ESTRO 2022 - Abstract Book

S1547

Abstract book

ESTRO 2022

The MP method is a general framework for automated treatment planning with the potential to substantially reduce planning workload while maintaining or improving plan quality.

PO-1746 Semi-automated treatment planning for low-dose total body irradiation (TBI)

R. van Leeuwen 1 , M. Barsegyan 1 , D. Verwegen 1 , E. van der Bijl 1 , R. Monshouwer 1

1 Radboudumc, Radiation Oncology, Nijmegen, The Netherlands

Purpose or Objective Total body irradiation (TBI) is a treatment used in the conditioning of patients prior to hematopoietic stem-cell transplantation. At the department of Radiation Oncology at Radboudumc, an extended source-to-surface distance (350cm), field-in-field technique was developed for TBI. Presently, treatment planning for TBI involves time-consuming forward planning of several multi-leaf collimator (MLC) beams to improve dose homogeneity and decrease dose to organs- at-risk. This research aims to develop and evaluate a computational tool to automatically generate such field-in-field beams for low-dose TBI thereby enabling faster treatment planning. Materials and Methods Patients were scanned on a CT scanner in supine position, with the knees bent to fit inside the maximum treatment range (160cm). First, the three-dimensional (3D) dose distribution was calculated using the CT scan and the Pinnacle treatment planning system (TPS, Philips, Best, The Netherlands), with two opposing open beams at the treatment distance and gantry and collimator angles. The dose distribution was then exported to our tool, developed in the Python programming language. Using this tool, the 3D dose matrix was projected to the midsagittal plane by calculating the mean along the left-right axis. In the resulting two-dimensional (2D) dose map, four discrete areas (1-4) with similar dose were defined (see Figure 1) using the histogram of the 2D dose map to determine transition dose values. The shapes of these areas were then used to set up four beams including an open beam and three MLC segments to increase the dose in areas 2-4. Monitor units (MUs) for the open beam were calculated aiming to adjust the average dose of area 1 to the prescription dose. For the MLC segments, the aim was to adjust the minimum dose of the corresponding area to the prescription dose. Beams from left and right directions of the patient were mirrored. Beam data were imported in the TPS and the 3D dose was calculated on the CT scan for evaluation. Performance of the software was evaluated quantitatively by analysing dose metrics and adherence to protocol requirements. A qualitative evaluation was carried out in the form of surveys with three experts involved with TBI treatment planning. The survey included a Turing test comparing automated plans and manually created plans, and rating several technical aspects.

Figure 1. Example of 2D dose map with four dose levels 1-4. A higher level corresponds to a lower dose.

Results The quantitative analysis showed that the treatment plans designed by the tool met protocol requirements. The qualitative surveys showed that the automatically generated plans are clinically acceptable according to experts. TBI plans could be produced by the tool within 30 seconds. Conclusion We showed that the developed software is successful in creating field-in-field beams for low dose TBI treatment that are acceptable according to protocol requirements and field experts. This method is now part of our standard procedure.

PO-1747 Linear approximation of variable RBE models using only LET

D. Wagenaar 1 , J. Langendijk 1 , S. Both 1

1 University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands

Purpose or Objective Variable relative biological effectiveness (RBE) models based on the linear-quadratic model for cell survival depend on the tissue-specific α / β value, fraction dose and the dependence on the dose-weighted average linear energy transfer (LET d ).