الفهرس 
ntroduction And Review of literature
Three-dimensional conformal radiation therapyOnly 14 pages are availabe for public view
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Abstract
Radiation therapy (RT) plays a critical role in the
management of cancer patients. The goal of radiotherapy is to
achieve tumor control without causing complications. The use of the
modern linear accelerator has become a very precise tool, capable of
depositing a defined dose to a specific volume of tissue. This has
been made possible by rapid advances in technology, including
intensity modulation and image guidance in real time. These
developments have been particularly useful in allowing sparing of
normal tissues lying in close proximity to tumors.
Each patient who undergoes curative radiotherapy has an
individualized treatment plan. Often the ideal plan cannot be created
and the chosen clinical approach represents a trade-off between
ensuring that the dose to the tumor is acceptable whilst minimizing
the risk of complications to normal tissue. Understanding how this
dose distribution translates into a biological effect is a key in
producing successful treatment plans.
Currently, the treatment planning process is defined and
evaluated only in terms of physical dose and physical volume, by
dose volume histogram and dose distribution homogeneity but this is
not enough because there are different factors affecting the treatment
outcome, these factors are;
Number of fractions and fraction size (dose fractionation).
Overall treatment time.
Type of tumor (it’s radiosensitivity)
Dose and volume of healthy tissueThese factors are considered as biological parameters. Biologic
indices represent an alternative method for evaluating treatment plans
to take in consideration the biological parameters. Criteria for an
optimal plan include both the biological and the physical aspects of
radiation oncology. By definition, an optimal plan should deliver
tumoricidal dose to the entire tumor and spare all the normal tissues.
These goals can be set, but are not attainable in the absolute terms. To
achieve quantitative biologic endpoints, models have been developed
involving biologic indices such as tumor control probability (TCP)
and normal tissue complication probability (NTCP).
So the aim of this work was to estimate a radiobiological method to
represent the outcome of different treatment plans, in external
beam radiotherapy and to apply the new method in evaluation and
comparison of different treatment plans and to use the biological
dose distribution to recommend an optimal fractionation schedule as
well as an optimal treatment plan.
In an attempt to launch a model to evaluate treatment plans in
advanced radiotherapy, we have studied some common evaluation
indices. In physical evaluation, we studied dose homogeneity indices
(MHI and HI), target coverage and conformity indices (PITV, TCI,
CI, and CN), dose gradient (GI and GM) and an index for overall
plan quality factor (QF), in addition to the total number of monitor
units. In Biological evaluation we studied TCP and NTCP for tumor
and critical structures, and P+ for free complication tumor control.
Evaluation has been performed for twelve plans, four rapidarc plans,
seven IMRT and one 3DCRT plan.
The used rapidarc plans are;One 300o arc from 210o to 150o with anterior 40oavoidance
sector, (1FRA).
One full rotation single arc (SA).
Two 130o lateral arcs (from 210o to 340o and from 20o to 150o)
(2HA).
Double Arcs with one full rotation (360o) arc and one (260o)
Arc: from 230o to 130o (DA).
Seven IMRT plans with different number of beams have been
studied. The number of beams range from 5 beams to 11 beams. The
beam angles in all plans were optimized using Eclipse IMRT
optimization module supplied with v13.5 of Varian Medical Systems
Eclipse planning software on which all plans have been performed. In
3D conformal techniques, five fields have been used.
A conventional schedule with a daily dose of 2 Gy for a total
dose of 76 Gy in 38 fractions over treatment time of 52 days has been
used. The other schedule was a hypofractionated schedule with a
daily dose of 3 Gy for a total dose of 69 Gy in 23 fractions in overall
treatment time of 31 days. In all techniques and schedules, dose
distribution was normalized and prescribed on mean dose.
We tried to convert the physical dose distribution of the twelve
plans under study into a biological dose distribution by adding two
tables representing the BED values of each dose level for the PTV
and OARs.
The dose distribution and DVH’s of the twelve plans for 18
patients of cancer prostate have been calculated and analyzed and
they were not sufficient to rank the different plans. The analysis of Dose statistical quantities of PTV showed a homogeneous dose
distribution in all plans. The same result was obtained by calculating
Seven different forms of HI. Target coverage indices and conformity
indices pointed out that all plans are well covered with conformed
dose. Gradient indices were almost equal in all plans. So we found
that all those physical indices are not enough in comparison of
different treatment plans. Therefore, we added all the physical
evaluation indices in a single factor. This factor is the quality factor
QF. The difference in plan quantitative quality was very clear and
statistically significant between different plans. The higher values of
QF were obtained in the four rapidarc plans.
Dose delivered to OARs were estimated and compared for
different plans. Both rapidarc and IMRT have lower doses to bladder,
rectum and heads of femur in comparison with 3DCRT. rapidarc plan
with avoidance sectors (2HA) is demonstrated to deliver the lowest
doses to all OAR’s.
In biological evaluation we pointed that, all plans have almost
similar TCP values. On the other hand, there is significantly
difference in the NTCP values and accordingly in P+ values. The
lowest value of P+ was obtained in the rapidarc teqnique with
avoidance sectors.
Both rapidarc and IMRT have lower doses to bladder, rectum
and heads of femur in comparison with 3DCRT. The reason of that
finding is the usage of inverse planning of both IMRT and rapidarc.
In IMRT technique we pointed out that the directions of the beams is
more critical in OARs dosimetry than the number of beams. IMRT
plans with beams facing any OAR produce a higher dose to that
organ regardless the number of beams. In rapidarc plans the reason ofThe low dose to OARs is the using of avoidance sectors in front of
the OARs.
In applying avoidance sectors in rapidarc treatment planning,
it is very important to notice that the starting and ending angels of the
treating arcs will affect the dose to OARs.
In order to evaluate treatment plans created in different
fractionation schemes BED distribution has been calculated. The
BED values of the PTV were higher in conventional dose
fractionation schedule than that in hypofractionation. In the same
time the BED values of OARs were lower in conventional dose
fractionation schedule than that in hypofractionation because of its
low value (3 Gy).This method of presenting the planning
outcome allowed us to judge both of the physical treatment planning
quality and the effectiveness of the fractionation schedule.
According to the results obtained in this study we concluded
that BED distribution is essential in physical and biological
evaluation of treatment planning and dose fractionation.