This document summarizes a practical report on radiation oncology. It evaluates plan verification for a prostate patient using VMAT and quality control of a multileaf collimator. For the plan verification, the gamma index was 98.7% passing strict 3mm/3% criteria. Quality control using a picket fence pattern showed good multileaf positioning with no detectable shifts. The accuracy of advanced radiotherapy techniques like IMRT and VMAT depends on dosimetric characteristics and quality control of the multileaf collimator.

1. Master in Medical Physics 2015 to 2016
Practical Report of Radiation Oncology
Francisco J. Hernandez Flores
International Centre for Theoretical Physics
franciscohernandez_f2010@hotmail.com
December 23, 2015
Abstract
In this practical session were evaluated two important point for the safety of
the patient during the complex plan like VMAT or IMRT in radiation therapy: the
first part evaluated was plan verification of the prostate patient and second part was
the quality control in multileaf using gafchromic film. Using the octavius phantom
with the appropriated software was evaluated the plan of VMAT technique when
te plan of patient is made with one of the advanced technique in radiotherapy is
requires a dedicated QA procedure for dosimetric verification of a planned dose
distribution to check for the agreement between a dose distribution calculated
by the Treatment Planning System (TPS) and the corresponding measured dose
distribution before to treat the patient. In this plan the acceptance criteria γ
was 98.7% taking in to account the strict analysis 3mm and 3% of dose pixel
by pixel. The second point was inspection of the static leaf positions of Multileaf
Collimator (MLC) devices is essential for safe radiotherapy deliveries in both
static and dynamic modes. The purpose of this quality control in multileaf is to
provide accurate delivered dose when the patient is treated with the most advanced
techniques in radiotherapy (3D conformal, IMRT, IGRT and VMAT), which use
irregular fields using multileaf collimators in a linear accelerator.
I. Introduction
Intensity-modulated radiation therapy (IMRT) and Volumetric Arc Therapy
(VMAT) these techniques requires detailed verification of the multileaf collima-
tor (MLC) leaves position, because geometric discrepancies between planned
and actual beam segments affect the treatment quality (increase the dose in the
patient), as demonstrated in literature both in terms of spatial and dosimetric
errors. For this reason, controls should be performed to verify leaves and jaws
settings when using several small fields, both during acceptance and subsequent
controls. [1]
The basic definition of the MLC leaf positions in the commercially available
treatment distance between the leaf tips and the central line of the radiation
systems is the radiation field. Accordingly, the MLC leaf positions inspection
1

2. Master in Medical Physics 2015 to 2016
should ideally be performed by measuring these distances practically and com-
paring it with the MLC file coming from planning system. Besides, the central
line of the radiation field should ideally intersect the mechanical isocentre of
the linac at the radiation center point.
II. Theory
Multi-Leaf collimators (MLC).
Is a machine collimation system that incorporates a set of computer controlled
leaves that allow the creation of user defined beam apertures.
Dynamic Multileaf Collimator
When MLC moves during treatment different parts of the field are shielded
resulting in different overall radiation levels delivered in different parts of the
beam: Intensity modulated radiotherapy.
• The transmission through the collimators should be less than 2% of the
primary (open) beam.
• The transmission between the leaves should be checked to ensure that it
is less than the manufacturer’s specification.
MLC configurations: Comparison of the leaf travel configurations of commer-
cially available MLCs fig.1. The maximum leaf extensions are compared to a
40cm by 40cm maximum field size. [2]
Figure 1: Leaf travel in total field 40 by 40 cm
Transmission Requirements: Interleaf Transmission.
Two important factors determine the cross-sectional shapes of the leaves
1. the leaves are focusing in the plane orthogonal to their travel and therefore
have to incorporate divergence
2

3. Master in Medical Physics 2015 to 2016
2. have to overlap their neighbors to minimize interleaf transmission.
The fig.2 illustration of different leakage paths between leaves and the effect
of leaf cross-section shape on penumbra along the size of an MLC leaf. The
overall pitch of the leaf pattern may be 1 cm, the profile of a strip irradiated
by the retraction of a single leaf is somewhat wider (W’) and has a penumbra
broadened by the leaf side design, being governed by (W − W)/2. [2]
In practice, the depths of the leaf side steps are only fractions of a millimeter,
and so the broadening is quite small.
Figure 2: leakage paths between MLC
III. Materials and Methods
Linac accelerator synergy Elekta, Phantom octavius 4D with the appropriated
software, Gafchromic film, red mosaiq and treatment planing system monaco.
The (common) VMAT quality control for verification in specific patient
process the VMAT patient plan would be approved before to do the quality
control of plan verification. Beams can remain as planed for the verification
plan, During verification process the planned treatment is transferred from the
patient to the phantom geometry, the treatment plan is recalculated on a CT
scan of the phantom and then the dose distribution is measured using octavius
device and compared the fluence of dose measure with the fluence dose of the
treatment plan and the acceptance criteria is take in to account if the plan is
approved or not depending of pass criteria.
In case of quality control of multileaf colimator there are, several form to
prove if the good movement of the multileaf, there is a test based in alphabetic
letter, in other case used software for to check the good movement of the
multileaf, but in our case we use the The analysis of the film irradiated with a
picket fence pattern, and visual behavior of the leaf in the film.
3

4. Master in Medical Physics 2015 to 2016
IV. Results and discussion
All plans of the patient made with the VMAT technique were analyzed using
the three parameter %DA (limit 3%), DTA (limit 3%), (limit 3 mm), and γ-index
with the 3% dose tolerance and 3 mm distance to agreement in relation to the
treatment planning system. The gamma criterion was considered fulfilled if γ
<1 in at least 95% of the points. Results confirmed a good agreement between
the two distribution with high and conformed dose to the target and low dose
to the organ at risk in our practice the plan was evaluated with strict analysis
3mm and 3% and obtaining a 98.7% of acceptance and pass criteria.
Qualitative examination of garden fence field. The analysis of the film
irradiated with a picket fence pattern shows that in both irradiated films the
shifts are easy detectable at visual inspection. Fig.3 displays the different
patterns used to perform the QA protocol.
Figure 3: Different fields pattern used in test of good positioning in multileaf collimator.
V. Conclusion
• An important benefit of this technique is the determination of a quality
measurement, termed the γ index, that indicates the difference between
the calculation and measurement relative to the acceptance tolerances.
• A γ-index distribution can be generated and displayed, providing a quan-
titative assessment of the quality of the calculation, both in regions that
pass and fail the acceptance criteria.
• The accuracy of the delivered dose in the patient with advanced techniques
depends on dosimetric characteristics of the multileaf collimators. There
is an option for optimizing the jaws to the irregular MLC field to reduce
the scattered radiation and interleaf radiation leakage beyond the field.
References
[1] Sharpe MB et al., Monitor unit settings for intensity modulated beams delivered
using a step-and-shoot approach., Med Phys. 2000 Dec;27(12):2719-25.
4

5. Master in Medical Physics 2015 to 2016
[2] Rossela Vidimari, Lecture of exercise in radiotherapy, ICTP Trieste Italy, 2nd
trimester 2015.
5