ESTRO 36 Abstract Book

S898 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Six prostate cancer patients with transperineal placement of 3mm-long 1mm-diameter polymer-based fiducials underwent 3mm slice thickness plan CT scan on day 14 after markers implantation, which is consider as a safe waiting time according to the literature. All patients were managed with the same IGRT protocol: before each daily treatment, two planar KV images were acquired with the OBI 1.4 system (Varian Medical Systems) at 45º and 315º. A manual marker match between the KV images and the planning CT DRRs was performed and automatically transfered to the treatment couch position to correct the patient position in the three translational directions (rotations were not taken into account). Weekly, after patient re-position and just before session delivery, a CBCT scan is acquired, that is used to assess rectum and bladder filling (slice thickness between 1mm and 3mm) These CBCT images, as being acquired in patient corrected position, have been used to evaluate the FM locations at different times during the course of treatment. A total of 37 CBCT images have been analysed to reconstruct the FM 3D coordinates. The displacement of each FM was calculated relative to its reference position on the reference planning CT, and also shift of the middlepoint of each 3 FM set. The distance between markers in each set at the time of planning CT and during specific evaluated treatment have also been computed. Results The average marker migration observed is 0.68±0.51 mm (range between 0 – 3.90 mm). This observation seems independent of the marker position inside the prostate, but not of the spatial coordinate: the antero-posterior direction presents the largest FM average displacement. Although the average migration observed is low, there are cases among the six patients where the migration observed an specific day was greater than 2mm. This observation may be directly related to the degree of prostate desplacement caused by the influence of the rectum and bladder, and also with the posible pelvic rotation in the moment of daily RT (not corrected with the 2D DRR vs KV image comparison). Changes in distance between pairs of FM in each set have been, on average, 0.12±0.11 mm (range between 0.02 – The low average FM migration observed is expectable, according to the waiting time between marker implantation and the planning CT scan procedure. A futher investigation should be done in order to reduce this waiting time. The fact of having observed cases among all patient with displacement greater than 2 mm should be taken into account in the CTV-PTV margins: an adequate expansión of margins might compensates for this set-up uncertainty. EP-1654 Clinical set up and first results of EPID in vivo dosimetry in an overload Chinese Radiotherapy J. Li 1 , A. Piermattei 2 , P. WANG 1 , S. Kang 1 , M. Xiao 1 , B. Tang 1 , X. Liao 1 , X. Xin 1 , L.C. Orlandini 1 1 Sichuan Cancer Hospital, Radiation Oncology, Chengdu, China 2 Fondazione Policlinico Universitario Agostino Gemelli, UOC Fisica Sanitaria, Rome, Italy Purpose or Objective In vivo dosimetry (IVD) is an important tool able to verify the accuracy of the treatment delivered. In an environment where several linacs of different types support daily heavy treatment workload over different shifts of therapists, physicists and Radiation oncologists, IVD checks can be strongly recommended to avoid 4.38 mm). Conclusion

important dosimetric discrepancies. The work describes the setup of IVD procedure with electronic portal imaging devices (EPID) in an overload radiotherapy clinical workflow, and the preliminary results obtained. Material and Methods 64 patients that underwent a VMAT or IMRT treatments for head and neck, brain, breast, lung, thorax, abdomen and pelvis where scheduled for in vivo dosimetry procedure with EPID. A commercial software (SOFTDISO, Best Medical, Italy) was used at this purpose. Two indexes were analysed: the ratio R between the reconstructed (Diso) and planned (Dtps) isocenter dose (R=Diso/Dtps) and Pγ% obtained performing a gamma analysis between the first EPID image and the next ones acquired. The acceptance criteria adopted for the ratio R was ±5%, while for the 2D γ-analysis in term of Pγ index, we adopted Pγ > 90% with a passing criteria of 3% global difference and 3mm distance to agreement for head and neck treatment and 5%, 5mm for the others districts. The percentage of patients P% with Rmean and Pgmean in the tolerance level P%(Rmean) P%(Pγmean)respectively, and the percentage of IVD test T% with R and Pγ in the tolerance level T%(R) and T%(Pγ), were evaluated. For each district P% take into account the patients with the mean values of the indexes within the tolerance levels, while the T% is referred to the number of tests. If one of the indexes resulted out of tolerance, corrective actions were performed and the test repeated at the next fraction. Results The results of 1211 IVD tests over 64 patients, were reported in Table 1. All the patients analysed shown both indexes (Rmean and Pγmean) in tolerance with the exception of breast and thorax treatments. For VMAT and IMRT thorax treatments P%(Pγ) decreased to 67%. The thorax patients were revised considering the high gradient regions of the isocenter and the positioning set up was optimized. For IMRT breast treatment, P%(Pγ) decreased to 50%: two (over four) IMRT breast patients were revised adjusting the bolus positioning over the mask in order to realign the reproducibility of the treatment (Pγ index) in the tolerance level. Adopting the appropriate corrections, the successive IVD tests guaranteed at the end of the treatment P% values within the tolerance levels. For thorax and breast treatments, due to the limitation of IVD tests acquired, the mean P%(Py) index values after the correction, were again out of tolerance but the effect of the correction was always efficient.