Depth dose measurement by using Al2O3 OSL dosimeters in high energy photons in the presence of air cavity and density inhomogeneity

Air cavities and tissue density inhomogeneity significantly affects the distribution of radiation doses, potentially resulting in adverse consequences such as cancer recurrence. This research aims to assess the accuracy of Al2O3 optically stimulated luminescence (OSL) dosimeters in measuring doses within varying thicknesses of air cavities (3, 5, and 8 cm) and tissue inhomogeneity of low and high density simulated by the lung and bone phantoms. An expanded polystyrene (EPS) was employed in homogeneous solid water® phantoms to simulate the air cavity. The percentage depth-dose (PDD) curves at 6 MV photons were obtained in both presence of air cavity and density inhomogeneity and compared to that in the EBT3 radiochromic film dosimeters and treatment planning system (TPS). The results indicated that the presence of an air cavity and tissue inhomogeneity affected the depth dose measured in OSL dosimeters, EBT3 films and TPS. OSLD and TPS showed good agreement at the centre of the cavity, which is within ±5% but could not estimate scattered radiation to the distal and proximal surfaces of the air cavity. The obtained p-values showed no significant differences of dose measured in OSL dosimeters compared to those in EBT3 films and TPS. The Kruskal Wallis test and Mann-Whitney showed no significant difference between OSL dosimeters, EBT3 film and TPS in the measurement of depth doses in the presence of density inhomogeneity. The overall results indicated the suitability of OSL dosimeters as indirect dosimeters for the measurements of depth dose in the presence of air cavity and tissue density inhomogeneity


Introduction
Dosimetry involves measuring and calculating the radiation dose received by a patient during treatment for ensuring accurate treatment dose delivery involving high energy photons.The ability of the dosimeter to accurately and efficiently measure the dose deposited in its effective volume is the main factor determining its applicability in dosimetry [1].It is prominence to ensure the planned dose are accurately delivered to region of interest as it may increase the probability of secondary malignancy risk due to unnecessary doses delivered to the patients.
The human body consists of inhomogeneous density which refers to the high-density tissues such as bones and low-density tissue such as lung tissues [2].The density differences have different physical and radiological characteristics giving significant impact of dose distributions.In addition, our body has a lot of heterogeneous interfaces such as lung-tissue and bone-tissue interfaces along the path of radiation beam causes increased or decreased transmission depending on the density and atomic number of the inhomogeneity medium [3].
The air cavities exist within the human body include the sinuses, the tracheobronchial tree, the oral cavity, the pharynx, and the nasal cavity.Additionally, the use of certain accessories like a mouth bite with an air tube for breathing purposes can also form large air cavities [4].The presence of air cavities in the human body can significantly impact the distribution of high-energy radiation beams.This is IOP Publishing doi:10.1088/1757-899X/1308/1/012011 2 due to the creation of electronic disequilibrium conditions near the interfaces between air and tissues, leading to insufficient doses at both the proximal and distal air cavity interfaces [5].
The Al2O3 optically stimulated luminescent (OSL) dosimeters have been extensively used as indirect dosimeter for the dosimetry works in radiotherapy and showed excellent agreement of dose readings involving high energy photons and electrons [6,7].The current study focused on the dose measurements and to study the effectiveness of OSL dosimeters for dose measurements in in the medium on high energy photons with the presence of density inhomogeneity and air cavity.The OSL NanoDot®s was used and the depth doses in mediums with the presence of density inhomogeneity and air cavities were measured.

Depth dose measurements in inhomogeneous density medium
The inhomogeneous density medium was prepared with the simulations of lung and bone tissues with the solid water phantoms.The lung was simulated by using cork slabs with average density of 0.24 g/cm 3 while the compact bone phantom with density of 1.85 g/cm 3 was used to simulate the bones.The lung and bone phantoms were sandwiched between the solid water phantoms to simulate the presence of low and high density mediums in solid water phantoms respectively as shown in Figure 1(a) and 1(b).The third set up of solid water, lung and bone phantoms are made to simulate the presence of all soft tissue, low and high density tissues at the same time as shown in Figure 1(c).The phantoms were irradiated to 6 MV photons with 300 cGy prescribed dose at 10 × 10 cm field size and 100 cm source to surface distance (SSD).Three sets of depth dose measurements were made on solid water-lung, solid water-bone and solid water-lung-bone setups with a bolus used to substitute solid water phantom at the locations of the OSL dosimeters to eliminate the air gaps.The depth doses were measured in several locations within the mediums including the interface regions (proximal and distal surfaces) and the interface between lung and bone.

Depth dose measurement in the presence of air cavity
An extended polystyrene (EP) with the density of 0.01 g/cm3 was used to simulate the air cavity [8].
The EP was sandwiched between the solid water phantoms with various thicknesses between 3and 8 cm as shown in Figure 2. The air cavities were simulated at thicknesses of 3, 5 and 8 cm.The phantoms were irradiated to 6 MV photons with 300 cGy prescribed dose at 10 × 10 cm field size and 100 cm source to surface distance (SSD).The depth doses were measured in several locations within the mediums including the interface regions (proximal and distal surfaces) between solid water phantoms and EP as well as within the EP.

Simulation by using treatment planning system
The phantoms of inhomogeneous density and the presence of air cavities were scanned by using a computed tomography (CT) scanner and determined of their depth doses by using a treatment planning system (TPS).The depth doses of the similar points to the OSL dosimeters were determined by using the Oncentra 4.3 TPS software used for the treatment planning for radiotherapy.All the depth doses measured in the OSL dosimeters were plotted and compared to those in the EBT3 film dosimeters and TPS.

Results and discussion
Figure 3 illustrated the PDD curve by using OSL dosimeters in comparison to the EBT3 film and TPS in solid water-lung, solid water bone and solid water-bone-lung respectively.The presence of lower and higher density tissue simulated by the lung and bone phantoms significantly altered the PDD curve in 6 MV photons compared to the homogeneous density solid water phantoms [9,10].The dose measured by the OSL dosimeters were in good agreement with the EBT3 film dosimeters with maximum percentage differences within 2.5% in all similar depths of measurements.The density inhomogeneity did not significantly affect the depth doses in TPS compared to the homogenous density phantoms.The depth doses by the OSL dosimeters were significantly lower in the lung-bone mediums compared to the EBT3 films although the two dosimeters showed similar trend of dose perturbations.Table 1-3 summarized the depth dose measured by the OSL dosimeters, EBT3 film dosimeters and TPS of the similar depths and locations in solid water-lung, solid water-bone and solid water-bone-lung phantoms.Figure 4 illustrated the PDD curve of OSL dosimeters in comparison to EBT3 film and TPS in in the presence of air cavities of 3, 5 and 8 cm thicknesses.The presence of air cavities significantly reduced the depth dose in all air cavity thicknesses [11].The depth dose in OSL dosimeters showed good agreement with EBT3 film dosimeters at all air cavity thicknesses.The depth dose in OSL dosimeters however were overestimated compared to the EBT with the maximum percentage of discrepancies of 15.46% when the thickness of air cavities increased.This is due to the depth dose measured in EBT3 films declined more significant compared to that in OSL dosimeters when the air cavities increased.It was observed also that the depth dose measured in OSL and EBT3 film dosimeters were not in agreement to the TPS declining of depth dose in TPS becoming less at increased air cavity thicknesses.Table 4 summarized the percentage dose measured by using OSL dosimeters, EBT3 films and TPS at similar depths with the presence of air cavities with thicknesses of 3, 5 and 8 cm.

Conclusion
The measurement of PDD showed good agreement of dose reading by using OSL dosimeters to EBT3 film dosimeters in inhomogeneous density medium and the presence of air cavity.The thickness of air cavities did not significantly change the sensitivity of OSL dosimeters.The overall results indicated the suitability of OSL NanoDots® to be used as inactive or indirect dosimeter for measuring the dose in mediums in the presence of inhomogeneous densities and air cavity.The overestimation of dosimetry by OSL dosimeters to the EBT3 films in air cavities shall be investigated in the future work to understand the dosimety behavior of OSL for in-air dosimetry.

Figure 1 .
Figure 1.The preparations of inhomogeneous density phantoms of (a) solid water-lung phantom (b) solid water-bone phantom and (c) solid water-lung phantom-bone phantom.

Figure 2 .
Figure 2. The phantom setup for the presence of air cavity simulated by using extended polystyrene (EP) with thicknesses between 3 and 8 cm.

Figure 3 .
Figure 3.The PDD in OSL dosimeters in comparison to the EBT3 film and TPS in (a) solid waterlung (b) solid water-bone and (c) solid water-lung-bone.

Figure 4 .
Figure 4.The PDD in OSL dosimeters in comparison to the EBT3 film and TPS in solid water-air cavity of (a) 3 cm (b) 5 cm and (c) 8 cm thicknesses.

Table 1 .
The depth dose measured in the OSL dosimeters, EBT3 film dosimeters and TPS at the similar depths in solid water-lung phantoms.

Table 2 .
The depth dose measured in the OSL dosimeters, EBT3 film dosimeters and TPS at the similar depths in solid water-bone phantoms.

Table 3 .
The depth dose measured in the OSL dosimeters, EBT3 film dosimeters and TPS at the similar depths in solid water-bone-lung phantoms.

Table 4 .
The depth dose measured in the OSL dosimeters, EBT3 film dosimeters and TPS at the similar depths in the presence of air cavities of different thicknesses.