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| ORIGINAL ARTICLE |
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| Ahead of print publication |
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Evaluation of surface dose calculations using monaco treatment planning system in an indigenously developed head and neck phantom
Mamta Mahur1, Munendra Singh2, Manoj Kumar Semwal3, Om Prakash Gurjar4
1 Department of Radiation Oncology, Delhi State Cancer Institute, Dilshad Garden, Delhi; Department of Physics, School of Basic Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India
2 Department of Radiation Oncology, Delhi State Cancer Institute, Dilshad Garden, Delhi, India
3 Department of Radiation Oncology, Army Hospital (Research and Referral), Delhi Cantonment, New Delhi, India
4 Government Cancer Hospital, Mahatma Gandhi Memorial Medical College, Indore, India
| Date of Submission | 16-Oct-2021 |
| Date of Decision | 30-Dec-2021 |
| Date of Acceptance | 30-Dec-2021 |
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Correspondence Address:
Mamta Mahur,
Department of Physics, School of Basic Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh
India
 Source of Support: None, Conflict of Interest: None DOI: 10.4103/mjdrdypu.mjdrdypu_827_21
Objective: The objective of this study is to evaluate the surface doses for 3-dimensional conformal radiation therapy (3DCRT), intensity-modulated radiation therapy (IMRT), and volumetric-modulated arc therapy (VMAT) treatment planning techniques using an inhouse designed head and neck (HN) phantom and to compare the measured surface doses with the doses calculated using the Monaco treatment planning system (TPS). Materials and Methods: An arbitrary clinical target volume was defined with 5 mm planning target volume (PTV) expansion on computed tomography images of an in house designed heterogeneous HN phantom. 3DCRT, IMRT, and VMAT plans were created using Monaco TPS for prescribed dose of 60Gy in 30 fractions to cover 95% of PTV volume. Dose measurements were performed using EBT3 Gafchromic films at 10 selected points on the surface of HN phantom, especially inside the treatment area. Percentage mean dose differences were evaluated between the TPS calculated doses and measured dose values at these identified points. Results: The average dose difference between the TPS calculated doses and film measurements were found to be varying from 11.66% to 19.73%. It was observed that TPS overestimated the surface doses in comparison to measured doses. The results also shows that Gafchromic films can be used for surface dose measurements in patients for in vivo dosimetry in areas where high skin dose is expected during radiotherapy treatment. Conclusion: The limitations of TPS should be considered while evaluating surface doses in radiotherapy plans.
Keywords: Monaco treatment planning system, phantom, surface dosimetry
How to cite this URL:
Mahur M, Singh M, Semwal MK, Gurjar OP. Evaluation of surface dose calculations using monaco treatment planning system in an indigenously developed head and neck phantom. Med J DY Patil Vidyapeeth [Epub ahead of print] [cited 2023 Mar 20]. Available from: https://www.mjdrdypv.org/preprintarticle.asp?id=339392 |
| Introduction | |  |
In many head and neck (HN) cancer patients, tumors are located in close proximity to the skin. Hence, in order to deliver adequate prescribed dose to the target, skin sparing limits the goal. Furthermore, due to the presence of many radiosensitive organs such as eyes, lenses, optical neves, brainstem, parotids, spine, larynx, etc., in the treatment area in HN region higher surface dose may lead to serious complications on the skin such as skin erythema, telengiectasia, necrosis, epilation, and desquamation.[1],[2],[3]
Hence, in vivo measurements of surface dose on patients undergoing treatment with advanced radiotherapy techniques become important part of assuring accuracy of radiation dose delivered. In modern radiotherapy treatment techniques, different types of pretreatment patient-specific dosimetric quality assurance (QA) techniques have been introduced to ensure the accuracy. Usually verification of radiotherapy treatment plan is performed on the Linear accelerator (Linac) without patient on a homogeneous round or cuboid shaped phantoms. Dose distribution of the treatment plan and measured dose distribution delivered by Linac using such homogeneous phantoms have been compared by many authors. Furthermore, skin dose on patients and measured dose on such homogeneous phantoms have been reported by several authors.[4],[5],[6] These standard phantoms are totally different in geometric structure as compared to patients as they do not reflect the different densities of structures as found in real patients. Thus resulting dose distribution is different as these standard phantoms are different in pattern due to different dose absorption and scatter interaction compared to real patients.
Furthermore, phantoms representing human anatomy are available commercially for performing QA procedures for complex radiotherapy plans. However, these commercially available phantoms are comparatively expansive and cannot be used to adopt to clinical situation of an individual patient. However, due to recent developments in prototyping, it has become possible to fabricate such phantom with specific clinical requirements. Furthermore, accuracy of calculated dose distribution in the buildup region by treatment planning system (TPS) is still a major challenge for most of the photon dose calculation algorithms. This accuracy is largely affected due to difficulties in modelling of the dose contribution from contaminated electrons originated from collimator assembly, flattening filter and to a lesser extent, secondary scatter photons form the linear accelerator head.[1],[7],[8],[9],[10]
Variation in surface doses calculated by TPSs has been reported by various researchers in homogeneous phantoms or commercially available anthropomorphic phantoms. Dogan and Chung et al.[4],[5] have reported lesser skin dose delivered as compared to conventional techniques. Chung et al.[5] also reported the inconsistencies between doses calculated by two TPS (PINNACLE3 and CORVUS) and those measured through film dosimetry on the surface and in buildup region using an inhouse designed HN compression film phantom made up of two semi cylindrical solid water slabs. Overestimations of up to 18.5% of the prescribed dose were reported. A study by Lee et al.[1] has reported increased skin dose during treatment by intensity-modulated radiation therapy (IMRT) techniques using anthropomorphic phantom and thermoluminescent dosimeters (TLD). Akino et al.[11] measured doses with film dosimetry at 3mm depth in breast phantom and found measured doses were higher than TPS calculated doses by 15%–30%. Oinam and Singh[12] evaluated the accuracy of anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) algorithm in the buildup region for eclipse TPS and compared the corresponding measured doses using the TLD. It was reported that both AAA and PBC algorithm overestimated the doses as compared to TLD measured doses. Court et al.[13] reported that the agreement between measured skin doses using metal oxide semiconductor field effect transistor (MOSFET) and calculated dose by PBC algorithm in eclipse TPS were within ± 20%. Rijken et al.[14] evaluated the surface dose in thorax phantom calculated by Pinnacle TPS and measured with radiochromic films. Measured surface doses were as much as double to the calculated doses due to the presence of vac bag immobilization device. Qi et al.[15] reported overestimation of up to 8.5% for surface dose between TPS calculations and MOSFET measurements in an anthropomorphic HN phantom.
Studies reporting comparison of TPS calculated doses and measured skin doses in actual clinical treatment conditions are few. Monte Carlo (MC) algorithm implemented in Monaco TPS has not been tested so far to check its accuracy in the dose calculations in the buildup region in actual clinical treatment situation on an indigenous HN phantom. In this study, we evaluated the accuracy of dose calculation by TPS in surface region that is skin, on an indigenously designed HN phantom.
| Materials and Methods | | |
Fabrication of phantom
A new cost effective heterogeneous HN phantom was fabricated to study the surface doses for different modern radiotherapy techniques. The phantom was constructed using material which met the requirements of human tissue equivalence of physical properties such as attenuation coefficient and computed tomography (CT) density and aid in quick manufacturing. Various researchers have investigated the property of paraffin wax and concluded that it exhibits similar radiation absorption properties due to similarity in chemical composition, mass density (0. 9g/cm3), and number electron per gram (3.44 × 1023e/g). Therefore, different types of waxes were tested for comparing their Hounsfield units (HU) number by acquiring their CT scan images using Biograph mCT Flow systems scanner (Siemens Healthcare, Germany).
The bone structure was constructed using synthetic anatomical models. Three dimentional (3D printing technique was used to prepare the moulds and were filled in with the bone equivalent material. Air-filled organs such as larynx and pharynx were simulated using conventional airy sponge. Conventional gel was used to simulate eyes. Molten wax was poured into the moulded structure and was subsequently allowed to solidify to minimize the air bubbles. The total weight of phantom was approximately closer to that of human head. The constructed phantom have physical dimensions of height 19.7 cm, length of 23.6 cm, and width of 14.5 cm.
For further validation of the constructed phantom, CT images were acquired with 0.1 cm slice thickness at 120 kilovoltage. Normal structures such as body, spine, eyes, brain, teeths, larynx, and mandible were contoured for the evaluation of geometric accuracy of the phantom. Volume and mean density of contoured structures were compared to one of the real patient representing close proximity of physical features of the phantom.
Simulation
Phantom was set-up on CT scanner couch using immobilization aides like HN base plate and head rests to acquire CT images. Images were acquired using biograph mCT Flow systems scanner (Siemens Healthcare, Germany) from vertex of head to C3 level of phantom with 3 mm slice thickness. Three lead markers were placed on phantom which were visible in one of transverse image to set the reference point. The acquired CT images were exported to MONACO TPS version 5.11 (Elekta, Impac Medical System, Inc., USA).
Radiotherapy planning
An arbitrary clinical target volume was contoured close to the skin on the acquired CT images of the phantom. A 5 mm planning target volume (PTV) expansion was generated with clipped margin of 1 mm from the surface. Organs at risk such as eyes, brain, brainstem, parotids, mandible, and skin were contoured according to the International Commission on Radiation Units and Measurements report 62.[16] Plans with 3-dimensional conformal radiation therapy (3DCRT), IMRT with step and shoot delivery technique (step-and-shoot intensity-modulated radiotherapy [SSIMRT]), IMRT with dynamic delivery technique (dynamic intensity-modulated radiation therapy [DIMRT]), and volumetric-modulated arc therapy (VMAT) with constant dose rate technique were created using Monaco TPS for a prescribed dose of 60 Gy in 30 fractions with planning isocenter defined at the center of PTV and for a minimum of 95% coverage of the PTV using 6 MegaVolt (MV) photon beam energy for Varian Clinac 600C (Varian Medical systems Palo Alto, CA) linear accelerator. 3DCRT plans were generated using simple bilateral field arrangement and were calculated using collapsed cone (CC) and MC algorithm along with field in field compensation for dose homogeneity. Five fields (5F), seven fields (7F), and nine fields (9F) coplanar field IMRT plans were generated for two different dose delivery techniques, i.e., step and shoot and dynamic multileaf collimator delivery with equidistant gantry angles for the three beam arrangements. Single arc (SA) and double arc (DA) VMAT plans were created for constant dose rate technique. SA plan was defined in clockwise direction and DA plan was defined with two 360 arcs in clockwise and counter clockwise direction with gantry angle increment of 30° (angular spacing between sampled fluence profiles in stage 1 of optimization) were created for constant dose rate technique. IMRT and VMAT plans were calculated using MC algorithm. Planning parameters of VMAT plan for optimization were consistent as much as possible with IMRT plans. All the plans were calculated for 3mm grid spacing and for statistical uncertainty of 1% per calculation.
Calibration of EBT3 gafchromic films
EBT3 Gafchromic Films (International Specialty Product, US) have been proven to be a viable tool for dosimetry in radiotherapy.[17],[18] These films are suitable to perform surface dose measurements and measurement in buildup region due to their high resolution, thin configuration and tissue equivalent properties. The EBT3 Gafchromic films were calibrated by irradiation with 6MV photons at source to surface distance (SSD) of 100 cm for 10 cm × 10 cm field size. Films were placed at 5 cm depth in solid water phantom with 10 cm slab of solid water phantom to provide sufficient back scattering. The irradiated films were scanned after a gap period of 24h by using Epson 10,000 ×l flatbed scanner (Epson America, Inc., CA). Films were analyzed using Film QA PRO software (National Institute of Health, USA), and calibration curve was obtained.
Radiotherapy delivery and dose verification
To evaluate the surface doses for the calculated radiotherapy plans, ten measurement points were identified at grid spacing of 3 cm in three axial planes of the CT images from planning isocenter of the HN phantom. The phantom was placed and set-up using the same immobilization aids as used in CT simulation procedure on the couch of Varian Clinac 600C linear accelerator (Varian Medical Systems, CA, USA). Planned and the machine isocenter were aligned with the help of lasers and three reference points for simulating the same setup as per the CT simulation process. The positional accuracy of the setup was further verified with the help of portal images before delivering the plans.
EBT3 Gafchromic films were placed at each identified measurement position [Figure 1] and plans were delivered on the phantom. Three repeated measurements were performed for each plan. After a gap of 24 h, irradiated films were scanned using EPSON flatbed scanner and optical densities were converted to doses by using FILM QA Pro software. Doses measured using films at all the measurement positions were compared to the corresponding calculated doses obtained from Monaco TPS. For a better interpretation of the results, the percentage difference between the dose calculated by TPS (DTPS) and the dose measured (DMeas) using EBT3 films were evaluated according to the equation (1)
| Results | | |
Head and neck phantom
CT images of different types of waxes were evaluated to find out the average HU and ED values. Paraffin wax was found to be the appropriate alternative for the reproduction of soft tissue with HU for HN with values ranging from −46 HU to −62 HU and ED values ranging from 1.005 to 0.992. The suitability of paraffin wax for the reproduction of soft tissue has been investigated by Rahman et al.[19] and reported the deviation in dose absorption of 3% for 6MV in comparison to water phantom. [Figure 2] shows one of the transverse slice of CT image of the in house fabricated HN phantom [Figure 1]. HU values were evaluated for the different contoured structures using the image analysis tool of Monaco TPS. HU values obtained from the Phantom were found to be in close approximation to real patient in terms of the HU values for respective structures, as shown in [Table 1]. From the comparison of HU values, it is evident that the inhouse designed phantom depicts the cost-effective alternative of costly Rando phantom. | Table 1: Comparison of houns.eld units values of phantom and real patient
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 | Figure 2: Transverse sections of inhouse fabricated head and neck phantom
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Evaluation of surface doses
3DCRT plans calculated using CC and MC algorithm, IMRT 5F, IMRT 7F, IMRT 9F plans for SSIMRT and DIMRT delivery, VMAT SA and VMAT DA treatment plans calculated using Monaco TPS were delivered on HN phantom with EBT3 films placed on the identified measurement positions, as shown in [Figure 3]. Mean surface dose at ten measurement positions was obtained from three repeated measurements for each delivery technique using EBT3 films and are plotted in [Figure 4]. The doses obtained were evaluated and compared with the respective TPS calculated doses. [Table 2] summarizes the mean percentage difference between TPS calculated doses and corresponding films measured doses at ten measurement points. In particular, the mean percentage difference between the calculated and measured values varies from 11.97% to 18.98%, 11.66% to 18.28%, 12.55% to 19.44%, 11.67% to 18.79%, 13.17% to 18.17%, 12.12% to 18.86%, 12.79% to 19.54%, 15.44% to 19.73%, 15.20% to 18.56% and 13.22 to 18.28% for VMAT SA, VMAT DA, SSIMRT 5F, SSIMRT 7F, SSIMRT 9F, DIMRT 5F, DIMRT 7F, DIMRT 9F, 3DCRT CC and 3DCRT MC plans respectively. Average variation in percentage mean doses difference between TPS calculated and measured dose using films were found to be 15.91%, 14.69%, 16.20%, 16.12%, 15.62% 15.75%, 16.09%, 16.76%, 16.72%, and 15.36% for VMAT SA, VMAT DA, SSIMRT 5F, SSIMRT 7F, SSIMRT 9F, DIMRT 5F, DIMRT 7F, DIMRT 9F, 3DCRT CC, and 3DCRT MC plans, respectively. In particular, it was observed that point of interest lying in the area of the skin near PTV that is P2 showed a large difference error of 19.73%. | Figure 4: Mean surface dose values at ten measurement points for different dose delivery techniques
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 | Table 2: Mean percentage difference between treatment planning system calculated doses and measured film doses
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| Discussion | | |
In this study, we evaluated the accuracy of dose calculation in the surface region that is in skin region by Monaco TPS on an indigenously designed HN phantom. The HN phantom was designed to represent approximate adaptation to a patient individual clinical situation like to address dosimetric issues related to the presence of dental impants, maxillofacial prosthetics or nonunimormity of skin caused due to surgery. The inhouse designed phantom also met the requirements of human tissue equivalence and proved to be an easy and cost-effective method of verifying in vivo dose measurement applications in actual clinical conditions. In this study, minor problems were encountered regarding filling of phantom with wax to minimize air bubbles. In conclusion, the new cost effective in house designed tissue equivalent phantom can be used for patient-specific dosimetry QA.
Surface dose comparison was performed at various points of interests identified on the phantom and CT images for treatment plans created using VMAT, SSIMRT, DIMRT, and 3DCRT delivery techniques. The surface dose in VMAT plans were found to be slightly lower than those of IMRT plans which shows that VMAT has better skin sparing capability.
According to the considered points of interest, the obtained difference between measured and calculated dose values for different dose delivery techniques ranges from 11.66% to 19.73%. It is evident from [Figure 4], that all the measurements are within the range of ± 20%, which corresponds to the acceptance value as recommended by the AAPM-TG 53[20] for the dose calculation in buildup region by TPS. In general, points located in steep dose gradient presented large dose differences for all the delivery techniques. In general results obtained in this study are in agreement with the previous reported studies and recommendations of American Association of Physicists in Medicine (AAPM).
Furthermore, it was observed that CC algorithm overestimated the dose in comparison to calculated values from MC algorithm in case of 3DCRT plan. As MC algorithm uses statistical method to simulate the tracks of individual particles using probability distribution. Hence, MC algorithms are considered as the gold standard for dose computation in radiation therapy. However, in Monaco TPS CC algorithm is recommended for 3DCRT plan calculations as wedges and blocks are not configured during the beam data commissioning.[21]
| Conclusion | | |
Accurate knowledge of surface dose calculations by TPS at every radiation treatment facility center allows for a sensible comparison of skin dose between patients and plans. Compared to the previous phantom studies, this study provided in vivo surface dose dosimetry in clinical situation using an indigenously designed HN phantom. The major limitation of this study was relatively small sample size and nonrandomized design. From the measurements, the difference was observed to be within the acceptable limits of ±20%, the value referred in the AAPM-TG 53 for the TPS calculation imprecision in the buildup region.
Acknowledgment
The work has been supported by Delhi State Cancer Institute.
Financial support and sponsorship
This study was financially supported by Equipment support from Department of Radiotherapy, Delhi State Cancer Institute (East), Dilshad Garden, Delhi.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]
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