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 Table of Contents  
Year : 2020  |  Volume : 11  |  Issue : 4  |  Page : 188-193

Dual-isocentric volumetric modulated arc therapy in synchronous bilateral breast cancer irradiation: A dosimetric study

Department of Radiation Oncology, Faculty of Medicine, Recep Tayyip Erdogan University, Rize, Turkey

Date of Submission25-Jun-2020
Date of Acceptance07-Aug-2020
Date of Web Publication30-Dec-2020

Correspondence Address:
Dr. Sema Yilmaz Rakici
Department of Radiation Oncology, Faculty of Medicine, Recep Tayyip Erdogan University, Rize
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jrcr.jrcr_32_20

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Introduction: In synchronous or metachronous breast cancer, bilateral breast irradiation is one of the major problems of radiation oncology. In this study, dosimetric radiation treatment of bilateral breast cancer (BBC) in two cases of synchronous bilateral mammary cancer will be discussed. In this study, a conformal radiotherapy (RT) plan for bilateral breast irradiation is discussed over two cases with synchronous BBC (sBBC). Materials and Methods: Two cases with sBBC requiring irradiation of dual breasts and regional nodal areas were chosen. Data from bilateral breast irradiation studies were used rather than dose recommendations used in unilateral breast irradiation for organs at risks. The dual-isocentric four-half-arc option was utilized in volumetric-modulated arc therapy (VMAT) plans. The dynamic sliding window method with fixed gantry angle was utilized in intensity-modulated radiation therapy (IMRT) plans. Results: Both IMRT and VMAT plans were chosen for the patients. D95% of the planning target volume supplied the desired dose prescription in the VMAT and IMRT plans of Case 1 and in the IMRT plan of Case 2, whereas the VMAT plan of Case 2 did not achieve the desired dose prescription at the target. The conformity index (CI) was the ideal plan in Case 1's VMAT plan, while Case 2's VMAT plan deviated greatly in terms of the CI value. The VMAT and IMRT plans of Case 1 and the IMRT plan of Case 2 were considered to be more homogeneous plans than the value closest to zero in terms of homogeneity index, whereas the VMAT plan of Case 2 was not considered homogeneous. Conclusion: It is recommended that in sBBC RT confined to breast only dual isometric, four half arc VMAT is an appropriate plan.

Keywords: Dual isocenter, partial volumetric modulated arc therapy, synchronous bilateral breast cancer

How to cite this article:
Rakici SY, Cinar Y. Dual-isocentric volumetric modulated arc therapy in synchronous bilateral breast cancer irradiation: A dosimetric study. J Radiat Cancer Res 2020;11:188-93

How to cite this URL:
Rakici SY, Cinar Y. Dual-isocentric volumetric modulated arc therapy in synchronous bilateral breast cancer irradiation: A dosimetric study. J Radiat Cancer Res [serial online] 2020 [cited 2021 Apr 17];11:188-93. Available from: https://www.journalrcr.org/text.asp?2020/11/4/188/305723

  Introduction Top

There has been a statistically significant increase in cases of synchronous bilateral breast cancer (sBBC) or metachronous BBC (mBBC) over time. The rate of BBC increased from 2.6% of all breast cancers in 1975 to 7.5% in 2014, and the rate of sBBC increased from 1.4% to 2.9%, and the rate of mBBC increased from 1.2% to 4.6%.[1] This increase in sBBC and mBBC rates emerges as a problem for an irradiation technique to be used in cases needing bilateral breast irradiation. In bilateral breast irradiation, radiotherapy (RT) planning becomes quite complex compared to unilateral irradiation due to concerns for creating an ideal RT plan with a conformal technique, the complex structure of the breast, the widening RT field with the addition of the regional nodal area, the doses taken by surrounding critical structures and the risk of overlapping areas. This requires choosing more advanced technological treatment modalities such as volumetric modulated arc therapy (VMAT), intensity-modulated radiation therapy (IMRT) and even intensity-modulated proton therapy (IMPT) rather than three-dimensional conformal radiation therapy (3DCRT) plans.[2] While the size of the area is a primary problem in synchronous irradiation, the edge of the old RT area, dose uncertainties and overlapping areas are considered as the main problem in metachronous irradiation. A number of dosimetric studies involving IMRT, 3DCRT, VMAT and helical tomotherapy have been carried out to overcome such a problem.[3],[4],[5],[6] Moreover, treatment techniques such as fixed-jaw IMRT, tangential VMAT and IMPT have been reported to be methods that can be effective in solving the sBBC problem.[2],[7] It has also been pointed out that better results are achieved in IMPT applications.[2]

Although mono-isocentric treatment approaches are regarded as a relatively easier choice, it can be said that they have certain limitations. The dual-isocentric four-half-arc VMAT option that we utilized in the present study may be a solution for the treatment of such patients. This technique can be recommended, especially in cases with sBBC who would undergo irradiation for their breasts only. However, for targets with larger volumes where the regional nodal area is irradiated, it appears that RT of sBBC will continue to be a problem.

  Materials and Methods Top

All procedures, including the informed consent process, were conducted in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2000. Two patients were chosen: Case 1 had undergone breast-conserving surgery, had early-stage breast cancer with both breasts at stage T1N0M0, and was to undergo irradiation for both breasts only; Case 2 had BBC with local stage T4N3, her both regional nodal areas were involved, and she was to undergo irradiation for the breast and regional nodal area. Clinical target volume (CTV) for breast and lymph node (LN) was shaped by the contouring  Atlas More Details of the Radiation Therapy Oncology Group.[8] The volume covering the entire breast was defined as CTV-breast for the breast target volume. The LN region, including axillary levels I, II, III, and infraclavicular, supraclavicular and mammaria interna nodes was defined as CTV-LN. Planning target volumes (PTVs) were expanded 5 mm in all directions from CTV and pulled from the skin with a 3-mm skin gap from the surface. The entire lungs, heart and left anterior descending artery (LAD) were considered organs at risks (OARs). PTV was prescribed to receive a total dose of 50 Gy consisting of 2 Gy fractions per day. It was aimed that 95% of the recommended dose would cover 95% of PTV. For dose recommendations for OARs, data from BBC patient studies were used rather than dose recommendations used in unilateral breast irradiation [Table 1].[4]
Table 1: Dose recommendations for target and organs at risks in bilateral breast irradiation

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Photon bundles of 6 MV obtained from Varian Trilogy Clinac iX treatment device were used for treatment planning. The treatment device Millennium 60 Paired Multileaf Collimator offers 40 paired 0.5 cm leaf resolution at isocenter and 1.0 cm leaf resolution at the remaining area. Its maximum leaf speed is 2.5 cm/s, and leaf transmission is 1.6%. In this study, RT plans created for the two cases were designed by the same planner in the Varian Eclipse (Version 13.6 Varian Medical Systems, Palo Alto, CA, USA) treatment planning system (TPS). VMAT plans were optimized by choosing a maximum variable dose rate of 600 MU/min. A constant dose rate of 400 MU/min was selected in intensity-modulated RT (IMRT) plans. For all plans, the analytical anisotropic algorith (AAA) version 13.6.23 was implemented in the Eclipse (Varian Medical Systems, Palo Alto, CA, USA ) photon dose calculation algorithm was used.[9],[10] The photon optimization (PO) (version 13.6.23) algorithm was recently released by Varian Medical System (Palo Alto, CA, USA) was used to optimize photons.

Design of intensity modulated radiation therapy plans

The dual-isocentric four-half-arc option was utilized in VMAT planning. For both breasts, the isocenter was automatically allowed to be determined by the device on the midline location of PTV-breast. The two-half-arc option (one being clockwise and the other being counterclockwise) with a gantry angle of 300°–179° was used for the left breast. The two-half-arc option (one being clockwise and the other being counterclockwise) with a gantry angle of 60°–181° was used for the right breast. During arc rotation, a 30° collimator angle was used to minimize the effect of the edges of the area and to take advantage of the direction of rotation of the leaves.

Design of intensity modulated radiation therapy plans

Dynamic sliding window method with fixed gantry angle was utilized in IMRT plans. The plans were designed by placing them in two separate isocenters in the midline, 3 cm above the chest wall on both breasts. The gantry angles that were used in Case 1 were 55°, 40°, 0°, and 235° on the right breast, and 310°, 325°, 0°, 140°, and 125° on the left breast [Figure 1]. In Case 2, the gantry angles that were used were 50°, 35°, 345°, 205° and 220° on the right, and 325°,315°, 15°, 135°, and 120° on the left for bilateral PTV-LN and PTV-breast planning [Figure 1]. The priority value of the other side was increased in comparison to that of the first site when the plans were made so that the other side did not take any doses. Then, the plans that were needed for the other side were made. In the second plan, the first plan of the side was chosen and included as a “base plan” in optimization. The results were assessed in terms of plan sum.
Figure 1: VMAT and IMRT area and dose images of both cases. Beam eye view and isodose distribution on transverse, coronal, and sagittal views of VMAT and IMRT plans of cases. Case 1 VMAT (a), Case 1 IMRT (b), Case 2 VMAT (c), Case 2 IMRT (d). Abbreviations, VMAT: Volumetric modulated arc therapy, IMRT: Intensity modulated radiation therapy

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IMRT and VMAT plans were optimized to produce the desired distributions of dose in all areas and at all targets at the same time. The same dose restrictions (constraints) were used for normal tissues. In both cases, bolus or extension out of the body (skin flash) was not utilized. [Figure 1] shows 3D field and dose distribution images of VMAT and IMRT plans. Dosimetric data of the target and of OARs, which were obtained through the TPS and dose-volume histograms (DVH), were compared. Conformity of the plans was assessed by calculating conformity index (CI) and homogeneity index (HI) values through formulas (CI = VRI/TV, HI = D2% - D98%/D50%).

  Results Top

A total of 4 plans, consisting of one VMAT and one IMRT for each case, were created for the two cases. [Table 2] summarizes the dosimetric comparison of the target and OAR doses of the plans.
Table 2: Comparison of target and organs at risk doses of two patients

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D95% of PTV provided the desired dose prescription in VMAT and IMRT plans of Case 1 and IMRT plan of Case 2 (D95% = 4858.3 cGy, 4867.1 cGy, and 4807.6 cGy, respectively). The VMAT plan of Case 2 did not achieve the desired dose prescription in the target (D95% = 4521.6 cGy). Highest maximum doses (Dmax) occurred in the IMRT plans of Case 2 (5686.8 cGy for 1 cc). Other plans had similar doses of Dmax (5465.2 cGy, 5491.5 cGy and 5456.2 cGy for the VMAT and IMRT of Case 1 and the VMAT of Case 2, respectively). The ideal plan in terms of CI was the VMAT plan of Case 1 (CI = 1.056). The VMAT plan of Case 2 had a big deviation in terms of the CI value (CI = 0.894).[11],[12] The IMRT plans of the two cases were considered to be conformal plans because their CI values were in the range of 1–2 [Table 2].[11],[12] The VMAT and IMRT plans of Case 1 and the IMRT plan of Case 2 were considered to be more homogeneous plans than the closest value to zero in terms of HI (0.101, 0.106, and 0.133, respectively). The VMAT plan of Case 2 was not considered homogeneous than the other plans (HI = 0.188).[13] MU values of the VMAT plans were found to be smaller than those of the IMRT plans.

Total lung V5Gy doses were higher in the two VMAT plans when the plans were compared in terms of OAR doses (80% and 100%). V20Gy doses were 11.2% in the VMAT plan and 17% in the IMRT plan of Case 1. In both plans of Case 2, the V20Gy values were found to be over 30%. The heart doses of the plans were at the conformal dose limits, except for the IMRT plan of Case 2. The lowest V25 Gy value was found in the VMAT plan of Case 1 (3.7%) [Table 2]. Maximum LAD doses were highest in the IMRT plan of Case 2 (3709.7 cGy) [Table 2].

DVHs created separately for the target and OARs of both cases are shown in [Figure 2]. The VMAT plan target coverage of Case 2, which included the regional nodal area, in particular, was found not to be conformal.
Figure 2: Dosimetric comparison of the target and OARs of the plans based on DVH. PTV optimizations for both cases are shown in red in DVH. OAR doses are highlighted in yellow (total lung), green (LAD) and orange (heart). ▴ is used for VMAT plans and ◾ is used for IMRT plans. Case 1 VMAT and IMRT (a), Case 2 VMAT and IMRT (b) Abbreviations, OARs: Organs at risk, DVH: Dose-volume histograms, PTV: Planning targetvolume, LAD: Left anterior descending artery

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  Discussion Top

All stages of RT are required in BC cases in radiation oncology clinics. Application of RT in advanced localized BC has become a standard.[14] Budrukkar et al.[15] 100% of the oncologist participated in a survey found RT to be relevant in local advanced mammary cancer. While 92.2% of the participants found adjuvant RT to be appropriate for early-stage MKC, 70% recommended RT only. In addition, even though it is a controversial subject, 82% recommended RT for metastatic breast cancer in order to alleviate the symptoms. There is a great need for RT in breast cancer RT clinics. This requires specialized techniques. These difficulties get complicated further in bilateral mammary CA. This study provides easier application of sBBC radiation. If irradiation of only one breast is required, the dual centered, dual arched VMAT technique can be applied in the first instance. If nodal radiation is required, IMRT is the first choice.

In cases with BBC requiring irradiation, we need to focus on more advanced technological planning techniques rather than 3DCRT. 3DCRT methods often require sacrificing target coverages for the sake of meeting the dose constraints set for OARs. Target coverage can be improved by using IMRT or VMAT approaches, but increases may occur in low volume OAR doses.[2]

In sBBC RT, use of VMAT + IMRT combination therapy is recommended. After IMRT of the whole breast, application of a booster VMAT is also recommended.[16] This way the targeted dosage penetrates the intended area better while at the same time OARs dosages can be reduced in both applications. Currently, IMPT studies are being carried out, especially due to difficulties in creating the desired dose prescription for OARs. In relation to this, in a very recent study, it has been reported that VMAT and IMPT techniques both offer excellent target coverage in patients with sBBC, whereas proton therapy is superior in terms of lowering OAR doses.[2] In that study, the IMPT method with a single-area optimization approach has been proposed. In proton therapy, the mean heart dose was 2.12 Gy and total lung V5 doses were 39.8%. In the VMAT plans, these doses were 9.98% and 97.9%, respectively. V5% doses were seen to be quite high in the VMAT plans. In our study, while the heart doses were similar in the VMAT plan of Case 1, the lung doses were found to be lower. The reason why the OAR doses obtained in our study were lower was that our plans were dual-isocentric two-arc plans. Dual-isocentric plans require capturing 2 images (2 image-guidance captures per fraction) per fraction in each application. They not only take longer time to implement but also require more careful work compared to mono-isocentric plans in terms of planning. Despite all these difficulties, we can say that it is a method worth applying in selected patients because it can improve the doses of the target volume and reduce OAR doses.

For the millennium MLC system used for VMAT on the Varian iX device, the maximum area sizes in the Eclipse planning system are X-jaw = 15–16 cm and Y-jaw = 38–40 cm. Unfortunately, Varian linear accelerators have a mechanical limitation due to these area sizes.[17] The target volume for VMAT must be smaller than this area size. In scenarios where the target volume is greater than this limit, IMRT, tomotherapy or IMPT (<28 cm × 38 cm) and multi-isocentric VMAT technologies may be more appropriate. A single isocenter was utilized under the sternum in most dosimetric studies on systems designed for sBBC, such as IMRT, VMAT or helical tomotherapy.[3],[4],[5],[6] This approach can provide a plan that is simpler and easier to administer, but its clinical use may be limited, especially for obese patients and patients with larger volume.[7] Huang et al. have pointed out fixed-jaw IMRT and tangential VMAT treatment techniques as effective methods that can solve the sBBC problem by claiming the advantage of local hot spot control in PTVs.[7] In that study, a mono-isocentric VMAT technique was utilized. sBBC nodal irradiation inclusive for the VMAT plan generally single isocenter was used.[18] In addition, in this patent group, dual isocenter VMAT plan was employed. In this method, 2–5 mm transition errors occurred between the dual-isocenters. However, this did not result in clinically significant dose dispersion alteration, and also the choice of dual isocenter was found to be slightly better than mono isocenter for sBBC.[19] In this study, the inclusion of nodal areas was found to be a relevant plan. In cases with very small areas lacking nods, sBBC irradiation, the dual centre VMAT plan is better suited.

The number of areas or arcs is known to have significant effects on the results of treatment plans in IMRT and VMAT planning. When treating sBBC with IMRT, treatments with more than 10 areas can provide an ideal dose distribution for the target and OARs. However, there are a number of drawbacks in the use of irradiation due to the increase in irradiation duration and MU.[4] In this sense, it is quite logical to limit the number of IMRT areas to 5–7 areas in general. However, MU increase in VMAT plans is at a moderate level compared to that in IMRT. It has already been reported that two-half-arc VMAT plans have better dose distribution compared to one or two-full-arc therapy plans.[4]

Inclusion of local nods to sBBC irradiation, modified hybrid plans were tried to obtain optimal VMAT.[18] Hybrid plans involving VMAT and modified 3D-CRT, PTV diffusion and protection of OAR were improved on. In this work, in addition, it is clarified that hybrid plans can be adjusted by the clinician and to develop a better plan more evaluation has to be made. There are also other studies concerning nodal irradiation and sBBC for VMAT plans.[5],[19] sBBC irradiation plans containing nodal areas prove to be challenging for the physicians. Our results, experience and recommendations for this patent group indicate that application dual center VMAT is more beneficial.

Kim et al. have pointed out that in certain initial TPSs, it was difficult to calculate the exact dose emitted from two separate isocenters when planning RT for sBBC; they indicated that it would, therefore, cause difficulties in understanding the scope of the total target dose, and that it is safer to choose mono-isocentric plans to accomplish the correct therapy plan due to the low planning capacity in the Monaco TPS used for VMAT plans.[20] In that study, IMRT was found to be superior in terms of target and OAR dose distribution. We can say that this was due to low doses occurring at the edges of the areas in mono-isocentric VMAT plans. In dual-isocentric plans, these plans are avoided by being considered as a feared method due to the risk of hot spots and area overlaps. However, because the TPS used in our study was a more advanced version for VMAT planning, the dose given in dual-isocentric four-arc VMAT was administered by calculating the exact dose accurately.

There are limitations in this study: dosimetric comparison data were obtained from two patients. Data from more patients will strengthen this study. However, as there are no standardized guidelines for sBBC RT, this study offers one.

  Conclusion Top

In summary, based on the present study, we can say that it is difficult to determine which technique is suitable for all patients with sBBC, but we can recommend dual-isocentric four-half-arc VMAT as a priority technique in patients who are to undergo breast irradiation only. It can be said that there is no standard technique that can be the best for a suitable plan in cases with larger areas where the regional nodal area is included in the target volume, as expressed in previous studies.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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Yusoff S, Chia D, Tang J, Lu J. Bilateral breast and regional nodal irradiation in early stage breast cancer-a dosimetric comparison of IMRT and 3D conformal radiation therapy. Int J Radiat Oncol Biol Phys 2012;84:S223.  Back to cited text no. 3
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Seppälä J, Heikkilä J, Myllyoja K, Koskela K. Volumetric modulated arc therapy for synchronous bilateral whole breast irradiation A case study. Rep Pract Oncol Radiother 2015;20:398-402.  Back to cited text no. 5
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Huang JH, Wu XX, Lin X, Shi JT, Ma YJ, Duan S, et al. Evaluation of fixed-jaw IMRT and tangential partial-VMAT radiotherapy plans for synchronous bilateral breast cancer irradiation based on a dosimetric study. J Appl Clin Med Phys 2019;20:31-41.  Back to cited text no. 7
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Fogliata A, Vanetti E, Albers D, Brink C, Clivio A, Knöös T, et al. On the dosimetric behaviour of photon dose calculation algorithms in the presence of simple geometric heterogeneities: Comparison with Monte Carlo calculations. Phys Med Biol 2007;52:1363-85.  Back to cited text no. 9
Ulmer W, Pyyry J, Kaissl W. A 3D photon superposition/convolution algorithm and its foundation on results of Monte Carlo calculations. Phys Med Biol 2005;50:1767-90.  Back to cited text no. 10
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[PUBMED]  [Full text]  
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  [Table 1], [Table 2]


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