Abstract Book

S1126

ESTRO 37

were analyzed. In 13 sessions the setup exceed the threshold requiring repositioning. Data and statistical analysis of the mean systematic error, the standard deviation of the systematic setup error and the standard deviation of the random setup error of the two correction protocols are displayed in table 1. A statistically significant greater range of displacement in the on-line group was seen.

to each repeated CT scan. The dose delivered to 98% of the ITV (D 98% ITV) as well as the OAR doses were evaluated. The selected OAR dose parameters were bone marrow V 10Gy 1 and V 20Gy , sacrum D 50% 2 , whole bowel V 15Gy 3 and the entire DVH of the whole bowel. For all repeated CT scans the ITV target coverage (D 98% ) was calculated and presented as function of the bladder volume difference with the plan CT scan. The relative intrafraction bladder volume differences between the first two repeated CT scans were compared with the relative interfraction bladder volume differences between the plan CT scan and the other repeated CT scans. Results For both IMPT as well as VMAT treatment plans large spread in D 98% ITV was observed when the bladder volume difference between the plan CT and the repeated CT was more than 140 ml. IMPT and VMAT treatment plans resulted in comparable D 98% of the ITV (Figure 1) while the bone marrow V 10Gy and V 20Gy, the sacrum D 50% and whole bowel V 15Gy were lower in both types of IMPT plans compared to the VMAT treatment plans (Table 1). The DVH of the whole bowel showed a lower mean dose for the IMPT plans compared to that planned with the VMAT technique (Figure 1). The mean relative intrafraction bladder volume difference was 50,2% (range 19,2% - 87,6%) and the mean relative interfraction bladder volume difference was 158,0% (range -4,9% - 726,3%).

The required width of the margins CTV-PTV for the patients treated with our off-line NAL protocol after applying all the requirements of the formula were 6,4 mm/5,4mm/6mm in the AP/CC/LAT axes respectively. With the data obtained in the on-line group we calculated the required CTV-PTV margins: 3,8 mm/3,5 mm/4,3 mm in the AP/CC/LAT axes respectively. We have also calculated the width of the CTV-PTV required margins in the on-line group if we had not used the daily corrections of the protocol. These margins were: 8,3mm/7,8mm/8,2mm in the AP/CC/LAT axes respectively. Conclusion The CTV-PTV 6mm margin used in our institution is inadequate especially in the anterior/posterior axe when using a NAL off-line protocol despite its tight threshold. The On-line protocol is better in detecting gross deviations in the set up protocol with the clinical and dosimetrical consequential benefit. The analysis of the daily interfractional prostatic movements with the fiducial markers shows that the off-line NAL-3 correction protocol is suboptimal, requiring wider CTV-PTV margins. An On-line technique such as IGRT with fiducials markers deserves necessary for treating localized prostate cancer patients with IMRT. EP-2056 In silico evaluation of VMAT and IMPT against inter- and intrafraction changes for cervix cancer E.M. Gort 1 , J.C. Beukema 1 , M.J. Spijkerman-Bergsma 1 , S. Both 1 , J.A. Langendijk 1 , W.P. Matysiak 1 , C.L. Brouwer 1 1 University of Groningen- University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective Interfraction motion should be accounted for in effective treatment of cervical cancer using external beam radiotherapy, and is mainly caused by differences in bladder filling. This in silico planning study was performed to evaluate the robustness of two- and four- field IMPT and 2-arcs VMAT plans for locally advanced cervical cancer patients in terms of target coverage and OAR dose. Material and Methods Three patients having six repeated CT scans (five weekly CTs plus one scan acquired 10 minutes after the first repeated CT) were retrospectively included in the study. The ITV 45Gy was delineated according to EMBRACE II intermediate IGRT protocol and three different primary treatment plans were created: a two-field IMPT, a four- field IMPT and a VMAT plan. Each repeated CT scan was re-contoured and registered to the plan CT scan and subsequently the primary treatment plan was translated