ESTRO 36 Abstract Book

S889 ESTRO 36 _______________________________________________________________________________________________

1 Aarhus University Hospital, Oncology, Aarhus C, Denmark

The influence of the intrafraction breath hold motion on PTV/CTV dose coverage and OARs dose is simulated in Pinnacle TPS (Philips, Madison, WI, USA) with for each beam and each fraction a random shift in the anterior- posterior (AP) direction of each isocenter (with 6 beams for 16 fractions, 96 isocenters are needed). Random shifts were sampled from a Gaussian distribution with standard deviations σ ranging from 0, 2.5, 5, 7.5 and 10 mm. For each standard deviation 3 simulations were performed. Results Figure 1 shows the results of the simulations. As expected the PTV and CTV coverage decreases for higher σ values. The V95% dose coverage of the PTV and the CTV decreases below the clinically acceptable value of 95% when the σ of the anterior–posterior motion exceeds 2.5 and 7.5 mm, respectively.

Purpose or Objective To evaluate the safety of PTV margin reduction in the elective lymph node target (CTV-E) under condition of Volumetric Arc Therapy (VMAT) and daily IGRT. Furthermore, the benefit of margin reduction for Organs At Risk (OARs) was evaluated. Material and Methods Ten locally advanced cervix cancer patients treated from December 2015 until June 2016, were analysed. The patients were treated with 45Gy in 25 fractions with VMAT and whole pelvic irradiation according to the Embrace II protocol. Patients with para-aortic irradiation were not included in this study. Daily image guidance was performed with CBCT, bony fusion and couch correction (translational and yaw). The ITV-45 defined the combined elective lymph node CTV (CTV-E) and the ITV related to the primary tumour. Four different dose-plans with ITV-45 to PTV margin of 0, 3, 5, and 8mm were evaluated. The target constraints were: ITV-45 D99.99>42.75Gy and PTV- 45 D95%≥42.75Gy. CTV-E was assumed to move as a rigid structure as lymph nodes are located mainly in relation to bone and muscles. CTV-E was transferred from plan CT to each CBCT by rigid bony registration with 6 degrees of freedom. The propagated CTV-E was visually validated and transferred back to the plan CT in the position of the patient during treatment. For each of the 4 plans, the accumulated D98 and D99.9 (average of the DVH of the 25 structures) was evaluated. For the 4 plans, V30 and V40 were extracted for bladder, bowel and rectum as well as PTV volumes and body V43. Results Figure 1 shows the accumulated CTV-E D98 for all 10 patients with 0, 3, 5 and 8mm PTV margins. Generally, a wider margin allows a better CTV-E coverage. With a 0mm margin, CTV-E D98 is larger than 43.4Gy for 9 patients and reduced to 42.2Gy for one patient. As for the 3mm PTV margin, all 10 patients have an accumulated CTV-E D98 larger than 43Gy. CTV-E D99.9, with a 3mm margin, reaches its lowest value for patient 2 with 39.3Gy. The second lowest value is 40.6Gy for patient 7. For the eight patients left, a 3mm PTV margin allows a minimum of 41.8Gy for CTV-E D99.9. Table 1 shows for each of the 4 dose-plans the average DVH parameters for the targets and OARs. An 8mm margin (1621cm 3 ) results in increased PTV volume of almost 50% compared with a 3mm margin (1114cm 3 ). V43 is reduced by 469cm 3 when the PTV margin is reduced from 8 to 3mm, corresponding to a relative volume reduction of about 30%. The average volume of bowel receiving more than 30Gy is decreased by 82 cm 3 when the PTV margin was decreased from 8 to 3mm.

Figure 1: The V95% coverage (in %) for CTV (red cubes) and PTV (blue diamonds) based on simulations for different σ values. For σ higher than 7.5 mm the V95% of the CTV is below the clinical acceptable value of 95%. The difference in dose to the OARs σ ≤ 7.5 mm is Planning simulations in Pinnacle showed that the plans are robust and the influence of intrafraction breath hold motion on the dose becomes clinically relevant only when σ > 7.5 mm. Clinical data of the chest wall movement of patients in anterior–posterior motion measured using an in-house developed 3D camera breath hold monitoring system (ref abstract) showed a σ of approximately 2.5 mm. This suggests that for most patients residual intrafraction motion during breath hold as achieved in our institute is small enough to assure a good CTV coverage. In this work only anterior–posterior motion is taken into account for the simulations. Therefore, we need to combine this with other inaccuracies before we can clinically interpret these simulations for reducing the CTV to PTV. Moreover, further investigation is required to determine what threshold should be taken as warning signal of the in-room breath hold monitoring system. Currently we choose to set the threshold value to 7.5 mm (maximum – minimum breath hold). negligible. Conclusion

Electronic Poster: Physics track: Inter-fraction motion management (excl. adaptive radiotherapy)

EP-1640 Dosimetric consequences of PTV margin reduction in cervix cancer radiotherapy with VMAT and IGRT T. Berger 1 , M.S. Assenholt 1 , N. Jensen 1 , L. Fokdal 1 , J. Lindegaard 1 , K. Tanderup 1