P33. Dosimetric CT selection for lung cancers

Introduction Search the computerized tomography (CT) on which the dosimetric calculation is more representative of the delivered dose. Static fields were studied. Materials and Method For 5 patients with non-small cells lung cancer locally advanced (IIIA to IV TNM stage), 4DCT (taking account of respiration) and helical CT (CTHel) were carried out. 10 CT taking account a different respiratory phase (CTX), CTAIP (Average Intensity Projection) and CTMIP (Maximum Intensity Projection) have been reconstructed. A single Physician has delineated CTV and organs at risk (OAR: Spinal cord, Esophagus, Trachea, Lung, Heart) for all CT (X, AIP, MIP and Hel). The prescription was 66 Gy in 33 fractions over 1 cm PTV around the CTV. For each patient, 2 planning have been done: Planning on CTHel using volumes defined on CTHel ( PLAN Hel ) . PLAN Hel (with fixed monitor unit) was applied to the 10 CTX and CTAIP ( PLAN Hel _ AIP ) . Planning on CTAIP using volumes defined on CTAIP ( PLAN AIP ) . PLAN AIP (with fixed monitor unit) was applied to the 10 CTX and CTHel ( PLAN AIP _ Hel ) . The CTX calculations were averaged and the result was considered as the delivered dose ( PLAN AIP _ REF or PLAN Hel _ REF ) . DVH (Dose Volume Histograms) were exported and compared to PLAN AIP / Hel and PLAN AIP _ Hel / Hel _ AIP . For each organ/CTV, volume and dose differences were calculated between ≪ PLAN xxx _ Ref and CTAIP calculation≫. They were also calculated between ≪ PLAN xxx _ Ref and CTHel calculation≫. Were extracted from this 4 populations: mean difference, standard deviation, maximal difference, integral difference and total volume difference. These indexes were statistically compared using Mann–Whitney test. Results All calculated indexes are lower in the case of comparaison between ≪ PLAN XXX _ Ref and AIP calculation≫. All mixed patient, all mixed organs, 88.1% of differences between ≪ PLAN xxx _ REF vs CTHel calculation≫ and ≪ PLAN xxx _ REF vs CTAIP calculation≫ are significantly different. Conclusion Results combination show that the calculated on AIP CT is more representative of the delivered dose in the case of broncho-pulmonary cancers regardless target volume or organs at risk.