order for the 90% isodose to cover the tumor for similar geometries. The second set of simulations involved a 7!7 cm2 field for the same set-up in order to see the dependence of field increase on the examined parameters. Next, a lateral field of same weight was also added in order to examine the effects on dose distribution caused by the addition of the third field. The 7!7 cm2 field was the choice of treatment since, it includes the target volume allowing a margin of 1 cm in each direction. The isocenter was located at the center of the phantom for all simulations. For simulating a realistic patient case, a phantom was created using DICOM RT Toolbox [25] from the CT images of a patient. The phantom was introduced to DOSXYZnrc and the 7!7 cm2 field size for all photon beam energies was used. The isocenter was located at the upper right lung. AP/PA opposed and AP/PA and lateral field techniques with equal weight for the beams were used for the production of the dose distribution. The PTV was delineated as a 5 cm diameter cylinder of 5 cm length. In order to perform, the comparison with the phantom experimental data the 50–90% relative dose distance (fringe) was used as reference. Finally for the air-channel case, the calculations were performed using 6, 15 and 20 MV beam energies. The first set-up was a single lateral field and the second one consisted of two opposed lateral fields of 5!5 cm2. The isocenter was located 0.7 cm from the air–water interface to simulate the irradiation of a larynx tumor that covers 1.4 cm of depth next to the larynx interface.
Results Single and AP/PA beams for lung phantom The profiles at the center of the phantom are for single and opposed beams are exactly the same (profiles of the opposed set-up are displayed in Fig. 3), showing that the addition of a second opposed beam in such a geometry does not alter the penumbra/fringe width.
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The dose profile along the central beam axis is presented in Fig. 2. For the AP/PA configuration at 6 MV the dose profile along the central beam axis is quite homogenous For the 15 and 20 MV the second build-up is clearly seen when the beams enter the tumor part. Inside the tumor the maximum dose (100%) for the highest energy (20 MV) is delivered at the central part of the tumor. At the upper and distal edge of the tumor, because of the effect of the second build-up when the beams enter the tumor, the dose is lowered (Table 1). For the 5!5 field and 20 MV the difference at the edge of the tumor can reach 7% compared to the dose at the central part. The examination of a larger field (7!7) reveals a reduction of this difference at 5% as for the larger field the electronic disequilibrium is less intense. The profile degradation in relation to the energy used can be seen in Fig. 3. It is clear that for higher energies the penumbra/fringe width is getting larger, as expected, due to the increase on the electrons’ range close to the field’s edge. For the 7!7 field the 50% isodose curve is 3.5 cm away from the central axis. Using the data from Fig. 3 we can see that for 6 MV the 90% curve is at 3 cm, for 15 MV at 2.49 cm and for 20 MV at 2.27 cm. The tumor edge is at 2.5 cm and therefore, in order to have at least 90% coverage of the target for higher energies a larger field than the one used for 6 MV should be used.
AP/PA and lateral field technique for lung phantom For 20 MV, the addition of a third beam from the lateral direction (x-axis), while keeping the same field size (7!7), can result in an improvement in the target coverage as the 90% isodose curve moves from 2.27 to 2.6 cm. Table 2 shows that for z-axis there is no considerable improvement, but for x-axis the addition of the 3D field seems to be beneficial when it comes to the tumor coverage, especially for higher energies, where the difference at the edge of tumor can be as high as 13% when the AP/PA field set-up is used. If more strict criteria are used, such as the coverage of the target by the 95%
Fig. 2. Comparison of depth dose curves of 6, 15, and 20 MV 5!5 cm2 AP/PA fields set-up in the lung phantom.
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Table 1 Relative dose along the beam axis for AP/PA beams in lung phantom Energy (MV)
6 15 20
5!5 cm2
7!7 cm2
Edge of tumor (%)
Isocenter (%)
Edge of tumor (%)
Isocenter (%)
98 95 93
100 100 100
100 97 95
100 100 100
isodose, the addition of the third beam for the 20 MV when keeping the same field size, is still not enough to encompass the tumor.
the PTV. However, larger PTV volume seems to get higher doses for 6 MV. The right lung distribution does not have considerable difference between high and low energy configuration. The same holds true if we consider the DVH for both lungs. A careful comparison of the Dose Volume Histograms of a 6 MV opposed field technique and that of a 20 MV AP/PA from Fig. 5a and b shows that the PTV coverage is better with the 6 MV opposed technique as more volume of the PTV receives higher dose. On the other hand, the whole lung receives lower doses with the 20 MV AP/PA and lateral field technique. Considering only the right lung this difference is more pronounced. So, a compromise must be made in order to decide what set-up is preferable to use.
AP/PA fields for CT reconstructed phantom Based on the Monte Carlo dose distribution for the CT reconstructed patient phantom (Fig. 4a and b) the findings of the lung phantom can be confirmed. Despite the fact that for 6 MV the 7!7 field is adequate to cover the 5 cm PTV with the 90% isodose, for 20 MV there is a shrinkage of the isodose curve at the side, where the field edge passes through the lung.
AP/PA and lateral field for CT reconstructed phantom If we add a third field from the side (Fig. 4c and d), the PTV coverage from the 90% isodose is now complete for the higher energy. For 6 MV (Fig. 4c) a region of a relative high dose (50%) at the left side of the image is clearly seen. The DVHs for AP/PA and AP/PA and lateral field techniques for the available energies were also compared (Fig. 5a and b). For the AP/PA technique, the better coverage of the tumor and the higher dose distribution in the right lung are prominent for the lower energy. For the AP/PA and lateral field technique, the higher energy dose distribution seems to be more homogenous in
Single and lateral opposed fields for air-channel phantom Similar effects can be noticed in air-channel geometry because of the loss of the electronic equilibrium due to the air-tissue interface. For an air-channel set-up the second build-up at the airtissue interface can cause a dose reduction for single field and lateral opposed fields configuration (Fig. 6). For the higher energy and the lateral opposed fields configuration the difference of the interface dose to that of the isocenter can reach up to 7%. As for the lung case, because of the traveling of the beam through a low density material (air-channel), for the lateral opposed beams, the dose profile at the isocenter degrades with increasing beam energy (Table 3). A difference from the lung case that has to be noted for the air-channel set-up is that with the addition of an opposed field the penumbra/fringe width becomes smaller. This happens because the first beam passes completely through the air-channel causing the degradation of the profile at the isocenter but the opposed field passes through the tissue resulting in a smaller penumbra width.
Fig. 3. Comparison of off-axis ratios for 6, 15 and 20 MV AP/PA fields in the lung phantom.
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Table 2 Relative dose values along and perpendicular to beam direction for 7!7 cm2 field size Energy (MV)
6 15 20
Along beam direction
Perpendicular to beam direction
Edge of tumor (AP/PA) (%)
Edge of tumor (3 fields) (%)
Isocenter (%)
Edge of tumor (AP/PA) (%)
Edge of tumor (3 fields) (%)
Isocenter (%)
100 97 95
98 96 95
100 100 100
95 92 87
100 95 91
100 100 100
The results in Table 3 are derived from a dose profile at the depth of the isocenter.
Discussion We have investigated dosimetric properties of 6, 15 and 20 MV using Monte Carlo calculations in a simple lung phantom and in a realistic CT reconstructed patient phantom. The effect of penumbra enlargement in dose distribution was investigated in relation to the beam energy. As the beam energy increases, the electron range increases, so secondary electrons produced further from the field edge get to travel outside the field edge and carry energy further outside the field, making more intense the effect of electronic disequilibrium. In the case of a low density material this range increases further and the penumbra profile degrades more. The above effect is more intense when the edge of the field is close to the boundaries of a water-like material (tumor) and a lower density one (lung) because more side scattered electrons are produced in the high density material which then enter the low density material.
After the comparison of a single anterior field set-up and a similar one with an additional opposed field in a phantom containing a low density inhomogeneity, we observed a decrease in the penumbra/fringe width when one of the opposed beams passes completely through water like material like in the case of the air-channel. This can be extended to a tumor close to the edge of lung, where one of the beams passes through lung and the other through water like material before entering the tumor. This is what happens in the geometry studied by White et al. [28]. In the case, where the tumor is in the middle of a low density area like when the tumor is completely encompassed by lung, the addition of an opposed field offers no improvement in the penumbra enlargement. The underdosage at the field edge when higher energy is used is clearly seen in the realistic patient phantom case, where there is shrinkage of the 90% isodose in the opposed field set-up. A conventional treatment planning system that does not model at all or does not model correctly the electron transport would not have shown this shrinkage [22] but it would have shown the relative high energy region (50%) produced by the lower energy beam. Although, the choice of energy for a plan is related to
Fig. 4. 7!7 cm2 fields in in CT reconstructed phantom, (a) AP/PA set-up 6 MV, (b) AP/PA set-up 20 MV, (c) AP/PA and lateral field set-up 6 MV, (d) AP/PA and lateral field set-up 20 MV.
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High vs low energy beams in low density tissues
Fig. 5. Comparison of DVH’s for 6, 15 and 20 MV beams in the CT reconstructed phantom, (a) AP/PA set-up, (b) AP/PA and lateral field set-up.
various parameters (such as patient thickness, planned treatment dose), with no doubt, this fact could be a misleading factor for the choice of a plan consisting of a beam of higher energy without any modification of the field size. Additionally, the effect of the second dose build-up that is presented as the beam enters from the lung material to the tumor was examined in relation to the energy and field used. This effect is also not modelled by a conventional algorithm. Recently, a paper was published by Osei et al. [20] which presents Monte Carlo calculations for different energies on a phantom of similar geometry. Our results agree with their general conclusions about the effects of small field sizes and high energies on the dose distribution in target. We see the incomplete coverage for the higher energy for the opposed field configuration if the field size is not adjusted, an effect which is not shown on TPS with simple heterogeneity correction algorithms. A disagreement between Osei et al. and our work is that their results show that when an opposed field is added, the build-up effect at z-axis is reduced. Our results show the opposite. The difference between the dose in the middle of target compared to the dose at the edge is higher when an opposed field is added. Any differences in the coverage of the tumor by the field and the more pessimistic evaluation of Osei et al. is mainly because of the criterion they use for the target coverage, which is 95% instead of 90% which is in our work. For the opposed beam set-up, as it can also be seen from the produced DVHs, lower energy can lead to a better dose homogeneity in the tumor volume, as when higher energies and smaller fields are used, the second build-up at the lungtumor interface results at lower dose at the edge of the tumor. In this case, we have to counterweight the dose homogeneity with the increased dose at lower depths that the healthy tissues receive. When adding a lateral field perpendicular to the opposed fields, we can see that some of the effects that were noticed with the opposed fields for higher energies are rectified. For
example the underdosage at the tumor edge drops from 13 to 9% for the given field size (Table 2). The 90% isodose as seen in Fig. 4b, now encompasses the tumor edge (Fig. 4d). Nevertheless, the addition of the third field has two inversely related implications: (a) the improvement of homogeneity in tumor (PTV) and (b) the irradiation of the contralateral lung with a low dose. Which option is preferable, in terms of the expected rate of radiation pneumonitis risk, is still an unresolved problem [5] and in clinical practice is left to the discretion of the treating physician. For the air-channel set-up the penumbra enlargement and the edge underdosage due to the second build-up at the air-tissue interface, when the energy is increased, are again prominent. In this case, the addition of the opposed field is beneficial for all energies as the penumbra and fringe are decreased because of the traversing of the second beam through the tissue. It seems that the beam set-up may play an important role in minimizing the undesirable effects of the high energy beams that are mentioned in the introduction. Kan et al. [10] studied a similar geometry using Monte Carlo. However, in his study only low energies (4–8 MV) were studied and the isocenter was at the middle of the air-channel set-up. He showed that for 6 MV and for opposed fields, the second build-up at the distal interface of the air-channel is insignificant for a field of 5!5 cm2 and increases up to 3% for 4!4 cm2. These results agree with our data for the low energies although we calculate a higher build-up difference for the 5!5 cm2, which we believe is accounted for by the different placement of the isocenter. As most treatment planning algorithms do not take into account the electron transport [26], the calculation of dose for treatment plans that include high energy photon beams passing through low density regions in the patient body, could result in inaccuracies in dose delivery that may play a critical role for the treatment outcome. In order to avoid this problem, the user is left with the choice of using low energy beams in such regions, where the calculation results are more accurate; loosing the clinical advantages that a higher energy beam may offer. For typical AP/PA set-ups it
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Fig. 6. Depth dose curves for (a) single and (b) two lateral opposed fields of 6, 15 and 20 MV for the air-channel.
seems that the low energy should be the treatment of choice if the user is not willing to increase the treatment field size [20]. Our results show that this topic has to be extensively studied and the user of a TPS must be very cautious when dealing with calculated treatment plans of higher energy in low density regions. At present time, treatment planning systems that use Monte Carlo calculations or Superposition/Convolution algorithms are the most accurate [18]. However, careful testing of those must be done, as the implementation of an algorithm and/or the simplifications made for speed gain may be different in each of them. The new TPS with such advanced dose calculation algorithms can be used to evaluate the plan and might provide the information about the appropriate margin that has to be used in each case without compromising the dose delivered to the target and critical organs nearby [31].
We tried to investigate with our experiments the basic physical processes that take place when treating in regions with low density heterogeneities and thus, we used simple geometry for our beams and our set-up. The comparison with
Table 3 Penumbra and Fringe width for 5!5 cm2 single/lateral opposed fields in the air-channel phantom Energy (MV)
Penumbra width AP/ PA (cm)
Fringe width AP/ PA (cm)
Penumbra width single field (cm)
Fringe width single field (cm)
6 15 20
0.745 0.98 1.09
0.56 0.722 0.723
– – 1.91
– – 1.26
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a realistic patient phantom case was in good qualitative agreement with the results obtained for the phantom set-up.
Conclusions There is no doubt that high energy produces some effects that are undesirable when treating in regions, where low density heterogeneities are present. The examination of the influence of the set-up geometry, the relative position of the treated site to the low density interface and the criteria that one uses for accepting or rejecting a dose distribution, showed that the magnitude of these effects depend on the above parameters. Thus, special care should be taken in order to decide the optimal energy choice.
* Corresponding author. Miltiadis F. Tsiakalos, Medical Physics Department, University Hospital of Larissa, P.O. Box 1425, 41-110, Larissa, Hellas, Greece. E-mail address: [email protected] Received 10 May 2004; received in revised form 21 February 2006; accepted 22 February 2006
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Dosimetry
Monte Carlo dosimetric evaluation of high energy vs low energy photon beams in low density tissues Miltiadis F. Tsiakalos*, Sotirios Stathakis, George A. Plataniotis, Constantin Kappas, Kiki Theodorou Medical Physics Department, Medical School, University of Thessalia, Larissa, Hellas, Greece
Abstract Background and purpose: Low megavoltage photon beams are often the treatment choice in radiotherapy when low density heterogeneities are involved, because higher energies show some undesirable dosimetric effects. This work is aimed at investigating the effects of different energy selection for low density tissues. Patients and methods: BEAMnrc was used to simulate simple treatment set-ups in a simple and a CT reconstructed lung phantom and an air-channel phantom. The dose distribution of 6, 15 and 20 MV photon beams was studied using single, AP/PA and three-field arrangements. Results: Our results showed no significant changes in the penumbra width in lung when a pair of opposed fields were used. The underdosage at the anterior/posterior tumor edge caused by the dose build-up at the lung-tumor interface reached 7% for a 5!5 cm AP/PA set-up. Shrinkage of the 90% isodose volume was noticed for the same set-up, which could be rectified by adding a lateral field. For the CT reconstructed phantom, the AP/PA set-up offered better tumor coverage when lower energies were used but for the three field set-up, higher energies resulted to better sparing of the lung tissue. For the air-channel set-up, adding an opposed field reduced the penumbra width. Using higher energies resulted in a 7% cold spot around the air-tissue interface for a 5!5 cm field. Conclusions: The choice of energy for treatment in the low density areas is not a straightforward decision but depends on a number of parameters such as the beam set-up and the dosimetric criteria. Updated calculation algorithms should be used in order to be confident for the choice of energy of treatment q 2006 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 79 (2006) 131–138. Keywords: Radiotherapy; Heterogeneities; Energy choice
For most cancer centers, the choice of the energy for the treatment of lung tumors is the low energy photon beam of 6 MV, while higher energy photon beams are used for the treatment of deeper located tumors in lung [7,28]. Lateral scattered electrons out of the beam path and the second build-up effect in low density/tissue interfaces, are known to result in a reduction of dose near the beam edge and at the interface, especially for higher energy photon beams [3,7,13,14,20,30]. The magnitude of the above phenomena depends on the beam energy and field size and have been previously investigated [6]. It has also been shown that although most of the treatment planning systems (TPS) fail to accurately predict the dose distribution under such conditions [1,7,11,13,14,16,22,29,30], Monte Carlo calculations give very good results [8,19,23]. Effects of lateral electronic equilibrium loss have been reported for single fields, and it has been shown that there is a significant reduction in dose and an increase in the
penumbra width when low density materials are present for energies above 10 MV [7,15,21,28]. Based on the findings of the above investigators the Radiation Therapy Oncology Group (RTOG) 91–05 has proposed the use of energies in the range of 4–12 MV [21]. In the case, where two opposed fields (AP/PA) are the preferable choice of treatment for a lung tumor and the patient anterior–posterior separation is greater than 20 cm, it has been reported that the dose delivered close to the chest wall with 6 MV photon beams may exceed the dose to the prescription point at midline by 10% or more [9]. This high value of dose at small depths can cause skin reactions and even parenchymal lesions in the underlying lung. Increasing the photon beam energy to 18 MV the resulting central axis dose will be more uniform while there will be significant reduction in dose to the superficial structures [28]. Controversy about the choice of energy continues as various investigators supported that addition of an opposed
0167-8140/$ - see front matter q 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2006.02.012
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or multiple fields may reduce the magnitude of underdosage caused by penumbra enlargement [2,12,27,28] In this work, we will compare the dose distributions in a phantom consisting of lung equivalent material, a phantom reconstructed from CT images of a lung patient and an airchannel phantom, by means of Monte Carlo for 6, 15 and 20 MV beam energies, using different beam geometries. The Quality Index (QI) for the above beams is 0.667, 0.755 and 0.794, respectively.
Materials and methods A phantom simulating a lung tumor (Fig. 1a) and a realistic anthropomorphic phantom reconstructed using CT slices from a patient were used to study the dose distribution in lung. The resulting distributions are given through the profiles and the central axis dose distribution. Dose Volume Histograms (DVH) were calculated for the CT based phantom case in order to help evaluate the different techniques and isodose distributions. Finally, an air-channel phantom was used to investigate the dose distribution close to the airtissue boundaries. BEAM [24] code was used to model a Varian Clinac 2100 C/ D linear accelerator and phase space data for 6, 15 and 20 MV were created. The percent depth dose (PDD) and profiles of measured data in different depths were used for verification of each one of the beam energies. The agreement between measured and calculated data was better than 1%.
DOSXYZnrc The phantom simulating a tumor in the lung, was created using the DOSXYZnrc [17] graphical user interface. The dimension of the block shaped phantom was 14!14! 14 cm with 2 cm wall made of water like material (rZ1 g/ cm3). Inside the phantom, another block of 10!10!10 cm thick lung equivalent material (rZ0.3 g/cm3) was created that contained a 5!5!5 cm water like material (Fig. 1a). The voxel size for the phantom was 0.3 cm in all
directions. The dimensions were chosen in order to be able to compare the results with the work of previous investigators that used similar geometry [4,7,15,28]. The isocenter was always located at the center of the tumor. The resulting dose distributions had statistical uncertainty less than 2% in all cases. A full set of DICOM CT slices of a patient with lung tumor (average lung density 0.28 g/cm3). were used to build a realistic phantom that was inserted in DOSXYZnrc to calculate the dose distribution using the 7!7 cm2 field. The voxel size of the reconstructed phantom was 0.3!0.3! 0.4 cm. The MC simulation for the CT based phantom was performed for the same energies and beam set-up as the ones used for the cubic lung phantom. Finally, an air-channel (densityZ0.0012 g/cm3) phantom was also created that consisted of a 3!3!14 cm air-channel inside a 14!14!14 cm water phantom. The air-channel runs completely across the phantom (Fig. 1b), and a 5! 5 cm2 field was used at 6, 15 and 20 MV. The voxel size for this phantom was 0.3 cm for all directions.
Simulation set-up For the lung phantom we used a 5!5 cm2 field incident from the top for our first simulation. This set-up was used to compare the penumbra, the fringe [7] (distance between the 50 and 90% relative doses) and the second build-up effect of a single field when a low density material is surrounding a water-like medium and it was repeated for all three photon beam energies. Next, two opposed fields (AP/PA) of equal weight were used in order to examine how the penumbra/fringe and the build-up is affected by the presence of an opposed field. The choice of the 5!5 cm2 field was made in order to examine small fields, where the problem could be more pronounced. The field was designed to be adjacent at the tumor edge in order to initially investigate the magnitude of the penumbra/fringe enlargement in relation to the tumor edge. This enlargement provides an indication
Fig. 1. Phantom representing (a) 5!5!5 cm tumor inside lung (b) Air-channel phantom.
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about the optimal margin we should use clinically in order for the 90% isodose to cover the tumor for similar geometries. The second set of simulations involved a 7!7 cm2 field for the same set-up in order to see the dependence of field increase on the examined parameters. Next, a lateral field of same weight was also added in order to examine the effects on dose distribution caused by the addition of the third field. The 7!7 cm2 field was the choice of treatment since, it includes the target volume allowing a margin of 1 cm in each direction. The isocenter was located at the center of the phantom for all simulations. For simulating a realistic patient case, a phantom was created using DICOM RT Toolbox [25] from the CT images of a patient. The phantom was introduced to DOSXYZnrc and the 7!7 cm2 field size for all photon beam energies was used. The isocenter was located at the upper right lung. AP/PA opposed and AP/PA and lateral field techniques with equal weight for the beams were used for the production of the dose distribution. The PTV was delineated as a 5 cm diameter cylinder of 5 cm length. In order to perform, the comparison with the phantom experimental data the 50–90% relative dose distance (fringe) was used as reference. Finally for the air-channel case, the calculations were performed using 6, 15 and 20 MV beam energies. The first set-up was a single lateral field and the second one consisted of two opposed lateral fields of 5!5 cm2. The isocenter was located 0.7 cm from the air–water interface to simulate the irradiation of a larynx tumor that covers 1.4 cm of depth next to the larynx interface.
Results Single and AP/PA beams for lung phantom The profiles at the center of the phantom are for single and opposed beams are exactly the same (profiles of the opposed set-up are displayed in Fig. 3), showing that the addition of a second opposed beam in such a geometry does not alter the penumbra/fringe width.
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The dose profile along the central beam axis is presented in Fig. 2. For the AP/PA configuration at 6 MV the dose profile along the central beam axis is quite homogenous For the 15 and 20 MV the second build-up is clearly seen when the beams enter the tumor part. Inside the tumor the maximum dose (100%) for the highest energy (20 MV) is delivered at the central part of the tumor. At the upper and distal edge of the tumor, because of the effect of the second build-up when the beams enter the tumor, the dose is lowered (Table 1). For the 5!5 field and 20 MV the difference at the edge of the tumor can reach 7% compared to the dose at the central part. The examination of a larger field (7!7) reveals a reduction of this difference at 5% as for the larger field the electronic disequilibrium is less intense. The profile degradation in relation to the energy used can be seen in Fig. 3. It is clear that for higher energies the penumbra/fringe width is getting larger, as expected, due to the increase on the electrons’ range close to the field’s edge. For the 7!7 field the 50% isodose curve is 3.5 cm away from the central axis. Using the data from Fig. 3 we can see that for 6 MV the 90% curve is at 3 cm, for 15 MV at 2.49 cm and for 20 MV at 2.27 cm. The tumor edge is at 2.5 cm and therefore, in order to have at least 90% coverage of the target for higher energies a larger field than the one used for 6 MV should be used.
AP/PA and lateral field technique for lung phantom For 20 MV, the addition of a third beam from the lateral direction (x-axis), while keeping the same field size (7!7), can result in an improvement in the target coverage as the 90% isodose curve moves from 2.27 to 2.6 cm. Table 2 shows that for z-axis there is no considerable improvement, but for x-axis the addition of the 3D field seems to be beneficial when it comes to the tumor coverage, especially for higher energies, where the difference at the edge of tumor can be as high as 13% when the AP/PA field set-up is used. If more strict criteria are used, such as the coverage of the target by the 95%
Fig. 2. Comparison of depth dose curves of 6, 15, and 20 MV 5!5 cm2 AP/PA fields set-up in the lung phantom.
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Table 1 Relative dose along the beam axis for AP/PA beams in lung phantom Energy (MV)
6 15 20
5!5 cm2
7!7 cm2
Edge of tumor (%)
Isocenter (%)
Edge of tumor (%)
Isocenter (%)
98 95 93
100 100 100
100 97 95
100 100 100
isodose, the addition of the third beam for the 20 MV when keeping the same field size, is still not enough to encompass the tumor.
the PTV. However, larger PTV volume seems to get higher doses for 6 MV. The right lung distribution does not have considerable difference between high and low energy configuration. The same holds true if we consider the DVH for both lungs. A careful comparison of the Dose Volume Histograms of a 6 MV opposed field technique and that of a 20 MV AP/PA from Fig. 5a and b shows that the PTV coverage is better with the 6 MV opposed technique as more volume of the PTV receives higher dose. On the other hand, the whole lung receives lower doses with the 20 MV AP/PA and lateral field technique. Considering only the right lung this difference is more pronounced. So, a compromise must be made in order to decide what set-up is preferable to use.
AP/PA fields for CT reconstructed phantom Based on the Monte Carlo dose distribution for the CT reconstructed patient phantom (Fig. 4a and b) the findings of the lung phantom can be confirmed. Despite the fact that for 6 MV the 7!7 field is adequate to cover the 5 cm PTV with the 90% isodose, for 20 MV there is a shrinkage of the isodose curve at the side, where the field edge passes through the lung.
AP/PA and lateral field for CT reconstructed phantom If we add a third field from the side (Fig. 4c and d), the PTV coverage from the 90% isodose is now complete for the higher energy. For 6 MV (Fig. 4c) a region of a relative high dose (50%) at the left side of the image is clearly seen. The DVHs for AP/PA and AP/PA and lateral field techniques for the available energies were also compared (Fig. 5a and b). For the AP/PA technique, the better coverage of the tumor and the higher dose distribution in the right lung are prominent for the lower energy. For the AP/PA and lateral field technique, the higher energy dose distribution seems to be more homogenous in
Single and lateral opposed fields for air-channel phantom Similar effects can be noticed in air-channel geometry because of the loss of the electronic equilibrium due to the air-tissue interface. For an air-channel set-up the second build-up at the airtissue interface can cause a dose reduction for single field and lateral opposed fields configuration (Fig. 6). For the higher energy and the lateral opposed fields configuration the difference of the interface dose to that of the isocenter can reach up to 7%. As for the lung case, because of the traveling of the beam through a low density material (air-channel), for the lateral opposed beams, the dose profile at the isocenter degrades with increasing beam energy (Table 3). A difference from the lung case that has to be noted for the air-channel set-up is that with the addition of an opposed field the penumbra/fringe width becomes smaller. This happens because the first beam passes completely through the air-channel causing the degradation of the profile at the isocenter but the opposed field passes through the tissue resulting in a smaller penumbra width.
Fig. 3. Comparison of off-axis ratios for 6, 15 and 20 MV AP/PA fields in the lung phantom.
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Table 2 Relative dose values along and perpendicular to beam direction for 7!7 cm2 field size Energy (MV)
6 15 20
Along beam direction
Perpendicular to beam direction
Edge of tumor (AP/PA) (%)
Edge of tumor (3 fields) (%)
Isocenter (%)
Edge of tumor (AP/PA) (%)
Edge of tumor (3 fields) (%)
Isocenter (%)
100 97 95
98 96 95
100 100 100
95 92 87
100 95 91
100 100 100
The results in Table 3 are derived from a dose profile at the depth of the isocenter.
Discussion We have investigated dosimetric properties of 6, 15 and 20 MV using Monte Carlo calculations in a simple lung phantom and in a realistic CT reconstructed patient phantom. The effect of penumbra enlargement in dose distribution was investigated in relation to the beam energy. As the beam energy increases, the electron range increases, so secondary electrons produced further from the field edge get to travel outside the field edge and carry energy further outside the field, making more intense the effect of electronic disequilibrium. In the case of a low density material this range increases further and the penumbra profile degrades more. The above effect is more intense when the edge of the field is close to the boundaries of a water-like material (tumor) and a lower density one (lung) because more side scattered electrons are produced in the high density material which then enter the low density material.
After the comparison of a single anterior field set-up and a similar one with an additional opposed field in a phantom containing a low density inhomogeneity, we observed a decrease in the penumbra/fringe width when one of the opposed beams passes completely through water like material like in the case of the air-channel. This can be extended to a tumor close to the edge of lung, where one of the beams passes through lung and the other through water like material before entering the tumor. This is what happens in the geometry studied by White et al. [28]. In the case, where the tumor is in the middle of a low density area like when the tumor is completely encompassed by lung, the addition of an opposed field offers no improvement in the penumbra enlargement. The underdosage at the field edge when higher energy is used is clearly seen in the realistic patient phantom case, where there is shrinkage of the 90% isodose in the opposed field set-up. A conventional treatment planning system that does not model at all or does not model correctly the electron transport would not have shown this shrinkage [22] but it would have shown the relative high energy region (50%) produced by the lower energy beam. Although, the choice of energy for a plan is related to
Fig. 4. 7!7 cm2 fields in in CT reconstructed phantom, (a) AP/PA set-up 6 MV, (b) AP/PA set-up 20 MV, (c) AP/PA and lateral field set-up 6 MV, (d) AP/PA and lateral field set-up 20 MV.
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Fig. 5. Comparison of DVH’s for 6, 15 and 20 MV beams in the CT reconstructed phantom, (a) AP/PA set-up, (b) AP/PA and lateral field set-up.
various parameters (such as patient thickness, planned treatment dose), with no doubt, this fact could be a misleading factor for the choice of a plan consisting of a beam of higher energy without any modification of the field size. Additionally, the effect of the second dose build-up that is presented as the beam enters from the lung material to the tumor was examined in relation to the energy and field used. This effect is also not modelled by a conventional algorithm. Recently, a paper was published by Osei et al. [20] which presents Monte Carlo calculations for different energies on a phantom of similar geometry. Our results agree with their general conclusions about the effects of small field sizes and high energies on the dose distribution in target. We see the incomplete coverage for the higher energy for the opposed field configuration if the field size is not adjusted, an effect which is not shown on TPS with simple heterogeneity correction algorithms. A disagreement between Osei et al. and our work is that their results show that when an opposed field is added, the build-up effect at z-axis is reduced. Our results show the opposite. The difference between the dose in the middle of target compared to the dose at the edge is higher when an opposed field is added. Any differences in the coverage of the tumor by the field and the more pessimistic evaluation of Osei et al. is mainly because of the criterion they use for the target coverage, which is 95% instead of 90% which is in our work. For the opposed beam set-up, as it can also be seen from the produced DVHs, lower energy can lead to a better dose homogeneity in the tumor volume, as when higher energies and smaller fields are used, the second build-up at the lungtumor interface results at lower dose at the edge of the tumor. In this case, we have to counterweight the dose homogeneity with the increased dose at lower depths that the healthy tissues receive. When adding a lateral field perpendicular to the opposed fields, we can see that some of the effects that were noticed with the opposed fields for higher energies are rectified. For
example the underdosage at the tumor edge drops from 13 to 9% for the given field size (Table 2). The 90% isodose as seen in Fig. 4b, now encompasses the tumor edge (Fig. 4d). Nevertheless, the addition of the third field has two inversely related implications: (a) the improvement of homogeneity in tumor (PTV) and (b) the irradiation of the contralateral lung with a low dose. Which option is preferable, in terms of the expected rate of radiation pneumonitis risk, is still an unresolved problem [5] and in clinical practice is left to the discretion of the treating physician. For the air-channel set-up the penumbra enlargement and the edge underdosage due to the second build-up at the air-tissue interface, when the energy is increased, are again prominent. In this case, the addition of the opposed field is beneficial for all energies as the penumbra and fringe are decreased because of the traversing of the second beam through the tissue. It seems that the beam set-up may play an important role in minimizing the undesirable effects of the high energy beams that are mentioned in the introduction. Kan et al. [10] studied a similar geometry using Monte Carlo. However, in his study only low energies (4–8 MV) were studied and the isocenter was at the middle of the air-channel set-up. He showed that for 6 MV and for opposed fields, the second build-up at the distal interface of the air-channel is insignificant for a field of 5!5 cm2 and increases up to 3% for 4!4 cm2. These results agree with our data for the low energies although we calculate a higher build-up difference for the 5!5 cm2, which we believe is accounted for by the different placement of the isocenter. As most treatment planning algorithms do not take into account the electron transport [26], the calculation of dose for treatment plans that include high energy photon beams passing through low density regions in the patient body, could result in inaccuracies in dose delivery that may play a critical role for the treatment outcome. In order to avoid this problem, the user is left with the choice of using low energy beams in such regions, where the calculation results are more accurate; loosing the clinical advantages that a higher energy beam may offer. For typical AP/PA set-ups it
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Fig. 6. Depth dose curves for (a) single and (b) two lateral opposed fields of 6, 15 and 20 MV for the air-channel.
seems that the low energy should be the treatment of choice if the user is not willing to increase the treatment field size [20]. Our results show that this topic has to be extensively studied and the user of a TPS must be very cautious when dealing with calculated treatment plans of higher energy in low density regions. At present time, treatment planning systems that use Monte Carlo calculations or Superposition/Convolution algorithms are the most accurate [18]. However, careful testing of those must be done, as the implementation of an algorithm and/or the simplifications made for speed gain may be different in each of them. The new TPS with such advanced dose calculation algorithms can be used to evaluate the plan and might provide the information about the appropriate margin that has to be used in each case without compromising the dose delivered to the target and critical organs nearby [31].
We tried to investigate with our experiments the basic physical processes that take place when treating in regions with low density heterogeneities and thus, we used simple geometry for our beams and our set-up. The comparison with
Table 3 Penumbra and Fringe width for 5!5 cm2 single/lateral opposed fields in the air-channel phantom Energy (MV)
Penumbra width AP/ PA (cm)
Fringe width AP/ PA (cm)
Penumbra width single field (cm)
Fringe width single field (cm)
6 15 20
0.745 0.98 1.09
0.56 0.722 0.723
– – 1.91
– – 1.26
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a realistic patient phantom case was in good qualitative agreement with the results obtained for the phantom set-up.
Conclusions There is no doubt that high energy produces some effects that are undesirable when treating in regions, where low density heterogeneities are present. The examination of the influence of the set-up geometry, the relative position of the treated site to the low density interface and the criteria that one uses for accepting or rejecting a dose distribution, showed that the magnitude of these effects depend on the above parameters. Thus, special care should be taken in order to decide the optimal energy choice.
* Corresponding author. Miltiadis F. Tsiakalos, Medical Physics Department, University Hospital of Larissa, P.O. Box 1425, 41-110, Larissa, Hellas, Greece. E-mail address: [email protected] Received 10 May 2004; received in revised form 21 February 2006; accepted 22 February 2006
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