520 Application of an offline patient set-up procedure using MVCT for prostate cancer helical tomotherapy

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5222 plane (2D), threedimensional conformal (3D), and stereotactical (SRX) to be able to evaluate the time spent on each of them. Results: Flowcharts for different types of patients are presented, giving detailed information of the agendas and activities (Appointments and Tasks) involved. Waiting times beetween all steps are measured monthly, the total waiting time for a new patient has an average value of 36.2 days. An analysis of the dosimetry step, for different complexities, shows delays of 1, 2.3, 2.5 and 1.8 days for 1D, 2D, 3D and SRX dosimetries respectively. Conclusion: The whole management of the Radiotherapy and Medical Physics department can be implemented using Varis 7.0 applications. An accurate tracking of time delays of different steps allows the analysis of trends, the adoption of correcting actions in the steps that proved to be unefficient, and enforces the process of continuous improvement. 520 A p p l i c a t i o n of an offline p a t i e n t s e t - u p p r o c e d u r e using MVCT f o r p r o s t a t e cancer helical t o m o t h e r a p y

S. Broqqi 1, C. Fiorino I, A. Tavilla 2,C. CozzarinP, N. Di Muzio2, G. Cattaneo I, F. Fazio2'3, R. Calandrino 1 1Institute H. San Raffaele, Medical Physics, Milano, Italy 2Institute H. San Raffaele, Radiotherapy Department, Milano, Italy 3IBFM CNR, University of Milano Bicocca, Ospedale S. Raffaele, Milano, Italy Aim: To estimate systematic and random set-up error for prostate irradiation by means of the MVCT of a Helical Tomotherapy (HT) unit with an off-line set-up correction protocol. M a t e r i a l and methods: 10 patients treated with HT with a hypofractionated schedule for prostate adjuvant radiotherapy were selected. MVCT scans of the P-i-V were consecutively acquired prior to treatment for the first 4 sessions and for each scan MVCT images were automatically matched on bone structures with reference KVC-I images, taking only translations into account. Set-up corrections were applied only if one of the estimated shifts was greater than 6mm; otherwise, no corrections were applied and systematic and random set-up errors were estimated as mean and standard deviation of the shifts reported for the first 4 fractions: a definitive set-up correction of systematic error was then applied at day 5 if it was found to be greater than 3 mm at least one direction. Set-up was then monitored with weekly MVCT, by re-acquiring the scan prior to treatment and rematching bone structures: if the estimated shifts were greater than 3 mm a set-up correction was re-applied; otherwise no shifts were implemented. Results: Considering the first 4 sessions, the average shifts relative to the planned scan images, were estimated equal to 0.24, -3.6 and 10.83 mm respectively in lateral (x), craniocaudal (y) and vertical directions (z). The vertical disagreement, explicable by a sag of the couch during longitudinal movement, is then reduced to an average shift <1ram, by considering the difference of the patient's position relative to the first fraction. For all patients, systematic setup error (standard deviation of the mean values) for the first 4 sessions was estimated equal to 2.9, 2.5 and 3.04 mm respectively in x, y and z directions with a corresponding random component (mean values of the standard deviations) equal to 1.7, 0.9 and 1.2 mm. The residual error, estimated by the post-correction weekly MVCT, was always within 3mm. The most variable movements were estimated in the x-direction, probably due to a different set-up procedure for the radiographer, relative to a conventional Linac. Conclusions: Using MVCT during the first week of the treatment was found to be effective in significantly reducing systematic set-up error. A new investigation with daily MVCT is now in progress in order to assess the minimum number of treatment sessions necessary to correctly estimate

systematic set-up error. 521 T r e a t m e n t planning c o m p a r i s o n b e t w e e n c o n f o r m a l r a d i o t h e r a p y and helical t o m o t h e r a p y in case of s t a g e I I I NSCLC

G. Cattaneo I, I. Dell'Oca 2, S. Broggi 1, S. Molinelli1'3, N. Di Muzio2, C. Messa4, F. Fazio2'4, R. CalandrinoI 1Institute H.S. Raffaele, Medical Physics, Milano, Italy 2Institute H.S. Raffaele, Radiotherapy Department, Milano, Italy 3Scuola di Specializzazione in Fisica Sanitaria, Universit~ degfi Studi di Milano, Italy 4Institute H.S. Raffaele, IBFM CNR, University of Milano, Bicocca Milano, Italy I n t r o d u c t i o n : Helical Tomotherapy (HT) is a novel form of IMRT delivered on a continuous helix. The radiation source (6 MV beam) is collimated to a fan beam which is modulated with a binary MLC. The degree of conformal irradiation and intensity modulation is achieved by a selection of fan beam thickness, pitch factor, and modulation factor. Aim: Subsequent to recent installation and clinical activation at our institute of an HT (HiArt 2) unit: to investigate the impact of HT upon the dose delivered to target volume (PTV) and organs at risk (OARs) when compared to our routinely delivered 3D conformal radiotherapy (CRT) in the case of patients affected by stage III NSCLC. M a t e r i a l s and methods: 6 stage III inoperable NSCLC patients were scheduled to receive 61.2-70.2 Gy, 1.8 Gy/f. Two treatment plans were developed subsequent to delineation of OARs (lung parenchyma, spinal cord, oesophagus, heart) and target volumes (CTV based on PETCT): a CRT plan (MLC, 6 MV beam, Eclipse Varian TPS) and an HT plan (fan beam thickness 2.5 cm, mean pitch value 0.3, modulator factor <3). Based on our previous CRT experience, dose volume constraints for P-rv coverage and OAR sparing were assessed for the HT inverse planning with highest priority PTV coverage and spinal cord sparing. A moderate modulation factor was used (_< 3) in order to achieve a reasonable HT delivery time. The two plans were compared in terms of DVHs and dose statistics on PTV and OARs; a statistical analysis was performed using nonparametric Wilcoxon matched pairs tests. Results: HT improved dose homogeneity when considering Prv (369_+137cc): the fraction of PTV receiving more than 100% of the prescribed dose increased from 83% (CRT) to 93% (HT) (p<0.05) and the standard deviation of dose distribution was 2.3 Gy (CRT) and 1.4 Gy (HT). The oesophagus was better spared by HT; Dmax decreased from 60 Gy (CRT) to 53 Gy (HT) (p<0.03) and Dmean from 27 Gy to 23 Gy (p<0.03). No important differences existed for spinal cord or heart: both plans resulted in Dmax < 40 Gy for the spinal cord and V45<16% for the heart. For lung parenchyma, Dmean was 19.4 Gy (CRT) versus 17.3 gy (HT); moreover, V30 was lower in each case with HT (26% vs 18%, p<0.03).The mean tomotherapy delivery time for 1.8 Gy was 5.6+0.8. Conclusions: Preliminary findings obtained in case of stage III NSCLC suggest that HT has the potential to improve therapeutic ratio for certain patients, when compared with CRT, by means of dose escalation strategy. 522 The e f f e c t of ionization c h a m b e r sleeves on the crosscalibration p r o c e d u r e r e c o m m e n d e d by the I A E A TRS3 9 8 for high e n e r g y electron beams

P. Carrasco, N. ]ornet, C. Pino, J. Mart[n-Viera, T. Eudaldo, M. Ribas Hospital de la Santa Creu i Sant Pau, Servei de Radiofisica i Radioprotecci6, Barcelona, Spain The IAEA TRS-398 provides a Code of Practice for reference