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ORIGINAL ARTICLE
Year : 2017  |  Volume : 13  |  Issue : 1  |  Page : 122-130

Dosimetric comparison of head and neck cancer patients planned with multivendor volumetric modulated arc therapy technology


1 Department of Radiation Oncology, Yashoda Hospitals, Hyderabad, Telangana; Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu, India
2 Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu; Department of Radiotherapy, All India Institute of Medical Sciences, New Delhi, India
3 Department of Radiation Oncology, Yashoda Hospitals, Hyderabad, Telangana, India

Date of Web Publication16-May-2017

Correspondence Address:
Vellaiyan Subramani
All India Institute of Medical Sciences, Ansari Nagar (East), New Delhi - 110 029
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.203600

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 > Abstract 

Aim: Purpose of this study is to dosimetrically compare head and neck (H and N) cancer patients planned with multivendor volumetric modulated arc therapy (VMAT) technology. VMAT treatment planning can be done using biological (treatment planning system [TPSB]: Monaco) or physical (TPSP: Eclipse)-based cost function optimization techniques. Planning and dosimetric comparisons were done in both techniques for H and N cases.
Materials and Methods: Twenty H and N patients were retrospectively selected for this study. VMAT plans were generated using TPSP (V11.0) and TPSB (V3.0) TPS. A total dose of 66 Gy (planning target volume 1 [PTV1]) and 60 Gy (PTV2) were prescribed to primary and nodal target volumes. Clinical planning objectives were achieved by both the optimization techniques. Dosimetric parameters were calculated for PTVs, and quantitative analyses were performed for critical organs. Monitor units were compared between two TPSs, and gamma analysis was performed between I'matriXX measured and TPS calculated.
Results: Clinically, acceptable VMAT plans showed comparable dose distributions between TPSB and TPSP optimization techniques. Comparison of mean dose, homogeneity index, and conformity index for PTV1 showed no statistical difference (P - 0.922, 0.096, and 0.097); however, in PTV2 statistically significant difference was observed (P - 0.024, 0.008, and 0.002) between TPSB and TPSP. TPSB optimization showed statistically significant superiority for spinal cord and brainstem (D1% P - 0.0078, 0.00002) whereas improved parotid sparing was observed in TPSP optimization (mean dose P - 0.00205). Gamma analysis illustrated that both systems could produce clinically deliverable plans.
Conclusion: VMAT plans by TPSP and TPSB offered clinically acceptable dose distributions. TPSB-based optimization showed enhanced sparing of serial organs whereas TPSP-based optimization showed superior sparing of parallel organs.

Keywords: Eclipse, head and neck, Monaco, volumetric modulated arc therapy


How to cite this article:
Kathirvel M, Subramani V, Subramanian V S, Swamy ST, Arun G, Kala S. Dosimetric comparison of head and neck cancer patients planned with multivendor volumetric modulated arc therapy technology. J Can Res Ther 2017;13:122-30

How to cite this URL:
Kathirvel M, Subramani V, Subramanian V S, Swamy ST, Arun G, Kala S. Dosimetric comparison of head and neck cancer patients planned with multivendor volumetric modulated arc therapy technology. J Can Res Ther [serial online] 2017 [cited 2019 Nov 22];13:122-30. Available from: http://www.cancerjournal.net/text.asp?2017/13/1/122/203600


 > Introduction Top


Squamous cell carcinoma of the tongue, hypopharynx, larynx, and tonsil are the most common cancer arising in the head and neck (H and N) region. Hypopharyngeal and larynx cancer usually does not give rise to symptoms until late in the course of the disease. For these reasons, squamous cell carcinoma of the hypopharynx and larynx is a highly malignant disease with a generally poor prognosis as well as the high incidence of early metastasis, and the survival rate is possibly the lowest of all cancer sites in the H and N. Treatment options available for carcinoma of tongue, hypopharynx, and tonsil are surgery, chemotherapy, and radiotherapy. Except for very early stage (T1) cancers of this region, treatment has primarily been surgery. Some early stage (T1 and T2), low-volume, exophytic pyriform sinus carcinomas have been successfully treated with radiation alone.[1],[2],[3] Single-modality therapy for advanced stage hypopharyngeal cancer, with either surgery or radiation therapy, has resulted in consistently poor survival.[4],[5],[6],[7] Combined-modality treatments are considered to be best for patients who present with Stage III or Stage IV disease.[2],[8] Neoadjuvant chemotherapy and radiation therapy may increase the chance for local control in selected advanced presentations to a level approaching that of resection.[2] Combined chemotherapy and radiation therapy may offer better tumor control with organ preservation than radiation therapy alone.[9],[10],[11]

Radiotherapy has been considered a major treatment modality which is combined with surgery or chemotherapy for advanced stage H and N cancers. Radiotherapy can be administered very safely using modern treatment techniques such as intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) with simultaneous integrated boost (SIB). VMAT treatment technique was introduced in 2007 by Varian Medical Systems (RapidArc, Palo Alto, CA, USA) and Elekta (Elekta AB, Stockholm, Sweden). VMAT is a radiation therapy technique that allows delivering more conformal doses to tumor in a very short time. VMAT delivers a continuous intensity modulated beam of radiation as the linear accelerator rotates in varying gantry speed, different dose rate, and changing multileaf collimator (MLC) position. Conformal dose distribution to tumor will spare surrounding healthy tissue and reduce unwanted dose to organs and nearby critical structures.[12],[13] The VMAT technique produces plan quality and dose distributions that are comparable and often superior to those of fixed-field step-and-shoot or sliding window IMRT for a wide range of disease sites.[1],[4],[14] Moreover, the advantage of the VMAT technique is the improved efficiency of the treatment in terms of better target coverage with sparing organs at risks (OARs) as well as significant reduction of the number of monitor units (MUs) and the shorter delivery time.[1],[2],[3]

Eclipse (Varian Medical Systems, Palo Alto, CA, USA) treatment planning system uses physical (TPSP)-based cost function during VMAT plan optimization to achieve clinically acceptable plans. Physical constraints method to represent dose effects in parallel critical structures is to use dose-volume histogram (DVH) constraints, which takes into consideration volume dependence relatively. The use of multiple DVH constraints may also be indicated in some cases (Carol et al. 1997), and DVH constraints can be used for the target volume and serial critical structures (spinal cord, brainstem, etc.,) as well.[15] The advantage of physical criteria, such as dose-volume criteria, is that, because they are simply and clearly defined, they can be used easily in clinical protocols.

Monaco (Elekta AB, Stockholm, Sweden) TPS uses biological (TPSB)-based optimization which offers three biological cost functions noted as Poisson statistics cell kill model, serial complication model, and parallel complication model. The Poisson cell kill model has been made a mandatory cost function for targets. Biological constraints for OARs are provided by serial complication model which uses equivalent uniform dose (EUD) objective to control maximum organ dose effectively and as for parallel OARs constraints, parallel complication model is used which uses mean organ damage constraint to control mean organ dose effectively.[6]

This study was initiated to compare two different types of optimization and dose calculation algorithms for the reason that, any patient present with H and N cancer has full freedom to choose desired hospital for radiotherapy treatment. Although the aim of any hospital is to provide best treatment, they need not have the same arrangement of equipment such as TPSP with Varian linear accelerator or TPSB with Elekta linear accelerator for treatment. In our study, we attempt to compare if same set of patients were planned into two different scenarios what will be the best possible dosimetric outcome of VMAT plans offered from two different set of equipment.

For this study, the planning goal has been set in such a way to deliver prescribed dose to planning target volume (PTV) as well as normal tissue doses to be kept within the tolerance doses. Dosimetric analyses were performed for targets and critical organs. Results were analyzed statistically to find out the effectiveness of two treatment planning techniques. Very limited papers have been published in comparison between TPSB and TPSP in the treatment of head and cancers. The purpose of this study is to find out the efficiency of TPSB and TPSP dose optimization algorithm in VMAT treatment planning for the treatment of squamous cell carcinoma in H and N.


 > Materials and Methods Top


Patient selection and preparation

Twenty advanced carcinoma of H and N patients were retrospectively selected from the pool of treated patients in this study. Cases include carcinoma of the tongue, hypopharynx, larynx, and tonsil. These patients presented with advanced Stage III disease, no distant metastases or previous H and N cancers. All patients were immobilized in the supine position with arms by side using 5-clamp H and N thermoplastic masks. Three-millimeter slice thicknesses of plain and intravenous (IV) contrast computed tomography (CT) scans were taken on a Biograph 16 Slice positron emission tomography (PET)-CT scanner (Siemens Medical Systems Concord, CA, USA). All twenty cases underwent PET scan. PET imaging typically using 18F-fluorodeoxyglucose can provide data on metabolically active tumor volumes. There is strong evidence that PET imaging is valuable in detecting distant metastatic disease and is better than conventional CT imaging.[16] Images were transferred via Digital Imaging and Communications in Medicine (DICOM) network to TPSP workstations for contouring. Target volume and critical structure were contoured on plain CT with the help of IV-contrast CT and PET-CT in TPSP.

Contouring and dose prescription

Target and OARs delineation was performed based on the International Commission on Radiation Units and Measurements (ICRU-83) guidelines. The gross tumor volume (GTV) was defined as the initial extent of gross tumor and involved lymph nodes, based on clinical examination and imaging at presentation. To generate PTV1, 5–7 mm margin was added around GTV. The PTV1 was subsequently modified in such a manner that it did not extend beyond the skin or encroach on the spinal cord to avoid overdose. Subclinical high-risk nodal volumes defined as PTV2 for prophylactic treatment. OARs such as spinal cord, brainstem, the bilateral parotids, and oral cavity were outlined. SIB technique was adopted for all cases. In this study, as all selected cases were presented with advanced stage, maximum dose prescribed to PTV1 was 66 Gy and to PTV2 was 60 Gy in 33 fractions. PTV1 receives radiation dose of 2 Gy/fraction while PTV2 receives 1.82 Gy/fraction (5 fractions/week).

Treatment planning and delivery

VMAT treatment planning was performed using 6 MV photon beam in TPSP (V11). Plans contained two arcs ranging from 181° to 179° in clockwise and counterclockwise direction with a collimator angle 345° and 15° respectively. VMAT plans were optimized using progressive resolution optimizer-III, and final dose calculations were performed using analytical anisotropic algorithm (AAA) with 2.5 mm grid size resolution. Treatment plans were created in such a way to achieve at least 95% of PTV volume (D95) receives 100% of prescription dose, and 2% of PTV volume (D2) receives not more than 107% of prescribed dose. Critical organs doses were kept as low as possible at the same time not exceeding tolerance limit doses as shown in [Table 1]. Treatment was performed using 6 MV photon beam from dual energy Clinac iX (Varian Medical Systems, Palo Alto, USA) which is equipped with sixty pairs millennium 120 MLC (5 mm width MLC at center 20 cm and 1 cm width MLC on outer sides), maximum leaf speed of 2.5 cm/s, maximum gantry speed of 5.54°/s, and maximum dose rate of 600 MU/min.
Table 1: Planning objectives for organs at risk and target volumes

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To compare the planning quality, all twenty patients CT scan images and structure sets were exported to TPSB from TPSP through DICOM transfer. Planner had full freedom to achieve best possible plan for each cases, acceptable all final plans should meet clinically target dose coverage and OAR tolerance criteria. Planning was performed using 6 MV photon beam in TPSB (V3.0). Plans created using two full 360° arc gantry rotation, and collimator angle of 3°. VMAT plans were optimized with the beamlet width of 0.4 cm. The first stage of the optimization was performed with fast pencil beam algorithm, whereas final dose calculation was performed on 3 mm grid with Monte Carlo-based algorithm (3% variance per se gment). Treatment planning was performed using clinically acceptable criteria as similar to Eclipse plans. The Elekta-Infinity (Elekta AB, Stockholm, Sweden) linear accelerator is equipped with forty leaf pairs MLC with 1 cm width, maximum leaf speed of 2.5 cm/s, maximum gantry speed of 6°/s, and maximum dose rate up to 500 MU/min. The intent of study is to compare H and N VMAT planning and delivery in multivendor environment; therefore in our study, both TPS are independent in terms of beam data and machine configuration.

Plan evaluation index

Plans were qualitatively assessed as preliminary verification by visual assessment of plan constraint and dose-color washes in CT axial, coronal, and sagittal views to eliminate major errors in the plans. The second level of plan evaluation was performed in terms of quantitative analysis using DVH to define target dose homogeneity index (HI), conformity index (CI), and OAR sparing. Target dose coverage was analyzed using the criteria recommend by ICRU-83 report and other methods. The evaluation indices are described as follows.

Conformity index

CI defines how well the prescription dose conforms to the primary PTV. The CI evaluates a plan's ability to spare normal tissue from the high doses delivered to the treatment volume.

CI = VR/VT.

where VR is the volume of the reference isodose (95% of the prescribed dose) and VT is the volume of the target. The optimal value is “1.” Since optimal plans have uniform doses in their treatment volumes, the conformity of each PTV was evaluated.

Homogeneity index

HI is a fast, simple scoring tool that analyzes and quantifies dose homogeneity in the target volume. It is used to evaluate and compare the dose distribution of treatment plans and to choose the best plan among the available plans.

HI= (D2%/D98%).

where D2% represents near-maximum dose and D98% represents near-minimum dose. Once again “1” is the ideal value.

The PTV mean dose, PTV V95% and V105% (the volume of PTV receiving 95% and 105% dose) prescribed dose were also analyzed for target volume. For OARs, dose analysis was performed using DVH, serial OARs such as spinal cord and brainstem were analyzed by maximum dose D1% (dose received by 1% of volume) and D1.8cc (dose received by 1.8cc of the organ). Parallel OARs left, and the right parotids mean dose, D33% (dose received by 33% of the parotid volume), and D67% (dose received by 67% of the parotid volume) were analyzed. Oral cavity dose was analyzed by D1% and D33%. Treatment efficiency was analyzed using MU and treatment delivery time between two plans. Statistical analyses were performed using the Student's t-test (paired, two-tailed, and differences were considered to be statistically significant for P< 0.05).

To verify VMAT plan quality two-dimensional planar dose verification was performed using I'matriXX with multicube phantom. I'matriXX is a device which consists of a 1020 vented ion chamber array detectors arranged in 32 × 32 grids and can cover up to 24 cm × 24 cm field areas. Verification plans were created in the TPSP and TPSB. All plans were delivered as pretreatment verification by their respective machines, and planar dose was measured using I'matriXX. TPS calculated dose maps were compared with measured dose maps using gamma evaluation method with clinically acceptable criteria 3 mm distance to agreement (DTA) and 3% dose difference (DD). 10% of TPS maximum dose is used for low-dose cutoff in gamma calculation. For statistical analyses, TPS calculated was kept as reference. In this study, the correlation coefficients were also calculated between TPS calculated dose maps and I'matriXX measured dose maps.


 > Results Top


Planning target volume

Clinically, acceptable VMAT plans showed comparable dose distributions between TPSB and TPSP-based optimization technique for all the cases [Figure 1]. Combined DVH results for PTV and OARs were shown in [Figure 2], and [Table 2] demonstrates the results for both PTVs. Comparison of mean dose, HI, and CI for PTV1 (prescribed dose 66 Gy) showed statistically insignificant difference (P - 0.922, 0.096, and 0.097), on the other hand, V95% and V105% showed that difference was statistically significant (P - 0.010 and 0.012). Dosimetric analyses for PTV2 (prescribed dose 60 Gy) showed difference is statistically significant (P value – Dmean - 0.024, HI - 0.008, CI - 0.002, V95% - 0.0002 and V105% - 0.729), which demonstrated an advantage of TPSP optimization in all parameters except V105% over TPSB optimization.
Figure 1: Comparison of dose distribution in treatment planning system using biological and physical

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Figure 2: Combined dose volume histogram for planning target volumes and organs at risks

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Table 2: Comparison of mean dosimetric parameters for planning target volume 66 and 60 Gy

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Spinal cord

TPSB could achieve additional sparing of spinal cord in terms of D1% and D1.8cc compared to TPSP (P value – D1% - 0.008, D1.8cc - 0.008) [Table 3]. Maximum dose received by 1% of volume (D1%) was 40.04 ± 1.89 Gy and 35.97 ± 2.63 Gy by TPSP and TPSB optimization, respectively [Table 3].
Table 3: Dose-volume histogram analysis of critical organ doses

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Brainstem

TPSB showed statistically significant superiority for brainstem D1% and D1.8cc compared to TPSP (P value – D1% - 0.00002, D1.8cc - 0.0003) [Table 3] while both the plans did not exceed the planning objective for brainstem [Table 1].

Parotids

TPSP optimization showed more gain in sparing parotids in terms of mean dose and D67% [Table 3] except D33% dose. P values for mean dose - 0.0006 and 0.0035, D67% - 0.00008 and 0.00003, D33% - 0.1491 and 0.7205 for left and right parotids respectively.

Oral cavity

TPSP and TPSB optimizations could not yield advantages over another in oral cavity for all dosimetric parameters. Oral cavity mean, D1% and D33% dose comparison showed no statistical difference between two techniques. Statistical values (P value) for oral cavity mean dose, D1% and D33% were 0.470, 0.147, and 0.671, as shown in [Table 3].

Delivery efficiency

TPSB optimization technique showed almost 1.5 times of more MUs required to create clinically acceptable plans when compared to TPSP optimization technique as shown in [Table 3]. Two full 360° arcs were utilized in both TPSP and TPSB optimization to create clinically acceptable VMAT plans. MU difference showed statistically significant (P - 0.0001) [Table 3] between TPSP and TPSB. To deliver entire treatment plan, Varian linear accelerator required on an average 2.72 ± 0.11 min, and Elekta required 3.49 ± 0.34 min. Varian linear accelerator on an average required 22% lesser beam “ON” time compared to Elekta linear accelerator, which is statistically significant.

Gamma analysis

[Table 4] shows the average area gamma agreement of twenty patients. Gamma analyses performed between TPSP calculated and I'matriXX measured with criteria 3 mm DTA and 3% DD [Figure 3]. Similar analysis was performed for TPSB, P value calculated between two planning systems showed that there was not much statistical significant difference in treatment deliveries (P - 0.4862). Correlation coefficient analysis illustrated that both plan very well correlating with each other in terms of treatment delivery (P - 0.1442). Gamma evaluation result demonstrated that both optimization techniques could produce clinically deliverable plans.
Table 4: Gamma analysis (3 mm distance to agreement/3% dose difference)

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Figure 3: Gamma analysis for 3 mm distance to agreement and 3% dose difference criteria

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 > Discussion Top


This study investigated the performances of commercially available TPSB (Monaco) and TPSP (Eclipse) VMAT plan optimization systems. In the cases of complex, H and N cancers with SIB techniques, VMAT plans requires very strong MLC modulation due to the proximity of the OARs and large overlap of the parotid glands with PTV. For all cases, TPSB and TPSP could generate clinically acceptable VMAT plans. Even though two different TPS were involved in the VMAT plan creation, each technique has its own advantages and disadvantages. TPSP VMAT plans showed slightly better results in terms of homogeneous dose within the target volume and also meet the most dose-volume criteria compared to TPSB VMAT plans. PTV dose conformity indices showed that TPSP VMAT plans were better than TPSB VMAT plans, but the difference does not appear to be clinically significant. Quantitative and statistical analyses of PTV1 parameters mean dose, HI, and CI showed statistically insignificant difference (P > 0.05) with the exception of V95% and V105%. While analyzing PTV2 with the exception of V105%, all other parameters showed statistically significant difference (P < 0.05) between two TPS. Our study results were very well comparable with the results obtained in TG-116 study. For PTV, all TPSB plans resulted in substantially less homogeneous target dose distributions compared to TPSP plans. Inhomogeneous dose distribution in VMAT plan from TPSB can be due to (i) the compulsory cell killing objective which penalizes small cold spots less drastically than physical minimum dose penalties (ii) the practice of normalizing the treatment plans to the minimum dose in the target not to equivalent target EUD and (iii) in geometrically complex Head and Neck cases planned using 1 cm MLC leaf widths. However, the TPSP VMAT plan had the lowest maximum dose and highest minimum target dose implying a more homogeneous dose distribution within the target and in addition consistently achieved the greatest coverage of the nodal target for all patients.

TPSB optimized VMAT plans showed improvement in sparing of spinal cord and brainstem when compared to TPSP optimized plans (P - 0.008). This is because in TPSp-based optimization maximum dose cost function uses only a single point on DVH to control maximum dose for serial structures. On the other hand, in TPSB-based optimization serial cost function uses many points on DVH to emphasize on high doses which provide more control on maximum dose. Reduction of doses to the spinal cord and brainstem could be beneficial to H and N patients with recurrent or locally persistent diseases, especially when the second course of radiotherapy is necessary. Several clinical studies assessed the utility of intensity modulated treatments in parotid salivary gland sparing and in reducing xerostomia. In our study, TPSP optimization could reduce parotid mean dose as much as 20.4% lesser when compared to the TPSB optimization, which is statistically significant (P - 0.002). Parallel cost function in TPSB optimization controls parallel organ doses by means of mean dose but in TPSP-based DVH cost function controls doses by a single or multiple points on DVH. TPSP-based optimization gives traditional DVH constraint more flexibility than TPSB optimization. Better sparing of parotid glands is more important in all cases to preserve salivary function.[17] A study from Chao et al.,[18] about a correlation of the salivary output with the mean doses to the parotid glands were investigated. A prospective study of salivary function sparing in patients with H and N cancers receiving intensity modulated or three-dimensional radiation therapy showed that whole mouth salivary output dropped by 4% for each increase in the mean dose by 1 Gy. When using IMRT for irradiation of oropharyngeal cancer salivary function was less impaired, but the majority of the patients still suffered from some degree of xerostomia.[2],[3],[4],[19] In our study, both parotid mean doses were >26 Gy due to more overlap of the parotid gland with PTV as mentioned early all cases in this study was present with advanced Stage III disease. Volume of 67% (D67%) parotids received average dose of 18 Gy and 30 Gy for TPSP and TPSB optimization, respectively. Braam et al.[20] showed that the normal tissue complication probability at several time points after radiation therapy was <20% only if the mean dose to the parotid glands was lower than 25 Gy, a dose level that even with IMRT is often not achieved. Our study demonstrated that TPSP plans offer relatively improved parotids sparing than TPSB plans.

Two full arcs were used to create VMAT plans in TPSP planning system for all cases in this study. As reported by Guckenberger et al.,[4] single arcs are often not sufficient to yield the desired plan quality for complex targets. The purpose of two arcs was found to be essential to achieve the high degree of conformal voidance required by the planning objective. Adding the second arc provided the planning system additional freedom for achieving better treatment plans. Superior results could be possibly achieved by allowing TPS to create more control points per arc and as a result more MUs required for TPSB to achieve similar modulation as compared to TPSP VMAT plans. In TPSP VMAT plan, each arc contains 177 control points and two arcs put together total of 354 control points per plan was achieved whereas in TPSB VMAT plans, on an average 452 MLC segments were found in double arc plans. Total number of MUs required to deliver the dose of 200 cGy for clinically acceptable VMAT plans offered by TPSP is nearly 60% lesser than TPSB (TPSP mean MUs 490.4 ± 59.4 and TPSB mean MUs 791.6 ± 122.8). TPSP VMAT optimization algorithms hardcore has been designed such a way gantry rotates faster and makes use of maximum achievable dose rate to deliver treatment in shorter time whereas TPSB modifies all possible parameters to achieve better plan required more segments and MUs. The improved treatment efficiency may reduce secondary malignancies due to less scatter dose by reducing the MU's. The reduction of primary leakage dose (out-of-field doses due to the linac-head scatter and leakage radiation) using VMAT decreases the total body exposure and therefore might also decrease the risk of developing secondary cancers.[21],[22] Exposure of adult from radiotherapy in the abdominopelvic region may result in the induction of genetic disorders in future generations due to secondary scatter radiation.[23] Varian linear accelerator on an average required 22% lesser beam “ON” time compared to Elekta linear accelerator which is statistically significant. The average radiation beam on time for TPSP-generated VMAT plans (2.72 ± 0.11 min) was found to be significantly lower than TPSB-generated VMAT plans (3.49 ± 0.34 min). Reduction in treatment time would increase patient comfort as well as decrease uncertainty due to intrafractional organ movement. Patient-specific pretreatment quality assurance is strongly recommended for all patients to identify any potential errors in the treatment planning process and in machine deliverability, especially for intensity modulated treatment. Gamma analysis result for the pretreatment quality assurance showed good agreement between the TPS calculated and measured fluence for 3 mm DTA, 3% DD clinical acceptance criteria for both TPS. The average area gamma difference for TPSP plans (95.99% ± 0.95%) and TPSB (95.64% ± 1.04%) were statistically insignificant. Correlation coefficient comparison result illustrated the difference was statistically insignificant also confirmed that both plans were similar in treatment delivery even for complex H and N cases.

Although two different optimization techniques showed clinically acceptable VMAT plans, few limitations were found in our study. Two different types of dose calculation and optimization techniques were used in our current study. TPSP uses model-based algorithm (AAA) for dose calculation where as TPSB uses Monte Carlo algorithm where two different beam and linear accelerator configurations were used to create VMAT plans. Grid size plays a major role in final outcome of the VMAT plan. In TPSP, 2.5 mm default dose calculation grid size was used whereas in TPSB3.0 mm is the default grid size. Reducing grid size in TPSB leads to more time consuming in dose calculations. The tradeoff between time and grid size was adapted in all VMAT plans from both planning system. In this study, combination of 5 mm and 10 mm MLC (millennium 120) from Varian linear accelerator was used in TPSP whereas in TPSB Elekta linear accelerator with 10 mm MLC was used. However, direct comparison VMAT plans with the recently introduced 4 mm MLC from Elekta will definitely further reduces the differences between TPSP and TPSB in terms of OAR doses.[24] Last, all VMAT plans were done by two different planners having enough experience in respective TPS. Even though well-defined clinical goals were given and achieved by both planner; there may be a little influence of the planner involved in our study.

Choice of the TPS is important to avoid delays in the clinical patient schedule during planning process and also to be able to provide better quality planning solutions for the given clinical case. Both TPSB and TPSP VMAT optimization algorithms used in this study were robust and can produce plans with a high degree of target dose conformity and critical organ sparing for all complex H and N cases in both patient scenarios.





VMAT plans of TPSP and TPSB offered clinically acceptable dose distributions. TPSB-based optimization showed enhanced OAR sparing for serial structures whereas TPSP-based optimization showed enhanced OAR sparing for parallel structures. Gamma analysis showed good agreement between the planned and delivered dose in both TPS. The results pertaining to serial, parallel structures, and treatment time were statistically significant. Considering the uncertainty in the biological parameter used in the TPSB optimization and the limitation in single scoring objective functions in TPSP-based optimization an elaborative clinical outcome study is needed for further analysis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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