SU-E-T-07: Edema Induced Changes in Tumor Cell Survival Fraction and Tumor Control Probability in 131Cs Permanent Prostate Implant Patients.

PURPOSE: The purpose of this study is to estimate the effect of edema, developed during implant procedure, on tumor cell surviving fraction(SF) and tumor control probability(TCP) in the patients of prostate cancer who underwent 131 Cs permanent seed implants. METHODS: The impact of edema on SF and TCP, was calculated using LQ equation extended to account for exponential nature of edema decay, dose delivered to dematous prostate and inhomogeneous dose distribution. Where (1) S(D)=(1/V)Σi=1n [Vpi{1+M0 exp(-λe t)}Si (D)] Si (D)=exp[-αRi (0)∫0t [exp(- λt)/{1+M0 exp(-λe t)}τ/3]dt -ßq(t){Ri (0)∫0t [exp(-λt)/{1+M0 exp(-λe t)}τ/3]dt }2 ] and (2) TCP=exp[-ρVpS(D)] Following parameters, α=0.15Gy-1 , ß=0.05Gy-2 , α/ß=3.0Gy, Tp=42days, µ=61.6d-1 and ρ=1×106 are used to calculate SF and TCP for 31 patients of 131 Cs permanent seed implants for edema half lives(EHL) ranging from 4 days to 34 days and for edemas of magnitudes(M0 ) varying from 5% to 60% of the actual prostate volume. RESULTS: The dose reductions in 131 Cs implants varied from 1.1% (for EHL=4 days and M0 =5%) to 32.3% (for EHL= 34 days and M0 = 60%). These are higher than the dose reduction in 125 I implants, which vary from 0.3% (for EHL= 4 days and M0 = 5%) to 17.5% (for EHL= 34 days and M0 = 60%). As edema half life increased from 4 days to 34 days and edema magnitude increased from 5% to 60% the SF increased by 4.57 log, and the TCP decreased by 0.80. CONCLUSIONS: Compensation of edema induced increase in the SF and decrease in the TCP in 131 Cs seed implants should be carefully done by redefining seed positions with the guidance of post needle plans. The presented model in this study can be used to estimate the SF or the TCP for pre plan or real time permanent prostate implants using day 0 post implant CT images.

Concomitant boost radiotherapy compared with conventional radiotherapy in squamous cell carcinoma of the head and neck--a phase III trial from a single institution in India.

AIMS: To test the efficacy of an accelerated fractionation schedule (concomitant boost) against standard conventional fractionation in squamous cell carcinomas of the head and neck region in our patient population. MATERIALS AND METHODS: Patients were randomised to receive either conventional radiotherapy with 2 Gy/fraction/day, to a dose of 66 Gy in 33 fractions over 6.5 weeks or accelerated radiotherapy in the form of concomitant boost to a dose of 67.5 Gy/40 fractions over 5 weeks (phase 1: 45 Gy/25 fractions/5 weeks and phase 2: 22.5 Gy/15 fractions/3 weeks as a second daily fraction after a 6h gap). The primary and secondary end points were disease-free survival and locoregional control respectively. RESULTS: The compliance was 97.2% and 96.5% in the concomitant boost and conventional arms, respectively. Patients treated with concomitant boost had a better 2-year disease-free survival (71.7% vs 52.17%, P=0.0007) and locoregional control rates (73.6% vs 54.5%, P=0.0006) than with conventional fractionation. On exploratory subgroup analysis, the oropharynx (P<0.001), T4 lesions (P=0.017), N+ disease (P<0.001) and stage IV disease (P<0.001) were statistically significant prognostic variables in favour of the concomitant boost arm. Grade 3 mucositis was seen in 35% of patients in the concomitant boost arm, whereas in the conventional arm only 19% of patients had grade 3 mucositis (P=0.01). The median radiotherapy duration in the concomitant boost arm was 36 days (range 36-53 days), whereas in the conventional arm it was 46 days (range 46-64 days). The mean gap in radiation treatment in the concomitant boost arm was 1.68 days (range 0-14 days), whereas the mean gap in the conventional arm was 1.58 days (range 0-14 days). CONCLUSIONS: Concomitant boost is a therapeutically superior and logistically feasible accelerated radiotherapy regimen in advanced head and neck cancers, especially in the setting of a developing country.

Dosimetric and qualitative analysis of kinetic properties of millennium 80 multileaf collimator system for dynamic intensity modulated radiotherapy treatments.

The aim of this paper is to analyze the positional accuracy, kinetic properties of the dynamic multileaf collimator (MLC) and dosimetric evaluation of fractional dose delivery for the intensity modulated radiotherapy (IMRT) for step and shoot and sliding window (dynamic) techniques of Varian multileaf collimator millennium 80. Various quality assurance tests such as accuracy in leaf positioning and speed, stability of dynamic MLC output, inter and intra leaf transmission, dosimetric leaf separation and multiple carriage field verification were performed. Evaluation of standard field patterns as pyramid, peaks, wedge, chair, garden fence test, picket fence test and sweeping gap output was done. Patient dose quality assurance procedure consists of an absolute dose measurement for all fields at 5 cm depth on solid water phantom using 0.6 cc water proof ion chamber and relative dose verification using Kodak EDR-2 films for all treatment fields along transverse and coronal direction using IMRT phantom. The relative dose verification was performed using Omni Pro IMRT film verification software. The tests performed showed acceptable results for commissioning the millennium 80 MLC and Clinac DHX for dynamic and step and shoot IMRT treatments.

Analytical approach to estimate normal tissue complication probability using best fit of normal tissue tolerance doses into the NTCP equation of the linear quadratic model.

AIMS AND OBJECTIVES: Aims and objectives of this study are to get the best fit of the normal tissue tolerance doses to the NTCP model of the linear quadratic model. METHODS AND MATERIALS: To compute the NTCP, the modified form of the Poisson cell kill model of NTCP, based on linear-quadratic model, is used. The model has been applied to compute the parameters of the NTCP model using clinical tolerance doses of various normal tissues / organs extracted from published reports of various authors. The normal tissue tolerance doses are calculated for partial volumes of the organs using the values of above-said parameters for published data on normal tissue tolerance doses. In this article, a graphical representation of the computed NTCP for bladder, brain, heart and rectum is presented. RESULTS AND CONCLUSION: A fairly good correspondence is found between the curves of 2 sets of data for brain, heart and rectum. Hence the model may, therefore, be used to interpolate clinical data to provide an estimate of NTCP for these organs for any altered fractionated treatment schedule.

Many component model and its parameters to fractionated irradiation.

PURPOSE: In this study, a many component model has been formulated to address the contribution of various bio-molecules in radiation induced biological damage and this model is fitted to radiation isoeffect data to evaluate the parameter alpha/beta. MATERIAL AND METHODS: Every living cell contains a number of organic and inorganic molecules which may act as targets to lead radiation induced damage. During irradiation, many kinds of physical, chemical and biological products are produced which are due to active participation of aforesaid molecules. The many component model, hereinafter called linear-quadratic-cubic (LQC) model, is an attempt to provide a better mathematical formulation which could demonstrate the contribution of various kinds of biomolecules in biological effect of radiation. This model has only one independent parameter alpha/beta and a number of dependent parameters which are the derivatives of alpha/beta. RESULTS: Three methods, for testing the fit of the proposed model and deriving the values of alpha/beta, are employed using multifractionation isoeffect data. These methods, i.e. (logDm-logDn) versus (dn-dm) plot, SFe-plot and logD versus d plot, provide their respective values of alpha/beta as 12.50 Gy, 12.51 Gy and 12.54 Gy for spleen. 7.41 Gy, 7.79 Gy and 7.42 Gy for kidney, 15.87 Gy, 15.09 Gy and 14.69 Gy for colon with 3 h interval, 12.50 Gy, 12.84 Gy and 12.45 Gy for colon with 12 h interval and 8.47 Gy, 8.84 Gy and 8.51 Gy for colon with 24 h interval. Since other parameters of the model are the derivatives of alpha/beta therefore can be derived using the values of alpha/beta. DISCUSSION: Graphical representation of afore said sets of data show that the plotted points have a certain amount of scattering which is not at the wider scale and can be well or less well described by a straight line. CONCLUSION: This model eliminates the shortcomings of LQ model and provides a satisfactory explanation of various radiobiological processes occurring during and after irradiation.