ESTRO 37 Abstract book

S1100

ESTRO 37

and r d . Applying parameters for another cell-line gave a smaller effect on the SOBP than varying the HSG parameters separately. Still, an increasing dose was observed in the SOBPs in these cases. Hence, use of appropriate parameters in the MKM is important to obtain a flat plateau region in the SOBP. Parameter sensitivity analysis will further be performed using inhomogeneous patient anatomy. EP-2013 Predicting growth hormone deficiency after childhood cancer from hypothalamic-pituitary structures C. Stokkevåg 1 , L. Toussaint 2 , L. Muren 2 , C. Pedro 3 , R. Mikkelsen 4 , N. Birkebæk 5 , H. Schrøder 6 , Y. Lassen- Ramshad 6 1 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway 2 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 3 Instituto Português de Oncologia de Lisboa Francisco Gentil, Department of Radiotherapy, Lisbon, Portugal 4 Aarhus University Hospital, Department of Neuroradiology, Aarhus, Denmark 5 Aarhus University Hospital, Paediatric Department, Aarhus, Denmark 6 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Abnormalities in hypothalamic–pituitary axis (HPA) function are frequently seen in childhood cancer survivors after cranial radiotherapy (RT). Growth hormone (GH) deficiency (GHD) is usually the first and most frequent endocrine problem, with both incidence and severity increasing with time. There are known differences in tolerance doses to the hypothalamus and pituitary gland, but the combined effect and radio-sensitivity of small subunits of the HPA are less known. The aim of this study was to investigate feasibility of differentiating doses to HPA substructures and to explore predictive strategies of GHD based on radiation dose to individual and combined HPA substructures. Material and Methods Treatment plans of eleven paediatric patients (aged 4-11 years) receiving cranial photon RT with prescription doses of 40-60 Gy (1.6-1.8 Gy/fraction) were analysed. Only patients with tumors not invading the HPA were included. Five of the eleven patients were diagnosed with GHD after two year follow-up (patient VII-XI). Substructures of the HPA including the hypothalamus, pituitary gland, arcuate nucleus and the infundibulum were contoured on CT/MR images (Fig 1) and dose/volume data from the substructures and the full HPA were analysed. Level of GH and risk of GHD were estimated by three approaches: i) using a volume-based model (A) and a mean dose-based model (B) correlating peak growth hormone levels to hypothalamic dosimetry and time after RT (IJROBP 2002, JCO 2011); ii) estimating risk of GHD according to mean pituitary dose (model C) threshold at 30 Gy (J Pediatr 1991); and iii) adopting pragmatic radiosensitivity levels of the hypothalamus (16 Gy) or the pituitary gland (30 Gy) to both the arcuate nucleus and infundibulum to predict risk of GHD.

small, they increased when approaching the particle range. The effect of varying the parameters on the SOBP increased with increasing variation, especially when varying r d and R n which completely altered the SOBP (Figure 1B). However, the resulting percentage change in the dose distributions was generally less than the percentage change of the parameters (Table 1). Similar results were obtained for single SOBPs at other dose levels. Also, with two opposing SOBPs the dose gradients disappeared, giving a more homogeneous dose to target. When applying parameters related to the other cell-lines, there was an approximately constant difference in the z- values from the HSG z-values, throughout the beam (Figure 1C). The SOBPs estimated applying these parameters showed in general a higher dose than the HSG SOBP, with a slight increase in dose towards the particle range (Figure 1D).

Conclusion Variations in input parameters altered both the shape and magnitude of the SOBP, particular for variations of R n