The Estimation of Radiation Dose to Out-of-Field Points of Organs at Risk in Block and MLC Shielded Fields in Lung Cancer Radiation Therapy

Nastaran Sadat  Momeni; Saeed  Afraydoon; Nima  Hamzian; Abolfazl  Nikfarjam; Mahmoud  Vakili Ghasemabad; Sepideh  Abdollahi-Dehkordi; Masoud  Shabani; Mahdi  Dehestani; Adel  Heidari

doi:10.18502/fbt.v10i2.12223

Nastaran Sadat Momeni Department of Medical Physics, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Saeed Afraydoon Department of Medical Physics, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Nima Hamzian Department of Medical Physics, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Abolfazl Nikfarjam Department of Medical Physics, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Mahmoud Vakili Ghasemabad Department of Community and Preventive Medicine, Health Monitoring Research Center, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Sepideh Abdollahi-Dehkordi Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran
Masoud Shabani Department of Radiation Oncology, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Mahdi Dehestani Department of Radiation Oncology, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
Adel Heidari Department of Radiation Oncology, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran

DOI: https://doi.org/10.18502/fbt.v10i2.12223

Keywords: Out Of Field Organs; Multi Leaf Collimator; Block; Lung Cancer; Radiation Therapy.

Abstract

Purpose: Photon-field shaping in radiation therapy with cerrobend block or Multi-Leaf Collimator (MLC) leads to an increase in the scattered dose to the out-of-field Organ At Risk (OAR). This study aimed to measure and compare the healthy organs absorbed dose outside the cerrobend block and MLC shielded field.

Materials and Methods: Computed Tomography (CT) images were taken of a heterogeneous Thorax phantom while the target volume and organ at risk, including the spinal cord, contralateral lung, and heart were contoured. Conformal Treatment planning was performed (POP fields, total dose 40 Gy, 5 fx/week, and 2 Gy/fx) on the Prowess Panter Treatment Planning System (TPS). Irradiation was performed with 6 and 18 Mv X-ray of Siemens Oncor medical linear accelerator, once for the block-shielded field and again for the MLC-shielded field. At each energy, the radiation dose to the contoured out-of-field organs was measured by an ionization chamber and compared.

Results: At both 6 and 18 MV energies, the out-of-field dose in the MLC-shielded fields was significantly lower than in the block-shielded ones (P < 0.001). The out-of-filed dose for contoured organ at risk was not significantly different at 18 MV compared with 6 MV. The dose calculated by the treatment planning system showed that the healthy organs absorbed doses in all conditions were significantly lower than the dosimetry results.

Conclusion: The use of MLC to shield the lung cancer treatment filed reduces the out-of-field OARs dose compared to cerrobend block. This reduction is greater at 18 MV photon beam but this difference is not statistically significant.