4-Pi Non-Coplanar Converging Beams Yield Superior Therapeutic Ratio than
Conventional Coplanar Beams
Song Wang, DooKee Cho, Albert M. Sabbas, K. Clifford Chao, Jenghwa Chang
Department of Radiation Oncology, New York-Presbyterian Hospital, Weill Cornell Medical Center, New York, NY, 10065, USA
[email protected]
Objective:
A comparison dosimetric study was conducted between a
novel 4-Pi non-coplanar converging beam delivery system
and conventional IMRT; preliminary results are presented
for a prostatic case and a pancreatic case.
• A treatment plan comprises of several delivery cones.
Each individual beam in the plan is uniquely defined
by the delivery cone angle and beam angle, as shown in
Figure 2.
Results:
Background:
•The 4-Pi non-coplanar converging beam delivery system
is being developed at our institution. This system utilizes
X-ray source aiming at the target and rotating around the
superior-inferior axis in a conical fashion. The delivery
system is demonstrated in Figure1.
Figure 2. The definitions of the 4-Pi beam
parameters; each beam is specified with the cone
angle and the beam angle, and beams are evenly
distributed on a cone
• It has been derived and validated in our previous study
that the orientation of a 4-Pi beam can be realized by a
combination of LINAC gantry rotation and couch
rotation in Pinnacle TPS [1].
Figure 1. The 4-Pi non-coplanar converging beam
delivery system. The X-ray source is mounted on an
annular structure, and the structure is able to skew
with respect to the transverse plane and rotate around
the superior-inferior axis. This results in beams being
delivered on a conical trajectory.
• The advantages of the proposed system include the
follows:
• Many more beams are used in the proposed system
compared to the conventional IMRT. This leads to
lower dose to the organ-at-risks (OARs) and normal
tissues.
• The beams are delivered non-coplanarly. It is
foreseeable that OARs can be further spared in
some cases due to the non-coplanar beam
arrangement.
• Couch-kick is not necessary and couch-gantry
collision can be completely eliminated. This not
only greatly shortens the treatment time but
substantially reduces intra-fraction motions.
• A Pinnacle script was written to convert a 4-Pi
plan to a series of pairs of LINAC gantry and
couch angles and load them into Pinnacle. The
plan was then computed as normal Pinnacle
plans.
2. Patient Study
For each investigated case, a 4-Pi plan as well as a
conventional IMRT plan were made for SBRT treatment.
One segment per beam was used in the 4-Pi plan
optimization to mimic the type of optimization expected
for arc-like delivery. Fifteen segments per beam was used
for the IMRT optimization. Plans were made with similar
PTV coverage and maximum dose.
• The prostatic case
• Prescription: 600cGy×5 fx
• 5-field IMRT plan: gantry angles of 0, 75, 145, 215
and 285 degrees
• 4-Pi plan: 100 beams; 20 beams for each of cones
of 0, -10, 20, -30 and 40 degrees
• The pancreatic case
Methods and Materials:
• Prescription: 660cGy×5 fx
1. Implementation of the 4-Pi system in Pinnacle
• 6-field IMRT plan: gantry angles of 30, 90, 310 and
350 degree, and gantry angles of 30 and 340 with a
270 degree couch kick
• The angle of a delivery cone is defined as the angle
between the cone edge and the transverse plane. The angle
is positive towards the patient head.
• Discrete beams are evenly distributed on a delivery cone
and specified by the angles from the vertical axis clockwisely.
Figure 3. DVHs (solid, IMRT; dashed, 4-Pi). left: the prostatic
case. PTV, light blue; CTV, red; rectum, blue; bladder, sky blue;
lt femoral head, green; rt femoral head, purple; right: the
pancreatic case. PTV, orange; CTV, purple; small bowel, slate
blue; spinal cord, light orange; stomach, tomato; bowel, olive.
With comparable PTV coverage and maximum dose, the 4-Pi
plans always end up with more OAR sparing at the middle dose
range and greater fraction of low dose . In terms of the mean
dose, the 4-Pi plan can reduce up to 44.6% OAR dose in the
prostatic case (lt femoral head) and up to 21.4% in the pancreatic
case (small bowel).
• 4-Pi plan: 100 beams; 20 beams for each of cones
of 0, -10, 20, -30 and 40 degrees
Figure 4. Iso-dose distributions. Upper left: IMRT prostate;
upper right: 4-Pi prostate; lower left: IMRT pancreas; lower
right: 4-Pi pancreas. The 4-Pi plans result in more conformal
dose distributions.
Conclusions:
The 4-Pi non-coplanar converging beam delivery system
tends to reduce OAR doses and improve the PTV dose
conformality. Given the collision-free feature of the
proposed system, it can be further developed to a hypefractionation treatment modality with fast delivery, precise
targeting as well as reduced radiation toxicity.
References:
[1] S. Wang, K. Chao, J. Chang, “CT-Based Treatment Planning for 4Pi
Non-Coplanar Converging (4PiNC) Beams in An External-Beam Treatment
Planning System”, Snap oral presentation at AAPM annual conference,
Indianapolis, August 2013