A COMPARATIVE DOSIMETRIC STUDY OF LEFT SIDED BREAST CANCER AFTER BREAST CONSERVING SURGERY WITH 3D-CRT AND VMAT

Purpose: This paper aims to compare dosimetric differences based between three-dimensional conformal radiotherapy (3D-CRT), and volumetric modulated arc therapy(VMAT) techniques of radiotherapy plans for postoperative left breast cancer after breast conserving surgery. Materials and Methods: Nineteen patients with left breast cancer who had received breast conserving surgery without nodal involvement. Two separate treatment plans with an identical isocenter were created to each patient: 3D radiotherapy plan with 2 tangent isocentric beams with field-in-field technique andVMAT plan with Two duplicate sets of partial arcs. Results: The VMAT plan showed superior to PTV dose conformity index (CI), homogeneity


ISSN: 2320-5407
Int. J. Adv. Res. 8 (11), 1088-1093 1089 In addition, the recent meta-analysis of the Early Breast Cancer Trialists Collaborative Group (EBCTCG) showed that the addition of RT after BCS improve significantly the overall survival for patients with node positive breast cancer [3].
However, radiotherapy can cause side effects such as breast fibrosis, changes in the breast appearance and late pulmonary and cardiovascular complications [4].
In recent years, remarkable progress has been made in breast cancer as new surgical techniques, new systemic therapeutic options, better understanding of the biology of the disease and advances in the field of radiation oncology.
Historically, breast radiotherapy treatment planning consisted of two-dimensional planning, used a fluoroscopic technique to establish treatment fields. This was followed by 3-D CRT with two conventional tangential radiotherapy fields [5].
In the modern era of radiotherapy, Volumetric modulated arc therapy (VMAT) is a modern form of Intensitymodulated radiation therapy (IMRT) that delivers the radiation dose continuously as the treatment machine rotates. This technique is much faster than that for fixed field IMRT delivery concentrating the radiation dose to the tumor while minimizing the dose to the organs surrounding the tumor [6].
However, this techniquehas been reported to increase the low dose volumes in contra-and ipsilateral lung and contralateral breast increasing, consequently, the risk for second cancer.
This paper aims to compare dosimetric differences based between three-dimensional conformal radiotherapy (3D-CRT), and VMAT techniques ofradiotherapy plans for postoperative left breast cancer.

Patient selection
We included nineteen patients treated in our department during January 2020 and June 2020 The patients were chosen according to the following criteria:

Target and normal tissue delineation:
The patients were imaged with a CT scanner in treatment position (supine, arms up). CT images were acquired from the level of mandible to the basis of lungs with a slice thickness of 2,5 mm.
The breast Clinical Target Volume (CTV) was delineated based on the RTOG guidelines. Its included all visible breast parenchyma, retracted 5 mm from the skin surface. The Planning Target Volume (PTV) comprised the CTV with a 7 mm circumferential margin to allow for daily set-up variations and account for setup uncertainties and respiratory motion. The volumes outside the body contour and inside the lung were excluded from the PTV.
The organs at risk (OARs) included ipsilateral lung, contralateral lung, contralateral breast, heart, and the spinal cord were delineated.
1090 Treatment planning: Plans were normalized in terms of planning target coverage with a standard dose of 50 Gy in 2-Gy fractions according to the ICRU report number 83 recommendations. The treatment energie was 6 MV. Two separate treatment plans with an identical isocenter were created to each patient: 1. VMAT plans were optimized for the RapidArc technique in the Eclipse treatment planning system (Varian) .
Two duplicate sets of partial arcs were used. One set rotated counterclockwise with collimator rotation of 30° and the other set rotated clockwise with collimator rotation of 330° 2. 3D CRT radiotherapy consisted of the isocentric field-in-field technique plan included 2 tangent beams. Beam weights and wedge angles were adjusted to provide homogeneous dose distribution and conformal dose to the CTV and PTV.
All plans used the anisotropic analytical algorithm (AAA version 10.0.28) for dose calculation with 2.5 mm calculation grid and heterogeneity correction.

Dosimetric evaluation parameters:
1. For the PTV, the dose to 98 and 2 % of the volume (D98 % and D2 %, respectively) and the part of the PTV receiving more than 107 % of the prescribed dose (V107 %) were evaluated. 2. The plan qualities were compared by the homogeneity index (HI) and conformity index (CI) as proposed by Paddick et al, as follow:

Statistical analysis
All results were compared and analyzed using using SPSS software, version 10.0, and a statistical significance level of 0.05 was used (p < 0.05).

Ethical considerations
The different treatment techniques have been applied to the patients' dataset without any clinical application. This activity does not require an ethical approval

Dose analysis of planning target volume
We have summarized the dosimetric results of PTV in Table 1.
In contrast there was no statically significant difference in the volume receiving 10Gy.

Discussion:-
Volumetric modulated arc therapy (VMAT) is a radiation technique delivers radiation on a linear accelerator using a cone beam that continuously rotates around the patient, which can achieve highly conformal dose distributions with improved target volume coverage and sparing of normal tissues compared with conventional radiotherapy techniques [7].
It's also used fewer MU and reduced treatment time compared with conventional radiotherapy.
Certainly, arctherapy is an innovative technique which has shown its superiority over conventional radiotherapy in the treatment of several cancers such as: head and neck, prostate, brain tumors. However, its use to irradiate mobile tumors is controversial given the obligation to add large margins to the target volume in order to account for respiratory movements thus exposing healthy tissue to higher doses and therefore it is not considered a standard irradiation technique in breast cancer [8].
At present, the main methods of postoperative radiotherapy for patients with advanced breast cancer include threedimensional conformal radiotherapy.
In this dosimetric study, we demonstrated that arctherapy improved coverage of the PTV with ≥99.6 % of the PTV and the best target volume conformity, thus overcoming the problem of inhomogeneity encountered during irradiation of the breast in conformational radiotherapy with a significant reduction in the percentages of overdose "hot spots".
Planning is harder especially if the cancer is in the left breast, considering the risk of ischemic heart disease after radiotherapy for breast cancer increases linearly with the mean dose to the heart by 7.4% per gray (Gy) [8], [9].
Radiation-induced pneumonitis risk is a rare radiotherapy complication in breast cancer patients after radiotherapy not exceeding 2%. Willner et al. [10] suggested that the incidence of radiation induced pneumonitis increased by 10% for every 10% increase in V10Gy.
Our study confirms these findings especially in higher-dose areas. However,all low dose parameters of ipsilateral lung,including V14Gy, and V20 Gy, are non-significantly smaller in VMAT plans compared with those in the corresponding 3D plans.
Conventionally, the whole breast radiation treatment is delivered with patient in the supine positionand its the position we have chosen for our patients. The prone treatment position is often proposed for patients with large or penduluous breasts to reduce the dose to ipsilateral lung and heart, and to increase the dose homogeneity by displacement of breast tissue away from chest wall and torso. The usage of VMAT techniques to decrease the dose to heart might be beneficial also in prone position. However, further studies are needed to evaluate the potential of VMAT techniques in prone treatment position. [11] Conclusion:-In our study,we note that VMAT compared with 3D planhas demonstrated the combined advantages in PTV dose coverage with better conformity.
The doses to heart, the ipsilateral lung and spinal cord were significantly reduced with VMAT.
However, further studies evaluating the clinical outcome of treatments are needed to proof the clinical value of this radiation technique.