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Page 2 of 10 Dumane et al. J Cancer Metastasis Treat 2019;5:42 I http://dx.doi.org/10.20517/2394-4722.2019.08

Conclusion: This work demonstrates that KBP can be trained and efficiently utilized to help not only speed up the
planning process but also help standardize the treatment plan quality.

Keywords: Brain metastases, radiotherapy, volumetric modulated arc therapy, knowledge-based planning,
stereotactic, radiosurgery

INTRODUCTION
Volumetric modulated arc therapy (VMAT) is a radiation treatment planning and delivery technique that
has been investigated and clinically applied for all disease sites including malignancies of the head and
neck, thorax, abdomen and pelvis, and compared to static field intensity modulated radiation therapy
(IMRT), VMAT has been shown to reduce treatment time and monitor units (MU), making it an attractive
radiotherapy delivery technique . Recent reports have established VMAT as a treatment technique for
[1]
delivering cranial stereotactic radiosurgery (SRS) providing optimal dose distributions while improving
[2-4]
the efficiency of treatment delivery. Conventionally, SRS uses one isocenter per lesion and requires multiple
isocenters for multiple lesions. This not only prolongs the delivery time reducing patient throughput but
is also uncomfortable for the patient. The use of a single isocenter to treat multiple cranial metastases has
been accomplished using VMAT producing highly conformal dose distributions while reducing treatment
times. Target coverage and dose gradients produced by this technique have been shown to be comparable
to Gamma Knife-based SRS . After the initial feasibility studies, reports have been presented outlining
[5,6]
further refinement of the procedure, dosimetric indices as well as plan quality parameters for single isocenter
VMAT in radiotherapy treatment of multiple cranial metastases . Depending on the complexity of the
[7,8]
case however, planning for these cases can be very time consuming and reports have expressed the need to
shorten the planning time, thereby aiming to improve the efficiency.

Knowledge-based planning (KBP) is a new paradigm in treatment planning and is a shift towards the
direction of automating the planning process [9,10] . KBP utilizes the dose distributions of prior plans to
build a model that can predict the same for new patients. KBP has been effectively utilized to generate
high quality treatment plans with consistency utilizing IMRT or VMAT for malignancies at various sites
including prostate, lung, liver, head and neck as well as intracranial stereotactic radiotherapy [11-15] . Although
comprehensive studies describing prediction methodologies that make precise estimations of SRS plan
quality metrics have been made using in-house systems , clinical application of a commercial KBP system
[15]
for treatment planning of multiple intracranial lesions with VMAT using a single isocenter has yet to be
performed. Recently RapidPlan (TM), which is commercial KBP software from Varian Medical Systems,
Palo Alto CA has become available for clinical use. In this study we train a knowledge-based model using
RapidPlan for multiple cranial lesions treated at our institution with VMAT. We then retrospectively apply
this model to compare the results of previously planned and treated cases.

METHODS
Treatment planning
VMAT plans were generated for 91 patients with a total of 139 lesions (range 1-4, median 1) in Eclipse V13.6
(Varian Medical Systems, Palo Alto, CA USA) from June 2017 to November 2018. Each case was planned
with a single isocenter using 3 to 6 arcs. The location of the isocenter was based on the geometric center of
the lesions to be treated. The arcs used were non-coplanar. Their distribution in terms of full arcs vs. partial
arcs, couch angles, collimator angles were such that they best covered the group of lesions to be treated
within that single isocenter. The arcs were chosen to avoid situations where there were two targets in the
same leaf track in order to avoid excess dose to the brain. The algorithm used for optimization with VMAT
was the progressive resolution optimizer and dose calculation was performed with analytical anisotropic