|Year : 2019 | Volume
| Issue : 8 | Page : 110-114
Effect of scattering and differential attenuation on beam profile in the presence of high-density intensity modifying compensator
Sandeep Kaushik1, Rajesh Punia2, Atul Tyagi3, Anand Malik4
1 Department of Physics, Guru Jambheshwar University of Science and Technology, Hisar, India
2 Department of Physics, Guru Jambheshwar University of Science and Technology, Hisar; Department of Physics, Maharshi Dayanand University, Rohtak, India
3 Department of Radiation Oncology, BLK Super Speciality Hospital, New Delhi, India
4 Department of Physics, Chaudhary Bansilal University, Bhiwani, Haryana, India
|Date of Web Publication||22-Mar-2019|
Dr. Rajesh Punia
Department of Physics, Maharshi Dayanand University, Rohtak - 124 001, Haryana
Source of Support: None, Conflict of Interest: None
Aim: The aim of this study is to investigate the effect of scattering and differential attenuation on dose profile of 6 MV photon beam in the presence of cadmium (Cd)-free compensator which has been used in compensator-based intensity-modulated radiotherapy.
Materials and Methods: Totally, 10 slabs of Cd-free compensator having thicknesses ranging from 2.4 to 61.4 mm have been prepared. Dose profiles have been taken using computer-controlled radiation field analyzer for five field sizes from 30 mm × 30 mm to 200 mm × 200 mm and at three depths in water phantom. Off-axis dose variation (ODV) has been measured with off-axis percentage depth dose scan and with ion chamber by measuring point dose at two diagonal points with respect to dose at central axis point in a plane and at three depths.
Results: A decrease in beam flatness has been observed with increase in compensator thickness and depth in phantom. ODV has been found to increase with compensator thickness. Selective beam hardening has been observed due to differential attenuation from compensator. Point dose measurements show approximately 20% and 23% underdose region at 70 and 106 mm off-axis diagonal point, respectively, as compared to dose at central axis point for a field size of 200 mm × 200 mm at a depth of 15 mm, with 30.2-mm slab thickness. Significant increase in scattered penumbra has been observed with field size and thickness of compensator due to increase in scattered photon.
Conclusions: The presence of compensator changes photon beam mean energy along the cross-section resulted in decreased beam flatness and increased scattering. This may lead to overestimation of dose along off-axis within radiation field if change in flatness is not taken into account and more exposure to healthy tissues in penumbral region due to large-angle scattering.
Keywords: Beam flatness, compensator, differential attenuation, penumbra, scattering
|How to cite this article:|
Kaushik S, Punia R, Tyagi A, Malik A. Effect of scattering and differential attenuation on beam profile in the presence of high-density intensity modifying compensator. J Can Res Ther 2019;15, Suppl S1:110-4
|How to cite this URL:|
Kaushik S, Punia R, Tyagi A, Malik A. Effect of scattering and differential attenuation on beam profile in the presence of high-density intensity modifying compensator. J Can Res Ther [serial online] 2019 [cited 2020 Oct 26];15:110-4. Available from: https://www.cancerjournal.net/text.asp?2019/15/8/110/244228
| > Introduction|| |
In medical linear accelerator (LINAC), an electron beam after passing through target get converted into photon beam, which is peaked in forward direction. Photon beam was then made to pass through a flattening filter, which converts it into a flat beam throughout the cross-section. Flattening filters are conical in shape, i.e., thicker at center and then thickness decreases away from center. As photon beam is not monoenergetic, these conical shaped flattening filters cause selective beam hardening. Transmitted photon beam thus produced is comprised of the hard component in central/axial region with higher average beam energy and softer component in peripheral region with comparatively lower average beam energy., During radiotherapy of the patient for treatment, intensity-modulated radiotherapy (IMRT) profiles are produced either with the help of multileaf collimator or compensator.,, Compensators are the physical beam modulators. These compensators further modify the beam characteristics and average beam energy, i.e., mean energy. It also causes beam perturbation which results in scattering, beam hardening, and attenuation., In flattened beam, generally percentage depth doses (PPDs) are taken along the central axis of beam considering the beam as flat. However, compensator-attenuated beams could become un-flat and produce more scattered photons, thus altering the dose statistics along the beam cross-section. These changes due to attenuation and scattering of photon could lead to overestimation and underestimation of dose along beam cross-section. Hence, disregarding the change in beam flatness due to differential attenuation could cause imprecise dosimetry. Limited studies have been done so far to quantify the effect of differential attenuation due to high-density intensity-modulating compensator alloy. To see the impact of attenuation and scattering along the beam cross-section in the presence of compensator, the study has been designed to quantify the change in beam flatness and off-axis dose variation (ODV) within the field and in penumbral region using high-density cadmium (Cd)-free compensator alloy.
| > Materials and Methods|| |
Compensator slabs of Cd-free compensator alloy with a certain known percentage by weight composition of lead (Pb), tin (Sn), and bismuth (Bi) were prepared. The density of the alloy was found to be 9.083 g/cc. A total of 10 slabs having uniform thicknesses 2.4, 3.3, 4.3, 7.1, 10.0, 19.8, 30.2, 41.6, 50.0, and 61.4 mm throughout the cross-section of area approximately 20 cm × 20 cm were cast. Digital Vernier caliper (Aerospace Digital Caliper) was used to measure the slab thicknesses. Six megavolt beam energy of Varian Trilogy Tx LINAC (Varian Medical Systems, Palo Alto, CA) has been used for relative dose measurements to find out beam flatness and beam penumbra. Relative dose measurements were carried out with CC13S field and reference ion chambers (IBA Dosimetry, Germany) having a sensitive volume of 0.13 cm3. Point dose measurements have been taken using CC13 ionization chamber (IBA Dosimetry) having a volume 0.13 cm3. Compensator slabs were placed on a perspex tray in a slot of collimator accessory mount on LINAC head. All measurements were taken with zero gantry and collimator angle. A total of five field sizes (A) 30 mm × 30 mm, 50 mm × 50 mm, 100 mm × 100 mm, 150 mm × 150 mm, and 200 mm × 200 mm have been analyzed for beam flatness and penumbra changes at 15 (depth of dose maximum, Dmax), 50, and 100 mm depths in water.
Beam flatness and beam penumbra
Schematic diagram representing a different portion of a beam profile is shown in [Figure 1]. Radiation field is defined as full width at half maximum (FWHM), i.e., distance between 50% dose values on both sides of beam profile. The value of penumbra is the average value of the distance between 20% and 80% dose value on either side of beam profile.
Beam flatness has been calculated by relation:
where Dmax is the dose maximum and Dmin is the dose minimum in central 80% area of FWHM.
Beam profiles were taken with radiation field analyzer, RFA300 (IBA Dosimetry, Germany) with Omnipro Accept® v7.0 software (IBA Dosimetry, Germany). Dose profiles scan were performed at 15, 50, and 100 mm depths in water for open and compensated field. All profiles have been measured for different field sizes and thickness of compensator.
ODV has been measured by finding the percentage variation of dose at an off-axis point P (50 mm, 50 mm) or P' (75 mm, 75 mm) [Figure 2] in the radiation field with respect to dose at a point on central axis P0 (0 mm, 0 mm) at a given depth for a field size of 200 mm × 200 mm and in a plane perpendicular to the beam axis.
where DP and is the dose measured at off-axis pointPand P', respectively, and DPo is the dose measured at point P0 on the central axis at the same plane. ODV measurements have been done for compensator thicknesses of 19.8, 30.2, 41.6, 50, and 61.4 mm.
| > Results and Discussion|| |
Change in beam flatness with field size and thickness of compensator at three depths have been tabulated and listed in [Table 1]. It has been observed that beam flatness decreases with increase in thickness of compensator. This deviation in beam flatness is due to beam perturbation caused by the presence of compensator. It shows that scattering and differential attenuation of beam along the cross-section increases with increase in thickness of compensator. Underflattening of compensated beam profile is a result of further selective beam hardening due to compensator. This beam hardening is more in peripheral region due to lower beam mean energy and less in the central region due to higher beam mean energy.
|Table 1: Change in beam flatness with thickness of compensator for different field size at different depth in water|
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Further, it has been observed that flatness decreases at higher depth in water phantom. It is due to phantom attenuation causing selective beam hardening. With respect to clinical use of high-density compensator, there is a need of correction factor for accounting change in flatness. This decrease in un-flatness is very high with large compensator thickness (>20 mm). Tailor et al. presented a formula based on half value layer values to calculate an off-axis energy-correction factor for any clinical LINAC photon beam for open beam. In a similar way, the correction factor for change in flatness should be taken into account in absolute dosimetry, especially for large field sizes and thick compensators.
Off-axis dose variation
ODVs at three depths with different thickness of compensator have been presented in [Table 2]. Here, negative sign is due to underdose and positive sign is due to overdose. Perusing the data listed in [Table 2], it has been observed that ODV is not linear with compensator thickness. Dose variation increases suddenly for compensator thickness >19.8 mm. This variation may be due to cutoff of soft component of X-ray by this range of slab thickness. Beyond compensator thickness >30 mm, the variation is negligible. It may be due to hardening of X-ray photon beam. At a depth of 15 mm, compensator slab of thickness 30.2 mm cause approximately 20% underdose region at an off-axis point P (50 mm, 50 mm) and 23% at P' (75 mm, 75 mm) if change in flatness is ignored. This variation changes negligibly at higher depth in phantom. [Figure 3] shows the comparison of PPD along the central axis and PDD through off-axis point P and P' for a field size of 200 mm × 200 mm in the presence of compensator thickness of 30.2 mm. It is clear from PDD curve that there is a significant difference between central axis PDD and off-axis PDD [Figure 3]. Such a deviation may cause large uncertainty in dose estimation if differential beam hardening is ignored along the cross-section. As stated in earlier section, this variation is due to differential attenuation of flat photon beam when it passes through compensator. Hence, neglecting the spectral variation along the beam cross-section and considering the beam as flat in the presence of uniform large thickness high-density compensator may cause non-uniform dose in target or target may get underdose. Modern treatment planning systems may take it into consideration, provided the accurate characterization of high-density compensator alloy has been done.
|Table 2: Percentage off-axis dose variation verses thickness of compensator at different depths in phantom for a field size of 200×200 mm2|
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|Figure 3: PDD through beam central axis and off-axis points P and P' for compensator thickness (t) 30.2 mm and field size 200×200 mm2|
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Beam penumbra (P)
The physical penumbra is a function of beam energy, source to surface distance, source size, source to collimator distance, and depth in phantom. The change in beam penumbra with compensator thickness for a number of field sizes is shown in [Figure 4]. It has been observed that penumbral width increases with increase in thickness of compensator alloy and field size. Increase in beam penumbra may also be due to beam perturbation in the presence of compensator. This beam perturbation produces the amount of scattered photons. Increase in width of penumbral region is due to scattered photon produced from compensator. In the presence of compensator, scattered photon produces large deviation in dose on central axis as stated by Islam and Van Dyk. For small field size (30 mm × 30 mm and 50 mm × 50 mm), the effect is less significant due to smaller area of compensator in the field and hence lesser scattered component in beam, whereas the effect is greatest with highest compensator thickness (t = 61.4 mm) and largest field size (200 mm × 200 mm) investigated, because of maximum scattering condition. In the presence of compensator, the increase in penumbral width is attributed to scatter penumbra. At higher depth in phantom, penumbra is relatively larger due to the addition of phantom scatter and increased source to detector distance [Figure 5]. Hence, large angular spread of scattered photon from compensator and phantom increases low-dose volume in penumbral region. As the component of penumbral width (from 50% to 20% dose value) is not a part of radiation defining field size, therefore, increase in penumbral region results in unnecessarily exposure to healthy tissues. This low-dose region in IMRT may become the site for secondary malignancy in patients with long survival., Kase et al. showed that secondary radiation contributes very little dose, but beyond 60 cm from the central axis, it becomes the dominant component.
|Figure 4: Change in penumbra (p) with thickness (t) of compensator for different field sizes (A) at dmax|
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|Figure 5: Change in penumbra (p) with thickness (t) of compensator for 200×200 mm2 field size at different depths|
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| > Conclusions|| |
The flatness of photon beams is very sensitive to change in photon beam mean energy. The presence of uniform compensator thickness causes differential attenuation along the beam cross-section, which results in ODV due to decrease in beam flatness. This deviation could lead to non-uniform dose distribution or underdose in target if a change in flatness is ignored. Underflat beam profile is a concern of precise dosimetry for higher thickness of Cd-free compensator alloy. Large-angle scattered photon from compensator increases penumbral width, causing exposure to healthy tissues. Hence, understanding of physical parameters that influence the dosimetric properties of clinical photon beam and its accurate characterization in planning system is needed.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]