Evaluation of a new rotational intensity modulated radiotherapy technique and adaptive approaches in treatment planning

Abstract

Evaluation of HybridArc: Rotational IMRT promises superior treatment efficiency by similar or increased treatment plan quality compared to static gantry IMRT. HybridArc (HA) is a new planning strategy provided by BrainLAB which combines a dynamic conformal arc (DCA) with static IMRT beams. In this master thesis HA was benchmarked against Elekta's VMAT solution in terms of plan quality, treatment efficiency and dosimetric accuracy. CT data sets and contours of three primary prostate patients were used to create plans with HA and VMAT.<br />For both Monte Carlo dose calculation was applied in the respective TPS (iPlan and Monaco). The prescribed dose to the PTV was 78Gy and we aimed for a maximum dose of less than 110%, as well as a D95% of 74.1Gy.<br />In-house constraints were used for rectum and bladder. HA was evaluated at two different DCA vs. IMRT beam weightings, i.e. 70/30 (HA7030) and 30/70 (HA3070). To make a fair comparison between the three planning approaches (HA7030, HA3070, VMAT) pareto optimal fronts were created using the D95% for PTV; D50% for bladder; and D50% and D10% for rectum.<br />Treatment efficiency was evaluated assessing total number of monitor units (MU), static IMRT segments (just for HA) and beam-on time of clinically acceptable plans. For dosimetric verification of the treatment plans the Delta4 system (Σ36 plans; 12 plans each) and radiochromic EBT2 films (Σ12 plans; 4 plans each) was used. If 90% of points had a gamma value smaller than one (GPR-gamma pass rate) the accuracy was considered as acceptable (DTA-3mm, dose deviation-3%). In total 206 treatment plans were created. 110 out of those fulfilled our objectives and were therefore considered as clinical acceptable. For all three patients D10% pareto optimal fronts for rectum were superior for VMAT compared to HA7030 and HA3070. For rectum D50% the advantage was diminished and for bladder D50% both HA approaches were superior to VMAT in two patients. Comparing the two weighting schemes for HA, HA3070 was either equal or superior to HA7030 in all pareto fronts. The median treatment duration for HA7030 plans was 7:56min with 450 MU. Median number of segments for the HA IMRT part were 70. The numbers for HA3070 were 8:45min and 464 MU with 73 segments. VMAT was highly efficient with a median irradiation time of 2:56min and median MU of 434. For VMAT all twelve irradiated plans had a GPR of higher than 96% and for HA 2 out of 24 plans had a GPR of 89%, but all other plans were above 92% (Delta4).<br />For EBT2 evaluation four HA plans had a GPR less than 90% but all VMAT plans failed the [gamma]-criterion with GPRs<=64%. In summary treatment plan quality was superior for VMAT except for fronts created for the D50% of the bladder. The higher the weighting of IMRT beams in HA the longer the treatment duration, but unexpectedly it didn't decrease significantly with a higher arc weighting. This was due to the fact that the segment number was quite similar. VMAT treatment efficiency was counterbalanced by longer planning times. Dosimetric deviations were acceptable with minor advantages for the investigated VMAT implementation.<br />Evaluation of iPlan RT Adaptive: Adaptive radiotherapy (ART) is a buzzword in the radiotherapy community. But the implementation of this new concept varies largely, ranging from margin adaptation to repeated biological imaging. General software solutions that support ART are lacking. The second part of this master thesis aimed at the evaluation of one of the first commercial tools following the ART concept, BrainLAB's iPlan RT Adaptive Module. The tool provides structure history information and evaluates volume changes, which occur during radiotherapy via CBCT images and deformable registration w.r.t. the planning CT. Prior clinical implementation, the deformable registration algorithm was benchmarked against manually delineated structures. CT data sets with 4mm slice thickness and contours (prostate, rectum and bladder) of seven primary prostate patients were used. For the stabilization of the prostate an endorectal balloon (ERB) was utilized.<br />8-14 CBCTs per patient were acquired during the whole treatment using the XVI system (Elekta) and a specific prostate imaging protocol. CBCT images were reconstructed to 4mm slice thickness. All data was transferred to the iPlan RT Adaptive Module (BrainLAB). In a first step, planning CTs and CBCTs were rigidly registered using bony anatomy. In a second step, deformable image registration based on an adapted Demons algorithm was applied. Absolute volumes and center of volume shifts (COV) were compared to manual drawn structures by a radiation oncologist. In total 79 CBCTs were deformable registered and applied on the structures. The absolute volumes from deformable registration and manual contouring differed by -6.6±19.7ccm, 19.6±78.1ccm, 7.7±31.9ccm for prostate, bladder and rectum, respectively. Differences in COV for automatic contours and manual contours were -0.4cm, 2.4cm and 0.2cm for prostate in AP, CC and LR direction. Corresponding values for bladder and rectum were 3.2cm, 2.4cm and 0.6cm and 4.5cm, 1.6cm and 0.6cm. Due to the limited field-of-view of the CBCT in longitudinal direction (12cm) no trend could be evaluated in terms of absolute volume changes for rectum and bladder. A systematic shift of the rectum in AP direction followed a distinct pattern. Due to the misinterpretation of the ERB by the algorithm, the rectum was shifted in anterior direction and displaced the prostate, when acting as a posterior boundary. Since the prostate is fixed by the pubic bone the volume is reduced.<br />The workflow implementation of the module was handsome and within a couple of minutes the volume trend could be observed. Image quality of the CBCT strongly impacted the results of deformable registration and use of conventional CT is recommended. Automatically created structures need further manual adjustment and the implemented algorithm has to be fine tuned for prostate cancer treatment with an ERB before implemented into clinical routine.