Introduction
Prostate cancer is one of the most prevalent cancers
among men, with a significant impact on their health
and quality of life.[] The treatment of prostate cancer often involves radiation therapy, which plays a crucial
role in eradicating cancer cells and reducing the risk of
recurrence.[] In recent years, advancements in radiation
therapy techniques have greatly improved treatment
outcomes, allowing for more precise targeting of tumor volumes while minimizing radiation exposure
to healthy surrounding tissues.[]
One such technique that has gained considerable attention
is volumetric-modulated arc therapy (VMAT).
VMAT is an advanced form of intensity-modulated
radiation therapy that delivers radiation through a
continuous arc of motion around the patient. This
technique utilizes dynamic modulation of multileaf
collimators and gantry rotation to shape the radiation
beam precisely, conforming to the tumor"s three-dimensional
shape. VMAT offers several advantages over
conventional radiation therapy techniques, including
shorter treatment times, improved dose conformity,
and reduced radiation exposure to healthy tissues.[,]
The planning process in VMAT involves the optimization
of various parameters to achieve the desired
treatment goals. One critical parameter in VMAT
planning is the minimum segment width (MSW),
which determines the width of the individual radiation
beam segments used during treatment delivery. The
MSW directly influences the treatment plan"s quality
and efficiency by impacting factors such as dose distribution,
target coverage, organ sparing, and treatment
delivery time.[] VMAT plan with a higher value of
MSW has fewer monitoring units (MU), less delivery
time (PDT), and more delivery efficiency than VMAT
plan with a lower MSW.[-]
Some studies have demonstrated the superiority of
the VMAT plan with lower MSW over the plan with
higher MSW.[,] Several studies have suggested using
an MSW of 1.0 cm for the VMAT plan as compared
to an MSW of 0.5 cm.[-] Therefore, this study aims
to investigate the influence of the MSW on the planning
outcomes of VMAT in prostate cancer patients.
By systematically varying the MSW value, we will
evaluate its impact on various dosimetric parameters,
including target coverage, dose conformity, organ atrisk
sparing, and treatment delivery efficiency metrics.
The findings from this study will contribute to a better
understanding of the role of MSW in VMAT planning
for prostate cancer and help identify the optimal MSW
value(s) that can maximize treatment efficacy while
minimizing treatment time and potential side effects.
Methods
Patient Selection
The study included twelve patients (aged between 55
and 68 years) who were diagnosed with prostate cancer
and received VMAT treatment at our hospital from January 2022 to November 2022. To ensure accurate
treatment, all patients were positioned in the supine
position and immobilized using a 4-clamp thermoplastic
pelvis mask. A Siemens computed tomography
(CT) simulator was used to perform scans, with a slice
thickness of 3 mm.
The CT images obtained from the scans were then reconstructed
and imported into the Monaco planning system
version 5.51.10. The planning target volume (PTVP)
encompassed the prostate tumor and seminal vesicles,
with a 5-mm margin on all sides except for a 3 mm margin
posteriorly. The delineation of PTVLN, which included
the pelvic lymph nodes, was performed by experienced
oncologists following the institute protocol.
In addition, several organs at risk (OAR) structures
were delineated, including the bladder, rectum, femoral
heads, cauda equina, sigmoid colon, and bowel bags.
These structures were outlined to ensure their protection
during the treatment planning process.
Treatment planning
VMAT plans were designed for all patients using the
Monte Carlo (MC) algorithm in the Monaco Treatment
Planning System (TPS). The Elekta Synergy Linear
Accelerator with a 6MV X-ray photon beam was
utilized to deliver the plans. A dual arc of 360° rotation
was employed for each case, clockwise from 181°
to 179°. During gantry rotation, the collimator angle
was set to 0° based on the patient"s anatomy. The MC
algorithm had a statistical uncertainty of 3% per control
point, and the final dose calculation used a 3 mm
resolution for the calculation grid. Each plan consisted
of a maximum of 180 control points (CP).
Four VMAT plans were generated, namely MSW0.5,
MSW1.0, MSW1.5, and MSW2.0. These plans had corresponding
MSWs of 0.5 cm, 1.0 cm, 1.5 cm, and 2.0
cm, respectively. The remaining parameters and cost
functions were kept constant. The prescription dose
for PTVP was 68Gy delivered in 25 fractions, while for
PTVLN, it was 45Gy in 25 fractions. Table 1 displays
the cost functions.
Table 1: The optimization cost functions of VMAT plans for prostate cancer
Plan Evaluation
The dosimetric indices used to compare the different
MSW prostate plans included the homogeneity index
(HI), conformity index (CI), maximum dose of the target
volume, MUs, CP, and the dose volume histogram
(DVH) parameters concerning OARs. The CI and HI
were calculated as follows:
CI= (TVPI)2/(TVxVPI)
HI = (D5%)/D95%
In the above equations, TVPI represents the target
volume receiving the prescription dose, TV represents
the total target volume, and VPI represents the volume
receiving the prescription dose. Ideally, the CI should
be close to 1. The D5% refers to the minimum dose
received by 5% of the planning target volume (PTV)
according to the DVH, indicating the maximum dose.
Conversely, the D95% represents the minimum dose
received by 95% of the PTV, indicating the minimum
dose. A lower HI indicates better homogeneity.
Plan Verification
The MatriXX Universal Detector Array, manufactured
by IBA in Germany, was utilized to compare the plan
quality of all the plans in this study. The evaluation of
plan quality involved calculating the gamma index and
the gamma pass rate (GPR) by comparing the dose fluence
generated by the TPS with the measurements obtained
from the MatriXX detector.
The GPR was determined by dividing the number
of measurement points that met the pre-defined criteria
by the total number of measurement points within
the specified threshold. To eliminate low-dose signals,
a lower limit of 10% was set during the gamma calculation.
The analysis of measurements was conducted using
a 3% dose difference (DD) and a 3-mm distance to
agreement (DTA).
In this study, the global gamma indices were considered
clinically acceptable if the GPR for the 3%/3
mm criteria was equal to or greater than 95%. To facilitate the measurements, the immatrix detector was
inserted into the miniphantom and positioned on the
treatment couch. The iso-plane was set at the depth of
the effective point of measurement on the side of MatriXX
using the corresponding markers. MatriXX was
calibrated at the used photon energy and all plan intended
by calibration factors.
Statistical Analysis
To compare dosimetric parameters and measurement
results between different groups, the Wilcoxon signedrank
test was employed. The statistical analysis was
performed using SPSS 22.0 software. A p<0.05 was
considered statistically significant, indicating a notable
difference between the groups.
Results
Target Doses
Figure 1 shows the DVH graph of VMAT plans with
different MSW parameters for a typical patient. In
MSW2.0 cases, PTV (primary) and PTV (LN) from the
DVH failed to meet the clinical requirement, i.e., for
PTV primary D95% = 90%, and for PTV lymph nodes,
D95% = 91.2% (<95%).
Fig. 1: The dose volume histograms of four different VMAT plan with different MSW"s for a
typical prostate cancer.
VMAT: Volumetric-modulated arc therapy; MSW: Minimum segment width.
Table 2 shows the comparison of the mean dose,
maximum dose, HI, and CI of the target PTVs.
The dosimetric parameters of PTVs" mean dose
and maximum dose were comparable among four
MSWs plans, but conformity and homogeneity were poor as MSWs value increased from 0.5 to 2.0.
Therefore, all MSWs group plans were not comparable
in terms of HI and CI. On average, a 15% decrement
was found in conformity with MSW2.0 plans
as compared to MSW0.5.
Table 2: Dosimetric results of PTVs for prostate VMAT plans with different MSWs (n=12)
As shown in Figure 2, VMAT plans with MSW
of 0.5 cm and 1.0 cm were similar for all dosimetric
parameters of PTVP and PTVLN (p>0.05) except for
maximum dose to PTVP. Except for maximum doses
to both targets, VMAT plans with MSW of 0.5cm were
better as compared to VMAT plans with MSW of 1.5
cm and 2.0 cm (p<0.05). However, the maximum dose
to the PTV (Primary) in the MSW0.5 plan was 0.9Gy
higher than in the MSW1.0 (p=0.005).
Fig. 2: Graphical Representation of homogeneity index, conformity index, and mean dose with different MSW"s for both
PTV"s for n=12.
PTV P: Primary planning target volume; PTV LN: Lymph node planning target volume.
OAR Doses
Figure 3 compares V58Gy(%), V54Gy(%), V50Gy(%),
and V41Gy(%) doses to the bladder, rectum, and
V35Gy(%) of the femoral heads among the four MSW
groups. As MSW"s value increased, OAR doses decreased.
There were no statistical differences in the
OAR doses between the MSW1.0 and MSW0.5 plans
(p>0.05). For OAR, there was no significant statistical
difference in the OAR doses between the MSW1.5 and
MSW0.5 plans except V58Gy for rectum and V41Gy
for bladder as shown in Table 3. Except for bladder
doses in higher MSW plans, there were no significant
differences in doses between the four types of VMAT
plans in terms of other remaining OAR.
Fig. 3: Graphical Representation of OAR"s doses with different MSW"s for n=12.
OARs: Organ at risks.
Table 3: Dosimetric results of OARs for prostate VMAT plans with different MSWs (n=12)
MU and CP
As the MSW value increased, CP in the prostate cancer
of the VMAT plan decreased; the mean number
of CP for the plans with MSWs of 0.5 cm, 1.0 cm, 1.5
cm, and 2.0 cm was 274, 248, 204, and 188, respectively (Fig. 4). Moreover, the MUs of the VMAT plan
decreased as the MSW increased (Fig. 4). The mean
MUs for the plans with MSWs of 0.5, 1.0, 1.5, and 2.0
cm were 1503.7±80.4, 1102.4±76.7, 914.8±64.2, and
900.9±81.6, respectively.
Fig. 4: Graphical representation monitor unit, control points, GPR, and PDT with VMAT
plan of different MSW"s for n=12.
PDT: Plan delivery time; GPR: Gamma passing rate.
Dosimetric Verification and Plan Delivery Time
The evaluation involved a comparison between the
measured planar dose and the dose calculated by the
TPS, employing the gamma passing criteria with a 3%
DD and a 3 mm DTA. Table 4 illustrates the GPRs for
plans characterized by MSW of 0.5 cm, 1.0 cm, 1.5 cm,
and 2.0 cm. The GPR was most pronounced in the plan
using 1.5 cm MSW, while it was least pronounced in
the plan using 0.5 cm MSW. In addition, Table 4 shows the PDT for the 12 patients from the time the beam is
turned on to the time it is turned off. CP and MUs of
the VMAT plan decreased as MSW increased, as did
plan delivery time.
Table 4: Statistical Results MU, Control Points, PDT, and GPR of VMAT plans with different MSW values (n=12)
Discussion
The process of designing VMAT plans to treat prostate
cancer results in a significant number of long, small, and irregular segments.[,] The MSW plays a crucial
role in optimizing the formation of these apertures.
Unfortunately, these segments can sometimes lead to
challenges during clinical delivery, causing low verification
rates and even interruptions in the VMAT plan
delivery.[,] To address the issue of plan complexity,
we investigated the impact of varying MSW values on
the quality of prostate cancer VMAT plans. We compared
four different optimization schemes, each based
on a different MSW value. The evaluation of plan quality
involved assessing several parameters, including the
HI, CI, maximum, and mean doses to the planning
target volume (PTV), as well as the dose-volume indices
of organ at risk, MUs, and CP. Our findings revealed
that VMAT plans generated with an MSW of
1.0 cm exhibited similar dose distributions to plans
with MSWs of 0.5 cm (Fig. 2). However, we observed
that plans with MSWs of 1.5 cm and 2.0 cm displayed
slightly inferior quality, failing to meet the clinical requirements
adequately (Table 2).
In addition, the number of CP and MUs decreased
as the MSWs increased (Table 4). When compared to
the plan using an MSW of 0.5 cm, the mean MU reductions
in the plans using MSWs of 1.0, 1.5 cm, and 2.0
cm were 26.68%, 39.16%, and 40.0%, respectively, while
the total CP was decreased by 9.2%, 25.5%, and 31.3%,
respectively. Previous studies showed that decreasing
the MUs for treatment delivery reduces the constraint
factor of the leaves" trajectories, the complexity of intensity?
modulated radiation therapy plans, and treatment
time[-] Hence, as the MSW increases and
VMAT plan complexity decreases, the therapeutic efficiency
may improve as well. The average delivery times
of the plans using MSWs of 1.0, 1.5, and 2.0 cm were
decreased by 37.8, 49.8, and 49.8 s, respectively (a drop
of approximately 12%, 15.8%, and 15.8%, respectively),
compared to the plan with an MSW of 0.5 cm.
The measured and computed doses were assessed
through a matrix detector, and all treatment plans
demonstrated favorable GPRs. The average GPR >94%
with a 3% DD and 3-mm DTA threshold indicates strong congruence between measured and calculated
doses.[] Enhanced agreement between measured
and TPS-calculated doses was observed with a higher
MSW. This outcome was anticipated due to the decrease
in the number of small fields as MSW increased,
facilitating dosimetric verification.
Conclusion
We concluded that VMAT plans for prostate cancer
generated with an MSW of 1.0 cm demonstrated comparable
dose distributions to plans with MSWs of 0.5
cm. However, plans with larger MSWs showed a decline
in quality, raising concerns about their clinical
suitability.
Peer-review: Externally peer-reviewed.
Conflict of Interest: All authors declared no conflict of interest.
Ethics Committee Approval: The study was approved by
the Amity University Ethics Committee (date: 02/11/2023).
Financial Support: None declared.
Authorship contributions: Concept - N.J., A.K., As.K.;
Design - N.J., A.K., As.K.; Supervision - N.J., A.K., As.K.;
Funding - N.J., As.K.; Data collection and/or processing ?
N.J.; Data analysis and/or interpretation - N.J., A.K.; Literature
search - N.J.; Writing - N.J., A.K.; Critical review - A.K.
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