ESTRO 2020 Abstract Book

S813 ESTRO 2020

Figure 1: A model of the ideal prostate treatment plan. A KB of 562 clinical prostate VMAT treatment plans (generated and QA-ed in 2017 and 2018 according to a well-established protocols) were used to inform this model. The mean and 25 th percentile of the PTV-OAR dose gradients from the training dataset were used to generate two sets of optimisation objectives in Pinnacle and these were tested on a cohort of 142 patients from 2019. The new treatment plans for 10 of the test patients (for which all of prostate radiotherapy dose criteria were met) were reviewed visually by a medical physics expert experienced in prostate treatment planning. Results For 114 of the 142 test cases, at least one of the new plans (average or 25 th percentile) met all of the dose criteria for prostate radiotherapy at The Christie. Furthermore, for 105 of the test cases, at least one of the new plans was superior to the original clinical plan. In Figure 2, PTV- Rectum dose gradient is plotted against PTV-Bladder dose gradient for the KB training data (transparent red), clinical test data (solid red), average new treatment plans (yellow) and 25 th percentile new treatment plans (blue). Figure 2: Treatment plans generated using KB dose gradient optimisation (blue and yellow) show more consistent and steeper PTV-OAR dose gradients than corresponding clinical treatment plans (red). Visual review of the new treatment plans for 10 patients from the test cohort showed that all were considered clinically acceptable. Conclusion A novel approach to KB treatment planning for prostate VMAT has been proposed, trained and tested. Initial results show that treatment plans generated automatically from the predictions of the model are clinically acceptable in more than 80 % of cases and show superior and more consistent OAR sparing than their corresponding clinical treatment plans. Although there is scope for refinement of the predictions, this method shows promise for realising significant efficiencies within treatment planning departments and with the transfer of knowledge between centres. PO-1507 VMAT by means of a dual layer stacked multileaf collimator or helical tomotherapy for cervix uteri L. Cozzi 1 , P. Subhajit 2 , S. Jamema 2 , C. Supriya 2 , M. Arjun 2 , F. Antonella 1 , A. Jai Prakash 3 1 Humanitas, Radiotherapy, Rozzano, Italy ; 2 Advanced Centre for Treatment Research and Education in Cancer- Tata Memorial Centre, Radiation Oncology, Mumbai, India ; 3 Advanced Centre for Treatment Research and Education in Cancer- Tata Memorial Centre, Radiation Oncology, Mumbai, Italy Purpose or Objective To ascertain the dosimetric performance of a new delivery system (the Halcyon system, H) equipped with dual layer stacked multileaf collimator (MLC) for risk adapted targets in cervix uteri cancer patients compared to another ring- based system in clinical operation (Helical Tomotherapy, HT) Material and Methods Twenty patients were retrospectively included in a treatment planning study (10 with positive lymph nodes and 10 without). The dose prescription (45 Gy to the primary tumor volume and a simultaneously integrated boost up to 55Gy for the positive patients) and the clinical planning objectives were defined consistently as recommended by an ongoing multicentric clinical trial. Halcyon plans were optimized for the volumetric modulated arc therapy. The plan comparison was performedby means of quantitative analysis of the dose volume histograms

Table 1

p value Clinca l Plan vs Rapid plan

P value Clinic al Plan vs Hybri d Plan

Clinical plan

Rapid Plan

Hybrid

Parameter

Plan

PTV45V42.75G y[%] ITV45V42.75G y[%] Help contour max Bowel V40 Gy (cc) Bowel V30 Gy (cc) Bowel V15 Gy (cc) Bladder V40Gy (%) Bladder V30 Gy (%) Rectum V40Gy (%) Rectum V30Gy (%)

96.8 (1.2) 99.8 (0.3) 102.1 (1.1) 187 (164)

97.1 (2.0) 99.8 (0.3)

96.4 (0.5) 99.9 (0.2)

0.33 0.8

0.57 0.48

102.2(0. 5) 172(104 ) 374.3(1 74) 1426(42 7)

101.9(0. 4) 161(123 ) 342.5(1 91) 1212(38 9)

0.76 0.13

0.24 0.000

404.4(2 61)

0.45 0.000

1293 (413)

0.2 0.000

49(15) 50.0(12) 45(15) 0.64 0.000

72(11) 74(9)

61(13) 0.34 0.000

83(17) 78(9)

79(13) 0.23 0.01

95(7)

95(5)

93(7)

1.0 0.23

1.06(0.0 4) 1.56(0.0 7)

1.03(0.0 1) 1.50(0.0 6)

V43 / VPTV45 1.05(0.0 5)

0.45 0.000

V36 / VPTV 45 1.55(0.1 2)

0.92 0.000

Conclusion Knowledge based planning model was created and validated for VMAT cervical cancer. It was observed that the HP resulted in superior sparing for bowel & bladder and conformality, while maintaining the target coverage. It was found that the rapid plan model was robust and hence will be used clinically in the future. PO-1506 An automated knowledge based treatment planning solution for prostate VMAT J. Wood 1 , M. Aznar 2 , P. Whitehurst 1 1 The Christie NHS Foundation Trust, Christie Medical Physics & Engineering, Manchester, United Kingdom ; 2 The University of Manchester, Radiotherapy Related Research, Manchester, United Kingdom Purpose or Objective This work aimed to develop a novel approach to knowledge based (KB) prostate VMAT treatment planning. Material and Methods Converting well defined PTV dose criteria into optimisation objectives is relatively straightforward. OARs, however, present a greater challenge because they sit in steep dose gradients and their size, shape and position relative to PTVs vary between patients. Precisely predicting optimal OAR DVH parameters is therefore difficult and uncertainties in the predictions ultimately become manifest in the degree to which treatment plans are dosimetrically optimal. The optimal PTV-OAR dose gradient (i.e. dose fall-off per unit distance) is characterised primarily by delivery machine parameters and not patient anatomy. On this basis, a model of the ideal prostate treatment plan was developed – see Figure 1, where the colour gradients represent achievable PTV-OAR dose gradients.