Evaluation of an automated template-based treatment planning system for radiotherapy of anal, rectal and prostate cancer

Abstract

Background and purpose: The Ethos system has enabled online adaptive radiotherapy (oART) by implementing an automated treatment planning system (aTPS) for both intensity-modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (VMAT) plan creation. The purpose of this study is to evaluate the quality of aTPS plans in the pelvic region.

Material and methods: Sixty patients with anal (n = 20), rectal (n = 20) or prostate (n = 20) cancer were retrospectively re-planned with the aTPS. Three IMRT (7-, 9- and 12-field) and two VMAT (2 and 3 arc) automatically generated plans (APs) were created per patient. The duration of the automated plan generation was registered. The best IMRT-AP and VMAT-AP for each patient were selected based on target coverage and dose to organs at risk (OARs). The AP quality was analyzed and compared to corresponding clinically accepted and manually generated VMAT plans (MPs) using several clinically relevant dose metrics. Calculation-based pre-treatment plan quality assurance (QA) was performed for all plans.

Results: The median total duration to generate the five APs with the aTPS was 55 min, 39 min and 35 min for anal, prostate and rectal plans, respectively. The target coverage and the OAR sparing were equivalent for IMRT-APs and VMAT-MPs, while VMAT-Aps.demonstrated lower target dose homogeneity and higher dose to some OARs. Both conformity and homogeneity index were equivalent (rectal) or better (anal and prostate) for IMRT-APs compared to VMAT-MPs. All plans passed the patient-specific QA tolerance limit.

Conclusions: The aTPS generates plans comparable to MPs within a short time-frame which is highly relevant for oART treatments.

Keywords: AP, automatically generated plan; Automated treatment planning; CN, conformity number; CT, computed tomography; CTV, clinical target volume; DVH, dose volume histogram; FFF, flattening filter free; GTV, gross tumor volume; HI, homogeneity index; IMRT, intensity modulated radiotherapy; Intelligent optimization engine; KPB, knowledge-based planning; Linac, Linear accelerators; MCO, multi-criteria optimization; MLC, multileaf collimator; MP, manually-generated plan; MR, magnetic resonance; MU, Monitor Unit; OAR, Organ at risk; Online adaptive radiotherapy; PTV, planning target volume; Pelvic cancer; Plan quality; QA, Quality assurance; SD, standard deviation; Template-based Ethos TPS; VMAT, volumetric arc radiotherapy; aTPS, automated treatment planning system; oART, online adaptive radiotherapy.

Fig. 1

Schematic figure of the different steps of manually and automated plan generation. The VMAT-MPs were optimized by manually applying the dose-volume constraints to the CTV, PTV and OARs (Table 1) and a relative weight to each of them. Support structures, e.g. OAR minus PTV, ring around PTV and the normal tissue objective (NTO) tool in Eclipse, were utilized to shape the dose distribution and to reduce the dose outside the target. MUs were set to a maximum value of 400 MU to limit the plans complexity. The inverse optimization process was performed utilizing the photon optimization algorithm (v.15.6, VMS). The IMRT-AP and VMAT-AP were generated with Ethos TPS (v.1.0 MR1, VMS). Disease-specific treatment planning templates were optimized and defined based on five patient cases (including in each subgroups of this study) for each tumor site, i.e. anal, rectal and prostate cancer. Then, the final approved template was used to generate APs for all 20 patients in each site group.

Fig. 2

Median of the dose volume histogram difference (ΔDVH) between IMRT-AP minus VMAT-MP (red solid line) and between VMAT-AP minus VMAT-MP (black solid line) and the IQR (dotted lines) for the cohort of anal, rectum, and prostate patients. Data are shown for all the PTV volumes and the OARs. Note that the plots have different axes scaling for the PTV-E volume. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Doses to PTV and OARs for IMRT-APs (red circle) and VMAT-APs (black cross) as function of VMAT-MPs (x-axis); the blue line indicates the identity line, while the green line indicates our clinical threshold for each metric as define in Table 1. Note that the plots have different axes scaling. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)