ESTRO 37 Abstract book

S1083

ESTRO 37

A Monte-Carlo (MC) code which was initially developed to model radiation induced cell kill and cell repopulation [3] is modified to include natural cell death as well. Results We present in figure 1 distributions of the surviving cells before and after (bold lines) an irradiation for the last four fractions of a six-fraction regime characterized with cell survival probability per fraction p s =0.4 (p s ( T n-1 ) = (p s ) n ), b =0.16d -1 , d =0.06 d -1 , N= 30. Dotted lines represent the case when only cell repopulation is considered, solid lines correspond to the case of both cell repopulation and natural cell death being taken into account. The analytically calculated TCP is 14.74% and the one obtained using the MC method based on 1.5x10 5 simulations is 14.68%. If there is no natural cell death the TCP calculated via both methods is 2.88% and 2.91% correspondingly.

where salivary ejection fraction (sEF) is determined after lemon juice stimulation at 15 min after TcO 4 - activity administration. Patients received image guided IMRT with radical or postoperative intent using 6 MV photons. The mean dose within the total volume of parotid glands is used as a dosimetric parameter. Both functional parameters are measured at baseline and 6 months after radiotherapy. Response is defined as relative proportions rF = F(6 mo)/F(baseline) or rEF = sEF(6 mo)/sEF(baseline) from baseline values. Additionally, rF results are compared with data reviewed in the QUANTEC salivary gland article (Deasy JO et al. IJROBP 76: Supplement, S58-S63, 2010). Results Averaged data points arranged in five groups are plotted as a function of mean parotid dose (stimulated flow: both parotids averaged, scintigraphic ejection fraction: per each gland) (Fig 1). When linear model for both total saliva flow and single gland ejection fraction are fitted a few interesting findings arise: - both rEF and rF lines intersect x axis at around 50 – 55 Gy indicating total loss of salivary function. - the rEF line intersects y axis at 1.28; this is explained either by linear model inadequacy at low doses (sigmoidal better?) or functional compensation of the remaining functional gland at 6 months after therapy. - the rF line intersects y axis at 0.8; this is explained by the omittance of other glands (submandibular and minor glands) from the flow model: the parotids are responsible of about 70 % of stimulated flow. - D 50 values for both curves are almost equal when calculated from the intersect point with y-axis: D 50 (rF) = 25.0 Gy, D 50 (rEF) = 26.7 Gy

Conclusion The results from the MC simulations show perfect agreement with the TCP values calculated using the derived analytical expression for various values of the model parameters and various fractionation regimes. The difference between the distributions obtained with and without accounting for natural cell death implies that natural cell death may have a positive impact on the tumour treatment outcome depending on the values of cell birth and natural cell death rates. A future analysis of TCP experimental animal data would help to estimate the impact of natural cell death. 1. Zaider M, Minerbo GN. Phys Med Biol 2000;45(2):279-93. 2. Stavreva N, Stavrev P, Warkentin B, Fallone BG. Med Phys 2003;30(5):735-42. 3. Stavreva N, Stavrev P, Fallone GB. Physica Medica (2009) 25, 181-191 EP-1989 Linear model for salivary gland dose dose response M. Tenhunen 1 , L. Tuomikoski 1 , J. Collan 1 , V. Loimu 1 , K. Saarilahti 1 1 Helsinki University Central Hospital, Cancer centre- Department of Radiotherapy, Helsinki, Finland Purpose or Objective A linear dose response model is tested for stimulated total salivary flow and individual parotid gland function. Parotid glands are considered the main source of stimulated salivary flow (60 – 70 %) and at least one parotid gland can usually be restricted to a mean dose level < 25 Gy during radical radiotherapy of head and neck tumours. Material and Methods Fifty head and neck cancer patients were treated with bilateral radiation therapy in a prospective trial. Salivary gland function was determined using two different methods: stimulated total salivary flow (F , averaged over 15 min) and dynamic salivary gland scintigraphy,

Conclusion A linear model as a function of mean dose within parotid glands fits quite well with the observations of total stimulated salivary flow, and a single parotid gland function described as scintigraphic ejectionn fraction. No data point at low doses <10 Gy exists and the linear model needs to be confirmed with the more comprehensive data set (e.g. unilateral neck treatments) to cover the whole dose range. EP-1990 Skin DVHs predict cutaneous toxicity in Head and Neck cancer patients treated with Tomotherapy M. Mori 1 , I. Dell'Oca 2 , S. Foti 2 , G.M. Cattaneo 1 , R. Calandrino 1 , N. Di Muzio 2 , C. Fiorino 1 1 San Raffaele Scientific Institute, Medical Physics, Milan, Italy 2 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy Purpose or Objective For Head and Neck (HN) cancer patients the dose received by the skin is critical with respect to the risk of developing acute and severe skin toxicities. However,