ESTRO 36 Abstract Book

S441 ESTRO 36 2017 _______________________________________________________________________________________________

7-field 6MV IMRT for a conventional Elekta Agility linac (Elekta AB, Stockholm, Sweden), 6MV FFF single 360° arc VMAT using Pinnacle 9.10 (Philips Radiation Oncology Systems, Fitchburg, WI) for a non-MRL and CyberKnife treatment using Multiplan (Accuray inc, Sunnyvale, CA). Plans were acceptable if the 16 dose constraints of the PACE trial (NCT01584258) were achieved, without a major variation to the protocol. Results

though robustness for anatomical and posture variations is possibly an issue. Compared to conventional plans, the beams are not fully opposing and fields cannot be opened manually outside the outer contour of the breast and the body. Therefore, in this study we evaluated the robustness of both an IMRT and a VMAT technique for daily variations in patient positioning in comparison to our conventional technique. Material and Methods 20 Patients treated with a dose of 16x2.66 Gy using a conventional technique to the breast and axillary lymph nodes levels I to IV (Figure 1a) were replanned with both an IMRT and a VMAT technique using Pinnacle autoplanning. The IMRT technique consisted of 6 beams with 20 o spacing, while the VMAT technique consisted of opposing pairs of 24 o arcs (Figure 1). The delivered dose was calculated using the cone beam CT (CBCT) (Elekta XVI) images for each fraction to quantify the influence of patient positioning, both for an online and offline correction protocol. Contours were transferred from planning CT to CBCT by deformable image registration using Mirada RTx. Density overrides were applied to account for imperfections in Hounsfield unit values on the CBCT. IMRT and VMAT techniques were compared to the conventional technique for the V95%, conformity index (CI), mean lung dose and mean heart dose. The CTV-PTV margin used is 7mm. Since the setup error is accounted for when evaluating dose on the CBCT, we used the CTV for the evaluation. Results Evaluation of the treatment plans for 20 patients showed that V95% coverage of IMRT and VMAT plans was comparable to conventional plans (Table 1). Conformity was significantly higher for IMRT and VMAT. Mean lung dose was approximately 0.7 Gy lower on average, while mean heart dose increased by approximately 0.7 Gy using IMRT or VMAT. Robustness evaluation of the dose on daily CBCT’s using an online positioning protocol showed that V95% coverage remained stable for conventional, IMRT an VMAT. Significant conformity improvement was obtained using both IMRT and VMAT, and small differences in mean heart dose (+0.7 G) and mean lung dose (-0.8 Gy) were found. Evaluation of an offline positioning protocol showed similar results. Conclusion Presented IMRT and VMAT techniques show a similar robustness for interfraction motion in locoregional breast irradiation compared to the conventional technique, while conformity of the target volume is increased significantly. An offline positioning protocol would be sufficient for clinically acceptable set-up accuracy.

Clinically acceptable 7-field IMRT MRL plans (see Figure 1) were achieved in all ten patients. Clinically acceptable plans were also achieved for all ten patients using 9-field IMRT, non-MRL 7-field IMRT, non-MRL VMAT and CyberKnife treatment. Clinically acceptable 5-field IMRT MRL plans were only possible in seven patients. Table 1 summarises the number of exceeded constraints, mean rectal doses and mean bladder V37Gy for each plan type. Given the small patient group, exploratory ANOVA analyses were undertaken for the number of co nstraints missed, the rectum D1cc and the two most challenging constraints to achieve- rectum V36Gy and bladder V37Gy. For the MRL, 5-field IMRT MRL plans performed significantly worse in all these analyses compared to 7- field IMRT. 7-field IMRT MRL plans had significantly lower rectal doses compared to CyberKnife plans. No significant differences were seen between 9-field IMRT MRL plans and non-MRL VMAT plans compared to 7-field IMRT. Conclusion MRL IMRT plans for prostate SBRT achieved the PACE trial constraints in all patients with 9-field appearing similar to 7-field IMRT. 5-field IMRT in this set-up appears inferior for the MRL. All platforms could produce clinically acceptable plans. Further work is needed for dosimetric validation and feasibility of MRL delivery. PO-0829 Robustness of IMRT and VMAT for interfraction motion in locoregional breast irradiation R. Canters 1 , M. Kunze-Busch 1 , P. Van Kollenburg 1 , M. Kusters 1 , P. Poortmans 1 , R. Monshouwer 1 1 Radboud University Medical Center, Radiation oncology, Nijmegen, The Netherlands Purpose or Objective Conventional techniques for locoregional breast irradiation using field abutment are challenging, even more in combination with breath-hold irradiation and with hypofractionation, since over- or underdosage may occur more consistently in the abutment region in these circumstances. IMRT and VMAT techniques are likely to result in more conformal and homogenous irradiation,