Murat OKUTAN, Nazan DEDE

The Effect of CTV Shrinkage Margins in Treatment Planning Systems to the Breast Surface Doses

OBJECTIVEThis study aims to evaluate the skin dose variation on a breast phantom for patient with mastectomytreated with bolus intensity-modulated radiotherapy (IMRT) when the clinic target volume (CTV)shrinkage margin is 3 mm and 5 mm.METHODSAlderson Rando phantom computed tomography (CT) scan was performed for two situations: 1-cmbolus and no-bolus. After the CTV organ at risk (OAR) volumes were created in the treatment planningsystem (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. Thetreatment 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.RESULTSWhen the averages of the measurement data for 3-mm CTV shrinkage into the tissue are compared withthe 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-mmCTV shrinkage into the tissue, the difference in surface doses for no-bolus, 50%-bolus, and 100%-bolusplans was determined as 5.4%, 2.6%, and 2.9%, respectively.CONCLUSIONWe recommend that 5-mm shrinkage with 50% bolus (1-cm thickness) should be used for the betterTPS surface dose calculation because the accuracy of TPS calculations increases with the decrease indifferences between the TPS and TLD validation readings.

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