The Effect of CTV Shrinkage Margins in Treatment Planning Systems to the Breast Surface Doses
Murat OKUTAN1,Nazan DEDE2
1Department of Medical Physics, İstanbul University Oncology Institute, İstanbul-Turkey
2Department of Medical Physics, Dr. Lütfi Kırdar Kartal Training and Research Hospital, İstanbul-Turkey
DOI : 10.5505/tjo.2018.1806 OBJECTIVE
This study aims to evaluate the skin dose variation on a breast phantom for patient with mastectomy treated with bolus intensity-modulated radiotherapy (IMRT) when the clinic target volume (CTV) shrinkage margin is 3 mm and 5 mm.

Alderson Rando phantom computed tomography (CT) scan was performed for two situations: 1-cm bolus and no-bolus. After the CTV organ at risk (OAR) volumes were created in the treatment planning system (TPS) using phantom image, no-bolus, 50%-bolus, and 100%-bolus IMRT plans were studied. The treatment plans for these three situations were made separately for 3-mm and 5-mm CTV shrinkage into the breast surface. The energy photon beams of 6 MV were used for the treatment plans. The treatment plans were made using the IMRT technique to give a dose of 50 Gy in 25 fractions to CTV. Measurements were made with thin thermoluminescent dosimetry (TLD) chips. The TLD average readings and TPS readings at the same point were compared.

When the averages of the measurement data for 3-mm CTV shrinkage into the tissue are compared with the values obtained from the treatment planning system, the difference in surface doses for no-bolus, 50%-bolus, and 100%-bolus plans was determined as 20.3%, 18%, and 12.6 %, respectively. For 5-mm CTV shrinkage into the tissue, the difference in surface doses for no-bolus, 50%-bolus, and 100%-bolus plans was determined as 5.4%, 2.6%, and 2.9%, respectively.

We recommend that 5-mm shrinkage with 50% bolus (1-cm thickness) should be used for the better TPS surface dose calculation because the accuracy of TPS calculations increases with the decrease in differences between the TPS and TLD validation readings. Keywords : Bolus; CTV shrinkage; IMRT; surface dose; TPS accuracy