Locally advanced Rectal cancer debate: adjuvant chemotherapy following neoadj...
Doctoral Dissertation
1. IMRT and Rotational IMRT (mARC)
Using
Flat and Unflat Photon Beam
Doctoral Dissertation
Of
Amal Sheta
Klinik und Poliklinik für
Strahlentherapie
Advisors
Prof. Ulrich Wolf
Prof. Thomas Kuhnt
2. Outline
Introduction
Aim of the Work
Results
IMRT using photon beam with and without FF
mARC and IMRT
Conclusion
2
Effect of Flattening filter (FF)
Treatment Techniques
Dosimetric characteristics of FF and FFF beams
Two Planning comparison Studies
4. Effect of Flattening Filter (FF)
4
Softening of the x-ray spectra
Reduction in head scattered
radiation
Non-uniform beam profile
High dose rate
Uniform beam profile
Significant decrease in output dose rate
Beam hardening
A major source of scatter and
leakage radiation
Krieger, Hanno. Strahlenphysik, Dosimetrie und Strahlenschutz: Band 2: Strahlungsquellen,
Detektoren und klinische Dosimetrie. Springer-Verlag, 2013.
7. Determine the main dosimetric characteristics of
FFF beams of Artiste linacs
Assess the effect of FFF beams on S&S-IMRT
treatment plans in comparison with those of FF
beams.
Estimate the performance of various mARC
techniques and compare their performance with
S&S-IMRT.
Aim of the Work
7
9. Planning Comparison Parameters
Plan quality
Achievement of the clinical
goals for PTV and OAR
Dose volume Histogram
(DVH) Analysis
Conformity Number (CN)
Homogeneity Index (HI)
Treatment Eficiency
Treatment delivery time
(TDT)
Reading of beam on time
9
11. 6 MV FF, 7 MV FFF 10 MV FF, 11 MV FFF
Depth dose curves almost similar, match exactly at 10 cm ×10 cm F.S and slight
differences are observed for larger and smaller F.S.
The beam softening due to flattening filter removal is compensated by the higher
maximum photon energy (higher electron energy on the target) of FFF beams.
Dosimetric characteristics
PDD Curves
11
12. Dosimetric characteristics
Dose Profile
The dose profiles for small F.S are almost identical and for larger F.S the difference
becomes more obevious.
For FFF beams the high photon energy shows profiles of steeper gradient.
At large F.S the out-of-field scatter is reduced due to removing the flattening filter.
12
16. Better
Plan Quality
FF and FFF Beam
HI & CN Prostate, Prostate-LN and H&N
Better
The dose homogeneity of IMRT-FFF is better than IMRT-FF plans for prostate
and comparable for H&N and prostate-LN .
The IMRT FFF plans have better conformity than IMRT FF for all cases
16
17. Treatment delivery time is the same for IMRT plans using FF beams and FFF beams
The number of MUs/Fx of IMRT plans with FFF beams is higher than with FF beams
and the %-differences of the number of MUs increase with increasing the volume of
PTV
Treatment Efficiency
FF and FFF Beam
17
22. CN & HI of Prostate,Prostate-LN and H&N using IMRT(7&9B) and mARC (SA&DA)
Plan Quality
IMRT and mARC
22
23. The treatment delivery time of prostate, prostate-LN and H&N plans
due to IMRT(7&9B) and mARC (SA&DA)
Treatment Efficiency
IMRT and mARC
Technique
Prostate
Time(min)
Prostate-LN
Time(min)
H&N
time(min)
SA(4) (90seg) 6:22 8:26 8:10
SA(6) (60seg) - 6:10 6:00
SA(8) (45seg) 3:30 4:46 4:41
DA(6) (122seg) - 9:10 10:45
IMRT 9B (50 or
60 segments)
6:21 8:00 6:47
23
24. The number of MU required to deliver the planned dose for prostate,
prostate-LN and H&N by using IMRT and mARC
Treatment Efficiency
IMRT and mARC
24
25. The shapes of the profiles of FFF beams were conical and affected by
the field size and the photon beam energy.
The FFF beams produce PDD curves with similar characteristics to FF
photon beams.
IMRT-FFF plans are clinically acceptable and comparable with IMRT-
FF plans but need more MUs and the differences of TDT are between -
20% to +25% in comparison with that of IMRT-FF plans.
mARC has a various options to create clinically acceptable treatment
plans with comparable dose distribution with S&S-IMRT.
The main advantages of mARC technique are the lower MUs than
IMRT and the possibility to shorten the TDT to the half.
Conclusions
25
Editor's Notes
As we see in this schematic diagram, At the right side we see the FF deals with the forwared peak of bermsstraulng x-ray to flatten the beam and at the left side the beam profile is nonuniform due to FF removal. The flattening filter also absorbs a large fraction of primary photons from the beam and hence removes an amount of beam intensity leading to significant decrease in output dose rate. Also the FF causes beam hardening and it is the major source of scatered and leakage radiation.
On the other hand the ff removale .... Nonuniform beam profile . this should not be a problem in case of modulated technique because the actual beam shape
can be taken into account in the segmentation process and therefore the filter should not be
necessary at all. Also removing leads to increasing dose rate ( that is useful for SRT)
In our clinic artiste linac produce ff and fff photon beams with a possibilty to go from S&S IMRT to Rotational IMRT...as we see the radiation beam rotate continously around the patient in the range of 360 degree and the dose delivered only at discrete angles.
Estimate the performance of various mARC techniques for tumor sites of different complexity and volumes and compare their performance with S&S-IMRT with static beams.
Planning comparison of IMRT (FF&FFF) using (7&9 field) and mArc (SA&DA) for different tumors sites
Here our clinical cases which used in the comparison studies of IMRT and mARC. The PTVs are in red color and the OAR in case of prostate and prostate-LN are blader (color), rectum(violot and blue) and femur(green), in case of H&N the OAR which we are interested here are right- left parotids and spinal cord.
MU Calculated by TPS
Treatment time measured by Linac does not include patient set-up time or verificated treatment position time
The calibration of 6 MV FF, 7 MV FFF and 10 MV FF, 11 MV FFF were done under the same
conditions: 10×10 cm field size, 100 cm SSD, Reference dose at beam central axis.
Our measurments show that PDD curves
With FFF beam the addition of soft x-rays to the beam spectrum lowers its mean energy. To avoid this effect siemens introduced FFF beams with higher maximum energies to get a dose distribution close to FF beams
Cross plane of 6 MV, 10 MV FF and 7 M V, 11 MV FFF beams at field size 3×3 cm2 and 30×30 cm2 normalized to the dose at central axis. (b) Out-of-field dose at field size 30×30 cm2 of 6 MV, 10 MV and 7 MV, 11 MV.
It can also be observed that at large eld sizes the out-of-eld doses due to FFF
beams get less than that of FF beams, as shown in Fig.4.1.b, in consequence of the
out-of-eld scatter that is reduced due to removing the flattening filter.
We have an example of the comparison between IMRT-FFF and IMRT-FF technique.
We see here Dose distribution of transversal ct sections and DVH of prostate LN in addtion to The ptv clinical goals of all cases included in our study. All these parameters show that the fff beams produce acceptable imrt plans and comparable with that created by ff beams
According to HI dose distribution within the PTV is more homogenous by applying uf beam in case of prostate . For H&N and prostateLN as large and complex ptv both modalities produce nearly the same homogeniety within PTV. CN values indicate that the IMRT FFF have better conformity than IMRT FF plans for all cases.
That result prove our hypothesis that however the FFF beams have inhomogeneous fleunce distribution, the superpostion of beam segments produce homogeneous dose distribution within the PTV
The CN value close to 1 means better PTV coverage and less irradiation for healthy tissue.. That can be understood through The out-of-eld doses of FFF-beams which are lower than that of FF-beams especially for F.Ss larger than 1010 cm2 and subsequently less dose is delivered to the surrounding tissues and OARs leading to more conformal plans
UF MUs Prostate = 1.3 * F MU , UF MUs H&N=1.5* F MU , UF MUs Pro(LN) = 2* F M
The meausured treatment delivery time is the same for IMRT FF and FFF plans.
IMRT -FFF need more MUs than that of the IMRT-FF plans to deliver the same prescribed dose . increasing the PTV volume requires more MUs per field in the beam case of FFF beams to compensate for the lower dose in the lateral part
removing the flattening filter leads to an increase of dose rate so the beam-on time is reduced but this does not mean that TDT will be decreased. The main parameters, which affect the TDT are the total number of MUs required to deliver the prescribed dose, the number of elds, the number of
segments and the leaf travel time from one segment to another depending on the shape of the segments. The TDT is aected by the number of MU/seg too, which determines
the applied dose rate for each segment.
The maximum output dose rate of Artiste
operating in FFF mode is 2000 MU/min but in order to maintain the dose linearity for
segments with low MU, the Control Console will automatically switch from the high
dose rate to the low dose rate of 500 MU/min for segments with less than 10 MU [7].
That might increase the overall TDT of IMRT-FFF plans
IMRT -FFF need more MUs than that of the IMRT-FF plans to deliver the same prescribed dose especially for large PTVs like prostate-LN and H&N . increasing the PTV volume requires more MUs per field to compensate for the lower dose in the lateral part .So we can say that The high dose rates from the FFF X-rays are now being off-set by the larger MUs requirements.
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For large treatment fields, the dose uniformity within an irradiated treatment field will need to be “modulated” by MLC movements (IMRT) to cut down the higher beam intensity near the central portion of the FFF X-ray beam. Thus, larger MUs are required compared with a conventional (flattened) X-ray beam. Or, MLC movements (IMRT) are now being used to “flatten” the FFF X-rays to provide dose uniformity within those large PTVs. The high dose rates from the FFF X-rays are now being off-set by the larger MUs requirements.
Another part of this work is about the mARC technique . mARC is a novel technique of arc therapy. It has special properties in comparison
with other VMAT techniques
so it was important to study mARC features and its variable plan parameters and how they affect the plan quality and treatment efficiency
of different types of cancer in comparison with S&S IMRT.
Schematic overview of an mARC delivery in the clockwise direction.
The main componant of the arc is the arclet . The arclet is defined by an optimization point (OP) that is situated at its middle and an arclet angle, , which determines its span. The optimization point (OP) corresponds to a gantry incidence defined by the treatment plan. No.of arclet is supposed to be of great importance for plan quality and treatment efficiency
The arc divided to beam on interval where the dose delivered through the arclet and beam off intervals (silent period) that follow the arclets. while Radiation OFF, leaves move to their next position and the gantry speed is adapted to optimize the next arclet delivery.
mARC technique should combine the speed of arc therapy with step and shoot modulation so it would be reasonable
to assume an improvement in:
* Treatment Efficiency
* Plan quality
The values with green color means that thechnique achieve the clinical goals for all patients. The values of orange color means that the technique is could not fulfile but close to the clinical goals limits for some patients.
Here we see an example of the dose distribution and DVHs for the IMRT 9B and the and mARC plans SA-4 & DA -6 .
Based on Our data of The visual examination of the dose distribution of the transversal CT sections and the DVHs of the IMRT and the mARC plans show that all plans
are clinically acceptable for all patients.
The rectum and the bladder as OARs of
the prostate-LN and the spinal cord of the H&N can sometimes be spared more using
mARC techniques
mARC plans of the prostate-LN, H&N and prostate resulted in comparable PTV dose homogeneity and
dose conformity with IMRT plans
This table shows The measured TDT time of SA (4) and SA (8) of prostate and SA (4) , SA-6 , SA-8 and DA (6) of prostate-LN and H&N in comparison with the TDT of the IMRT 9B of all cases.
The TDT of mARC plan is affected mainly by the value of FGS. As we observe here gradual increase in F.G.S leads to gradually decreasing in TDT.Till 40 % when SA (8) of 45 arclets is used instead of SA (4) of 90 arclets.
For prostate, reducing treatment time is the main advantage of using mARC technique over IMRT because we got short TDT and a acceptable plan quality.
In case of prostate-LN, the TDT of SA (4) and DA (6) plans are comparable with that of the IMRT.
In case of H&N, and in contrast to prostate and prostate-LN, the TDTs required to deliver IMRT plans were less than those of SA (4) and DA (6)
plans. This result can be explained by the number of the arclets that is larger than the number of the IMRT segments in combination with the PTV complexity that leads to complex
shapes of the arclets and hence longer time is required for MLC to adjust the arclets shapes leading to lower gantry speed and long treatment time.
This Figure shows the number of the MU for all patient groups of all techniques.
As we see , for prostate-LN and H&N The number of MU required to deliver the planned dose of sa-4 is lower than IMRT with significant difference in case of prostate-LN .
For prostate, the number of the MU required for delivering SA (8) is lower than that required for SA (4) and IMRT 7B and 9B plans by about 20 % with signicant dierence.
The main advantage of mARC technique is the lower MUs than IMRT to deliver the same prescribed dose and the possibility to shorten the TDT, which allows to treat more patients per machine or to reduce the working hours. Furthermore, it reduces the risk of patient movement and increases the patient's comfort.
