Background: Accelerated partial breast irradiation (APBI) is a method in which just bed of lumpectomy with a margin of 1–2 cm is irradiated. Regarding advantages of APBI to whole-brain radiation therapy (WBRT) and limitations for performing other techniques, we compare external beam radiation therapy (EBRT) with three-dimensional conformal radiation therapy (3DCRT), as a type of APBI technique.
Methods: Dosimetric parameters including uniformity index (UI), conformity index (CI), and homogeneity index (HI) beside heart and lung doses were assessed and compared in two techniques. CT images of 24 patients with left-sided breast cancer after lumpectomy were selected. Patients were categorized into three groups based on the volume of breast, respectively, ≤ 1000 cc, 1000–1500 cc, and ≥ 1500 cc. CI, HI, UI and DVH were calculated by DosiSoftIsogray treatment planning software.
Results: Results show the value of UI in APBI method is more than EBRT method significantly (p=0.004). Moreover, that CI in APBI method was more than EBRT (p=0.0000) and nearer to 1. There was no significant difference between HI values between APBI and EBRT methods. As the volume of breast gets bigger, HI values rise, meaning worse homogeneity.
Conclusion: APBI method may be a good method for minimizing side effect and minimizing treatment periods.


Keywords: partial radiation therapy, external beam radiation therapy, breast cancer


INTRODUCTION


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The most common cancer in women is breast cancer among other cancer, with probability in order of 28.1 in 100,000.1 In 2016, 246,660 new cases of breast cancer were diagnosed comprising 29 percent of all cancers within which 40,450 died, meaning 14 percent mortality.2

Prognosis and treatment method generally depend on stage and vast of metastasis. Method of treatment principally includes surgery (lumpectomy and mastectomy), radiotherapy and systematic treatment (chemotherapy and hormone therapy).3,4

After breast conserves surgery, radiotherapy may reduce the incidence of recurrence to half and mortality rate to 1/6. In many cases, breast-conserving surgery can eliminate any diagnosed microscopic disease. Nevertheless, some microscopic tumors may persist and if not treated result in recurrence or metastasis or both.5 Radiotherapy after surgery reduces local recurrence from 27.2% to 8.8%.6

Breast-conserving radiotherapy is divided generally into two methods: external beam radiation therapy to the whole breast and partial breast radiation.7

External beam radiation therapy (EBRT) involves 50–55 total doses with a daily dose of 1.8–2 Gy to the breast in a period of 5–6 weeks.8

Disadvantages of EBRT include cancer induction to contralateral breast,9 dose receiving by adjacent anatomies like coronary arteries and other sensitive organs10 and requiring patients to be visited in the department for 6–7 weeks.9

Partial breast irradiation (PBI) has been proposed as a solution for the above problems.11 Accelerated partial breast irradiation (APBI) is the method in which just a bed of lumpectomy with a margin of 1–2 cm is irradiated, while fraction dose is escalated and target volume is diminished. The technique let treatment time to be shortened.12,13 As a smaller portion of the breast is irradiated, the number of time must have radiation is reduced, making it more comfortable for patients and their family.

Current researches show that PBI results in less local recurrence rate that is comparable to the recurrence rate of whole breast irradiation. Moreover, the cosmetic consequence was better than for most patients making it a worthy option for patients with smaller tumors.

APBI is performed by two methods: brachytherapy and 3D external conformal radiation therapy.14 Volumetric modulated arc therapy (VMAT), three-dimensional conformal radiation therapy (3DRT), intensity-modulated radiotherapy (IMRT) and proton beam therapy are among 3D external conformal radiation therapy techniques.15

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Because of the three-dimensional structure of the breast, an accomplishment of uniform dose distribution in breast volume is laborious. Treatment planning to achieve uniform dose, due to the volume and shape of breast and chest wall movement, is challengeable.16

Regarding the advantages of APBI to whole-brain radiation therapy (WBRT) and limitations for performing other techniques, we compare EBRT with 3DCRT, as a type of APBI technique. Dosimetric parameters including CI, homogeneity index (HI), and UI beside heart and lung doses were assessed and compared in two techniques.

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