|Ahead of print publication
Dosimetric evaluation of hybrid and volumetric-modulated arc therapy plan for left-sided chest wall irradiation in MONACO treatment planning system
P Sathiyaraj1, P Senthil Manikandan1, C Varatharaj1, KM Ganesh1, S Sathiyan1, M Ravikumar2
1 Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Sri Shankara Cancer Foundation, Bengaluru, Karnataka, India
2 Department of Radiotherapy, Sri Shankara Cancer Foundation, Bengaluru, Karnataka, India
|Date of Submission||28-May-2020|
|Date of Decision||16-Jul-2020|
|Date of Acceptance||15-Sep-2020|
|Date of Web Publication||15-Jun-2022|
P Senthil Manikandan,
Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bengaluru - 560 029, Karnataka
Source of Support: None, Conflict of Interest: None
Background and Objective: Radiation induced toxicities in heart and lungs are diminishes the survival rate of cancer patients. The purpose of this study is to evaluate the dosimetric parameters of hybrid plans for chest wall irradiation of left breast carcinoma patients using Monaco treatment planning system and compare with the volumetric-modulated arc therapy (VMAT) treatment plans. Materials and Methods: Fifteen carcinoma left breast patients were randomly selected to evaluate the advantage of hybrid plan over VMAT. Hybrid plans were generated with 70% and 30% dose contribution from forward intensity-modulated radiotherapy and VMAT, respectively, whereas VMAT had been done with full prescription. Conformity and homogeneity indices were evaluated for target coverage between hybrid and VMAT plans.
Results: Hybrid plan has proved its superiority over VMAT in terms of better organ at risk sparing and lesser low dose spillage and at the same time providing comparable target coverage. In low-dose spectrum, VMAT showed higher dose–volume than the hybrid plan. The maximum variation was found to be 44.75% at 7 Gy and the minimum dose difference was observed at 1 Gy (6.02%). Conclusion: This study suggests that the hybrid plan could be a better option for left-sided chest wall irradiation in regular clinical practice.
Keywords: Hybrid, Monaco treatment planning system, three-dimensional conformal radiotherapy, volumetric-modulated arc therapy
|How to cite this URL:|
Sathiyaraj P, Manikandan P S, Varatharaj C, Ganesh K M, Sathiyan S, Ravikumar M. Dosimetric evaluation of hybrid and volumetric-modulated arc therapy plan for left-sided chest wall irradiation in MONACO treatment planning system. J Can Res Ther [Epub ahead of print] [cited 2022 Aug 19]. Available from: https://www.cancerjournal.net/preprintarticle.asp?id=347680
| > Introduction|| |
Long-term survival of patients with breast cancer has been observed with proper radiotherapy treatment. Though post mastectomy radiotherapy significantly enhances the survival rate, radiation-induced cardiac toxicity such as myocardial infraction, coronary revascularization, and ischemic heart disease diminishes it.,, There is evidence in literature that the cardiac-related diseases are increased linearly with the mean dose of heart. Another study showed the estimated excess rate ratio for incident lung cancer of 0.11 per Gy of ipsilateral whole lung dose. Along with lung and heart, dose to the contralateral breast also should be restricted to avoid secondary malignancy. Irregularity of anatomy and spectrum of heterogeneity makes the chest wall plan always complex.
At present, different planning techniques such as three-dimensional conformal radiotherapy (3D-CRT), forward intensity-modulated radiotherapy (FIMRT) or Field-in-Field (FIF) technique, intensity-modulated radiotherapy (IMRT), and volumetric-modulated arc therapy (VMAT),,,,,, are used for breast irradiation. Typical tangential arrangement of fields in 3D-CRT restricts the spread of lower doses into the lung and heart volumes at the cost of certain high-dose volume in the same organs. In addition, due to the complex geometry of the chest wall, adequate dose coverage may not be obtained especially at the chest wall and supraclavicular fossa (SCF) field junctions. In addition, high dose spillage outside the planning target volume (PTV) is an issue with 3DCRT technique. Though VMAT improves the dose conformity and reduces the lung and heart doses, spread of low doses and high monitor units (MUs) are of concern.
Considering the advantages and disadvantages of each technique, Mayo et al. introduced hybrid plans, combining both 3DCRT and IMRT plans at optimum proportions. Currently, no adequate studies are available in literature regarding left-sided chest wall irradiation with hybrid plan, with Monaco treatment planning system (M/s Elekta Ltd., UK). In this study, we aim to investigate the dosimetric parameters of hybrid plans for chest wall irradiation of left breast carcinoma patients using Monaco treatment planning system (TPS) and to dosimetrically compare with the VMAT treatment plans.
| > Materials and Methods|| |
Fifteen carcinoma left breast patients were randomly selected to evaluate the advantage of hybrid plan over VMAT. The demographic data of the patients are shown in [Table 1].
All-in-one base board made of carbon fiber with appropriate cushion wedge (5°, 10°, or 15°) was used for immobilization. The arms were extended behind the head and supported with appropriate cushion wedges and grip pole to maintain the same position throughout the course of treatment. Computed tomography (CT) images of all patients were acquired in Philips big bore Brilliance CT scanner with 3-mm slice thickness and exported to Monaco TPS for delineation of target and critical organs and planning. Clinical target volume (CTV) was delineated with supraclavicular nodes based on radiation therapy oncology group breast cancer atlas. For PTV 5-mm margin was given from CTV. All organ at risk (OARs) and target were delineated and checked by an experienced radiation oncologist.
Chest wall and supraclavicular node were planned for a dose prescription of 50 Gy in 25 fractions. [Table 2] shows the planning objectives of PTV and OARs. All plans were generated by a single physicist in Monaco TPS (version 5.11.03) with Monte Carlo and collapsed cone algorithms for VMAT and 3DCRT, respectively. Monaco TPS offers a tool called bias dose plan for performing hybrid plan by accounting the dose of base plan.
Volumetric-modulated arc therapy plan
VMAT plans were created for a prescription dose of 50 Gy in 25 fractions with 2 coplanar partial arcs. The start angle of the arc varied from 310° to 330° and the arc length varied from 140° to 160°. The minimum segment width was set to 0.8 cm, and the total control point was set to 180. A grid size used for calculation was 3 mm. The uncertainty during calculation was set to 1% per plan. In order to account for chest wall motion, the auto flash margin was set to 2 cm. All VMAT plans were done using 6 MV photons. Both target equivalent uniform dose and target penalty cost functions were used for target dose constraints.
Hybrid plans were generated with 70% and 30% dose contribution from FIMRT and VMAT, respectively. For chest wall, tangential fields were used in FIMRT plans and the different positions of the isocenter are depicted in [Figure 1]. 6 MV photon beam was used for medial tangent field for lateral tangent (LT) field, and the energy was selected depending on the distance between the surface of the body and the posterior part of PTVs.
Approximately 2-cm flash margin was created outside the body by MLCs. For CW plan, FIF technique was used to reduce the maximum dose, also we used optimal number of FIF in order to minimize the interplay effect. Using Add RX tool, SCF plan was generated separately with sharing the same isocenter of chest wall plan. In the last step of hybrid plan, VMAT plan was created for a prescription dose of 15 Gy (30% contribution) in 25 fractions with the same plan parameters as described in the above section (VMAT plan) and sharing the same isocenter of CW and SCF plans. Dose of the FIMRT plan (CW + SCF) was accounted in VMAT optimization as the base dose plan.
Evaluation of dosimetric parameters
To analyze hybrid and VMAT plans, various dosimetric parameters were used to compare PTVs and OARs. The conformity and homogeneity of dose distribution were evaluated using the corresponding indices as defined below. Furthermore, tumor volume receiving 95% of prescribed dose, maximum dose to the target, and total MUs were analyzed.
Conformity index (CI) = PIV/TV
Where PIV = Prescription isodose volume, TV = Tumor volume
Homogeneity index, HI = (D2% − D98%)/DP,
Where D2% is the dose received by 2% of target volume, D98% is the dose received by 98% of target volume, and DP is the prescription dose.
Various dosimetric parameters were evaluated for ipsilateral lung, contralateral lung, Heart, and contralateral breast. For ipsilateral lung, percentage of volume receiving 5 Gy (V5), percentage of volume receiving 20 Gy (V20), and mean dose were analyzed. For heart, percentage of volume receiving 25 Gy (V25), percentage of volume receiving 30 Gy (V30), and mean dose were evaluated. For contralateral lung and contralateral breast, the mean dose was evaluated. A volume, body-PTV was created by subtracting the PTV volume from the body contour to analyze the spread of low dose (1 Gy to 20 Gy) in healthy tissue, which is of major concern in VMAT techniques. Statistical test of significance was used for dosimetric difference of hybrid and VMAT plans. Student's t-test was performed to find the statistical significance (P ≤ 0.05).
| > Results|| |
The dosimetric parameters of PTV and OARs are summarized in [Table 3]. [Figure 2] shows the 95% dose coverage to the target in coronal, sagittal, and transverse views in iso-fill mode. Both VMAT and hybrid plans achieved the target objectives (95% of prescription dose to 95% of target volume). In SCF, 95% of dose coverage was superior in VMAT than hybrid (P < 0.05). The calculated CI and HI for both planning techniques were observed to be statistically insignificant (P > 0.05). No statistically significant difference (P > 0.05) in maximum dose inside the PTV was observed between VMAT and hybrid plans. The comparison of low dose spillage of 5 Gy and 10 Gy is depicted in [Figure 3]. The low-dose spillage (1 Gy to 20 Gy) is also plotted against the percentage of patient volume irradiated and shown in [Figure 4]. In this low-dose spectrum, VMAT showed higher dose–volume than the hybrid plan. The maximum variation was found to be 44.75% at 7 Gy and the minimum dose difference was observed at 1 Gy (6.02%). The percentage difference in low dose–volume between the VMAT and hybrid plan showed a particular trend, with an initial increase, reaching a maximum and then decreasing, as shown in [Figure 5]. The best fit curve were noticed to be a second-order polynomial and the correlation values were 0.8945 and 0.9543 for the hybrid and VMAT plans, respectively. A statistically significant difference (P < 0.05) was observed in the mean dose of contralateral lung and heart, which is much lower in the hybrid plan than VMAT. No statistically significant difference (P > 0.05) was noticed in the volume of heart receiving 25 Gy (V25) between the plans, while a highly significant reduction in dose to the contralateral breast was found in the hybrid plan with a P value very less than 0.05.
|Table 3: Dosimetric parameters of planning target volume and organ at risks in hybrid and volumetric modulated arc therapy plans|
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|Figure 2: Planning target volume coverage (95% of dose) in transfer, sagittal, and coronal view of Hybrid (a), volumetric-modulated arc therapy (b)|
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|Figure 3: Low-dose spillage (5 Gy – pink, 10 Gy – yellow) in iso line mode of volumetric-modulated arc therapy (left) and hybrid (right)|
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|Figure 4: Comparison of low-dose volumes (1 Gy to 20 Gy) in hybrid and volumetric-modulated arc therapy plan|
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|Figure 5: Percentage difference of low-dose spillage in hybrid and volumetric-modulated arc therapy plan|
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| > Discussion|| |
Though appropriate radiotherapy offers long-term survival to the patients, it is always associated with the added risk of radiation induced cardiac toxicity and secondary malignancy in the contralateral breast. In the conventional technique, it is possible to block the beams entering the contralateral breast completely and thus reducing the probability of stochastic effects. However, blocking the contralateral breast from the tangential field may not possible for some patients due to their anatomical geometry, making it impossible to avoid high doses. Advanced techniques such as IMRT and VMAT help in this respect. In addition, it gives better conformity and dose homogeneity to the target. Hybrid plan technique is an attempt to utilize both VMAT and FIMRT techniques for their respective benefits. In this study, 70% of the prescription dose was contributed by the FIMRT technique, while 30% by VMAT.
A detailed review on different planning techniques is reported in literature, and several studies favor the use of hybrid plan for breast cases. Balaji et al. have done hybrid plans for left-sided chest wall irradiation combining 3DCRT and VMAT with different proportions of the prescription dose, and they have concluded that 70% to 80% of contribution from 3DCRT and the rest from VMAT are optimal for the hybrid plan. Rafic et al. reported the hybrid conformal plan for left-sided chest wall irradiation with FIF alone techniques and concluded that the planning strategy is feasible for day-to-day clinical practice. Local disease control depends on adequate target coverage. In this study, hybrid plan gives a competitive coverage, dose uniformity, and conformity when compared to a VMAT plan. In hybrid plan, FIMRT contributes 80%–90% weightage of the prescription dose (35 Gy) to the target, while the subsequent VMAT plan enables to boost the deficiency in target coverage without unduly increasing the OAR doses. This study strongly recommends that the OAR doses to be kept as low as possible in the FIMRT plan by blocking the overlapping portions of the heart, ipsilateral lung, and contralateral breast in the medial and LT fields. Such field blocking helps to reduce the OAR doses at the cost of target coverage. However, this is not of much concern because that disadvantage can be overcome in the subsequent VMAT plan optimization. It is noticed from this study that, if 80%–85% of PTV coverage is achieved with less OAR doses in FIMRT plan, then the hybrid plan would result in good quality with the help of VMAT.
The mean heart dose could be significantly reduced in the hybrid plan. It has been reported that V25 of the heart should not be more than 10%. However, in this study, we could achieve V25 <5.42% but not significantly differing from that of VMAT (5.54%). Reports show that the mortality due to cardiac issues is related to low dose levels., Blocking some portion of the beam during planning in the hybrid plan not only reduces the heart dose but to the ipsilateral lung as well. Hence, V5 of ipsilateral lung could be kept well below (48.84 ± 4.89 Gy) the dose that has been reported to cause lung pneumonitis., The independent VMAT plan showed a very high volume of lung receiving 5 Gy dose (67.48%). The V20 dose constraints are achieved well in hybrid plans. Because the secondary malignancy to the contralateral breast is a stochastic effect, dose should keep as low as possible. VMAT has a significant spread of low doses to the contralateral breast as shown in [Figure 3]. Because the hybrid plan has a maximum contribution of 30% dose from VMAT, a significant reduction in low-dose spillage into the contralateral breast (hybrid: 2.8 Gy ± 0.53 Gy, VMAT: 4.57 Gy ± 1.19 Gy) is observed. Even though the hybrid plan seems to be better, because the presence of VMAT, low-dose spillage has to be analyzed. In this study, we have analyzed doses from 1 Gy to 20 Gy, and the difference between hybrid plan and VMAT was found to be highly statistically significant (P > 0.05). Large number of MUs in VMAT plan would lead to increased head leakage that enhances the out-of-field dose. As hybrid plans yield lesser MUs than VMAT, it might reduce the low-dose spillage around the target.
In the present study, hybrid plan shows its superiority over VMAT in terms of better OARs sparing and lesser low-dose spillage and at the same time providing comparable target coverage. Mayo et al. reported that by reducing the contribution of IMRT dose to approximately one-third of the total prescription, the magnitude of errors in dose delivery can be reduced. The uncertainty in dose delivery is significantly reduced in this study as the dose contribution from VMAT is only 30% of the prescription dose.
| > Conclusion|| |
This study provides information on the advantage of hybrid plan over VMAT for chest wall irradiation of left breast carcinoma patients using Monaco TPS. Hybrid plan provides superior results over VMAT in terms of low-dose spillage and OARs sparing and at the same time providing comparable target coverage. Hybrid plan helps to reduce the intra-fraction uncertainties that are arising due to patient movement during treatment. After detailed evaluation, it can be concluded that the hybrid plan could be a better option for left-sided chest wall irradiation in regular clinical practice. The future of the study is follows: comparison of electron–photon mixed planning and photon's hybrid plan for chest wall.
Financial support and sponsorship
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], [Table 3]