Dose Calculation Alogorithm

Question 1

IMRT

Answer 1

Intensity Modulated Radiation Therapy

Question 2

DVO

Answer 2

Dose Volume Optimizer

Question 3

DVO’s 2 Phases

Answer 3

Gradient Evaluation & Line Search
Based on dose-volume objectives (upper and lower objective) specifying the portion of a volume that may receive a specified dose (e.g 50% of a volume may receive 50 Gy)

Question 4

Fluences in DVO

Answer 4

Either begin at zero or from previous optimization that can be used as initial guess

Question 5

Volume Representation

Answer 5

With point clouds generated from the contours using shape-based interpolation, quasi random sampling, and relaxation

Question 6

Target Masking

Answer 6

the point set for the target is projected to the fluence matrix. Only rays within 0.5 cm from the closest projected point are allowed to have non-zero fluence values. Field sizes are automatically determined.

Question 7

Dose Computation

Answer 7

based on multi-resolution 3D convolution of MC-generated point spread function kernels

Question 8

LMC

Answer 8

Leaf Motion Calculator

Question 9

DVO supports two delivery techniques for the dose dynamic delivery of IMRT?

Answer 9

Sliding Window (Varian MLC Devices)

Multiple Static Segments (MSS) - Varian, Siemens, and Elekta

Question 10

DAO

Answer 10

Direct Aperture Optimization

Question 11

Main Features

Answer 11

User specified Number of Beams, their directions and apertures (segments) per beam

Weight and shape of apertures optimized simultaneously

The optimization algorithm then varies the weights of the aperture well as the leaf positions on a pre-defined grid

Resulting plan is ready to deliver with no further sequencing steps

Results in DMPO in Pinnacle

DAO used in IMAT

Question 12

DMPO

Answer 12

Direct Mahcine Parameter Optimization

Question 13

Pinnacle Optimization Options

Answer 13

None - no parameters of this beam will be optimized. (Dose from this beam will be included in the objective evaluation)

Intensity Modulation - the opening density matrix of the beam will be optimized. This is the default type and is only available for photon beams

Beam Weight - the weight of the beam will be optimized. This is available for all beam types

Segment Weight: the weight of each segment of the beam will be optimized. This is only available for photon, step-and shoot, and motorized wedge beams

DMPO - this optimizations produces the MLC leaf positions and segmented weights. The final conversion process is not requirws. Dose can be computed once the optimization process is complete. This is available only for photon beams that do not contain dynamic wedges or blocks.

Question 14

DMPO Advantages

Answer 14

Answers a non‐convex problem; has a greater degree of non‐linearity, parameter
coupling and is subject to numerous linear constraints.

Starting point is fluence optimization, where an initial estimate is produced by
assigning a uniform fluence to the beam’s eye view of the target for each beam,
and the ODM elements are then optimized.

An integrated leaf sequencer is designed specifically to produce a suitable starting
point for the subsequent machine parameter optimization.

The sequencer ensures that all segments have a certain degree of uniqueness, and
it will also aim to produce regular aperture shapes in order to avoid the tongue‐
and‐groove effect.

Basic steps of the optimization :
The first few iterations are used to find an initial set of control points that
meets the user and machine specific requirements.
During the remainder of the iterations, the MLC leaf positions and segment
weights are optimized. When the optimization is finished, no post processing
is needed.

The dose calculation translates the current control points to equivalent ODMs
during optimization

Question 15

DMPO Leaf Sequencing Steps

Answer 15

1. The ODMs are resampled into a grid that
   matches the MLC leaves.
2. The fluence values are constrained to a
   number of equidistant levels, and the
   fluences are then decomposed into smaller
   elements using a combination of the ‘close
   in’ and ‘leaf sweep’ techniques
3. The elements are used to build the
   segments. In this process, all requirements
   on the leaf positions are considered,
   including shape regularity.
4. Jaw positions are assigned to the segments,
   and the remaining requirements are
   processed, including segment uniqueness.
   Segments that cannot fulfill all requirements
   are discarded.

Question 16

Intensity modulation Process

Answer 16

 Full intensity modulation: 2‐step process using pixel or fluence based
optimization
 First step: the optimizer generates a continuous intensity modulated
profile for each beam while minimizing the value of the cost function
 Second step: converts the intensity profile to deliverable MLC shapes
 Reduction of radiation efficiency due to many segments and dramatic
increase in MUs, latter leads to increase in leakage and scatter radiation
 Increased exposure to complex IMRT plans increases the probability of
radiation induced secondary malignancies

Question 17

DMPO Process

Answer 17

DMPO (DAO) : 1‐step process directly optimizes weights and leaf
positions with a pre determined number of MLC shapes for each beam.
 A continuous profile is generated for user defined number of segments‐
converted into specific number of deliverable MLC segments‐ and
further refined to minimize the value of cost function
 DMPO plans have lowest MUs.
 DMPO tends to create plans with larger segment areas.
 DMPO is able to create more delivery efficient plans by selectively
increasing the segment area.