Dosimetric Influence of Normalization Points on Post Mastectomy Chest Wall Teletherapy

Abstract

Background: Many advanced radiotherapy techniques had been employed in breast cancer teletherapy, purposely to significantly reduce dose to organs at risk (heart and lungs) with marginal or no compromise in planning target volume (PTV). Majority of used techniques yielded rewarding results in developed countries where facilities, manpower and skills are available. In Nigeria, significant number of post mastectomy breast cancer patients received chest wall irradiation using manual hand planning, therefore, the dose distributions to target volume and organs at risk (OAR) were uncertain. Sokoto centre being the first in the country to use treatment planning system (TPS) focused on identifying planning skills (normalization points) with good 95% dose coverage to PTV, and minimizing dose to OAR. Methods: Eighteen post mastectomy patients (ten rights and eight lefts chest walls) were simulated via computed tomography scan (CT-scan) in supine position with breast board and fudicial markers to demarcate tumour bed borders. Planning target volume (PTV chest walls) and OAR were contoured from the acquired CT images and bi-tangential portals were applied. The energy used from Elekta précised Linac was 6 MV, and dose of 50Gy in 25# was prescribed to each patient. The Upper 1/3rd normalization point (UNP), Lower 1/3rd (LNP) and Inter-field (INP) were sequentially applied as dose normalization points on each planning CT image, dose to PTV and OAR were evaluated using Clarkson and pencil beam calculation algorithms. Statistical analysis was conducted using SPSS Software to study dose distributions of the normalization points. Results: Patients simulated were between the ages of 29-56yrs with mean age of 42yrs. The mean percentage doses from normalization points on left chest walls ranged between 81.7-107.7% to PTV, 13.3-17.8% to the lung and 5.5-6.8% to the heart, the reported hot spots were between 110.7 to 141.5%. Similarly, the mean doses from normalization points on the right chest walls ranged between 81.5-108.8% for PTV, 17.8-23.5% to the lung and 2.7-3.7% to the heart, with hot spots of 108.8-137.9%. The statistical differences using independent-t- test for the normalization points on both left and right chest walls shows p-value < 5%. Conclusion: The three normalization points influenced dose distribution to PTV and OAR differently. The UNP and LNP showed a desired dosimetry with marginal compromise in 95% PTV coverage compared to INP.