One of the most common cancers among men in Poland is prostate cancer, and one of the methods of its treatment is Simultaneous Integrated Boost (SIB). Technique allows the simultaneous delivery of different dose levels to different target volumes within a single treatment fraction.
Due to the location between the sensitive organs, it is necessary to achieve a rapid dose gradient around Planning Target Volume (PTV) and one way to achieve this is to use The Normal Tissue Objective (NTO) in the Treatment Planning System (TPS) Eclipse (Varian Medical Systems, Palo Alto, CA, USA).
The study compares plans without NTO, automatic NTO and with different combinations of the priority p (50, 200 and 500) and the fall-off k (0.05, 0.2 and 1). For the evaluation of the plans, dose distributions, conformity index (CI) and gradient index (GI) as well as doses in organs at risk (OAR) were compared.
The dose distribution shows that NTO is an effective tool for dose reduction around PTV. The conformity index determines the adjustment of the isodose shape to the target area and remained at a similar level on all plans, with an average value of 0.529 ± 0.196. The gradient factor is the difference between the equivalent sphere radius of the prescription and half-prescription isodoses, so its lower value means a greater dose decrease beyond PTV. This is visible for higher priority plans, but unfortunately it is related to a decrease in PTV coverage. Dose analysis in OAR showed that NTO used in prostate cancer planning, probably due to its anatomical location, does not have a significant dose reduction effect.
Further research using a larger sample and different NTO parameters or changing the location of interest is needed.
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Caldeira A, Trinca WC Mr, Flores TP Ms, Obst FM, Brito CS, Grüssner MM, Costa AB. The Influence of Normal Tissue Objective in the Treatment of Prostate Cancer. J Med Imaging Radiat Sci. 2020 Jun;51(2):312-316
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