ESTRO 2020 Abstract Book

S306 ESTRO 2020

Conclusion These results suggest that magnetic focusing is a viable method of proton minibeam production. In this case a permanent Halbach quadrupole magnet assembly was used to generate the minibeams distribution without the need for cooling or complex power supply/control systems. Multiple minibeams could be delivered though precise motion control of the robotic patient positioner, while beam modulation can be achieved via control of either the incident proton energy or through the use of a range modulator.

interaction with beam modifying devices. The work presented here experimentally tests the ability of a single quadrupole magnet to focus an incident proton beam and deliver an acceptable minibeam distribution. Material and Methods A proton beam of 9.8 cm range in water was focused by a single permanent quadrupole magnet in this study. The quadrupole magnet was constructed from segments of radiation resistant Sm2Co17 rare-earth permanent magnetic material adhered into a Halbach cylinder with a nominal field gradient of 250 T/m and a length of 6.8 cm. Minibeam distributions were evaluated as a function of depth using EBT3 Gafchromic film located within a custom- made water tank. Both single and multiple minibeam distributions were assessed, with the latter delivered through the use of a laboratory jack and precision height gauge shifting the water tank vertically with respect to the incident proton beam (Figure 1). Lastly, modulated minibeam distributions were delivered by shifting the water tank vertically and modulating individual beamlets using appropriate range shifts and weightings.

Poster discussion: PH: Adaptive radiotherapy and inter- fraction motion management 2

PD-0552 Validation of an average anatomy model based on deformable image registration for lung cancer

G. Lim 1 , S. Van Kranen 1 , J. Sonke 1 , P. Remeijer 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands

Purpose or Objective Image-guided adaptive radiotherapy based on cone beam CT (CBCT) is used to detect differences with respect to the planning CT (pCT). Significant deformations are typically mitigated by creating a new treatment plan based on a repeat CT (rCT). The average anatomy model (AAM) is an adaptive strategy in which CBCTs from previous fractions are deformably registered to the pCT to estimate systematic deformations [1,2]. By averaging the deformable vector fields (DVFs) a new synthetic CT (sCT) can be created, in which the systematic deformations are mitigated. We aim to apply the AAM to lung cancer patients in which a systematic offset between initial tumor and lymph node positions can occur, typically several mm’s [2]. Here we present a validation study of the deformable image registration (DIR) and AAM algorithms, required for clinical introduction. Material and Methods Two validation methods were used to assess the geometric reproducibility of the DIR and AAM algorithms. • A modified version of the distance discordance metric [3] was evaluated on a set of 10 patients, each with 5 CBCTs and an rCT. Each CBCT was deformably registered to the pCT as well as to the rCT, producing two DVFs that were concatenated to derive a corresponding pCT-rCT DVF. Ideally, the resulting 5 pCT-rCT DVFs per patient are all identical, and differences between them are a measure of the registration error of the DIR algorithm. This error was quantified by the sample standard deviation (SD) per voxel of all 5 pCT-rCT DVFs per patient.

Figure 1: Experimental setup for proton minibeam delivery. From left to right components are: 1C=first collimator, FC=functional cone, 2C=second collimator, VS=vertical spacers, M=magnet, FH=film holder, LJ=laboratory jack, HG=height gauge, WT=water tank, DI=digital imager. Results The Halbach cylinder allowed for the delivery of focused proton beamlets with a narrow FWHM of 2.43 mm at entrance. Additionally, the experimental setup allowed for the delivery of these beamlets with precise lateral displacement (+/-0.02 mm) and individual weighting or modulation. At entrance, the peak to valley dose ratios were 9.8 and 9.1 for the multiple beamlet and modulated multiple beamlet cases respectively (Figure 2). Both the multiple beamlet and modulated multiple beamlet cases delivered a merged dose at the level of the Bragg peak (98 mm) with a FW90M of 15.47 and flatness of 4.25% and 2.78% respectively.

•

The AAM was used to generate an sCT for 16 patients based on 4-5 CBCTs per patient. The AAM was then reapplied a second time but using registrations to the sCT instead of the pCT, resulting in a second sCT with a corresponding average DVF (DVF 2 ). Ideally, both sCTs are equal since they are derived from the same set of CBCTs, and any residual values in DVF 2 are a measure of the registration error of the full AAM algorithm.

In both methods, only voxels in the intersection of the convex hull around the lungs and all CBCT field-of-views were considered. Results

Figure 2: Transverse profiles of single (left), multiple (center) and modulated (right) focused proton minibeams at entrance (top) and 98 mm depth (bottom).