Doctoral Dissertation

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<p>Comparing IMRT and VMAT for the Treatment of Prostate Cancer with LN</p> <p>IMRT and Rotational IMRT (mARC) Using Flat and Unflat Photon Beam</p> <p>Doctoral DissertationOf Amal ShetaKlinik und Poliklinik fr Strahlentherapie AdvisorsProf. Ulrich WolfProf. Thomas Kuhnt</p> <p> OutlineIntroduction</p> <p>Aim of the WorkResults</p> <p> IMRT using photon beam with and without FF mARC and IMRTConclusion2 Effect of Flattening filter (FF)</p> <p> Treatment Techniques </p> <p> Dosimetric characteristics of FF and FFF beams</p> <p> Two Planning comparison Studies</p> <p>INTRODUCTION3</p> <p> Effect of Flattening Filter (FF)4 Softening of the x-ray spectra Reduction in head scattered radiation</p> <p> 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 </p> <p>Krieger, Hanno. Strahlenphysik, Dosimetrie und Strahlenschutz: Band 2: Strahlungsquellen, Detektoren und klinische Dosimetrie. Springer-Verlag, 2013.</p> <p>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.</p> <p>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 shapecan be taken into account in the segmentation process and therefore the filter should not benecessary at all. Also removing leads to increasing dose rate ( that is useful for SRT) </p> <p>Treatment TechniquesStep and Shoot IMRT and Rotational IMRT (mARC)5</p> <p>In our clinic artiste linac produce ff and fff photon beams with a possibilty to go from S&amp;S IMRT to Rotational we see the radiation beam rotate continously around the patient in the range of 360 degree and the dose delivered only at discrete angles.5</p> <p>Aims of the work6</p> <p>Determine the main dosimetric characteristics of FFF beams of Artiste linacsAssess the effect of FFF beams on S&amp;S-IMRT treatment plans in comparison with those of FF beams.Estimate the performance of various mARC techniques and compare their performance with S&amp;S-IMRT.</p> <p>Aim of the Work7</p> <p>Estimate the performance of various mARC techniques for tumor sites of different complexity and volumes and compare their performance with S&amp;S-IMRT with static beams.</p> <p>7</p> <p>Clinical Cases </p> <p>8prostate with LNH&amp;Nprostate</p> <p>Planning comparison of IMRT (FF&amp;FFF) using (7&amp;9 field) and mArc (SA&amp;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&amp;N the OAR which we are interested here are right- left parotids and spinal cord.8</p> <p> Planning Comparison Parameters9</p> <p>MU Calculated by TPS</p> <p>Treatment time measured by Linac does not include patient set-up time or verificated treatment position time</p> <p>9</p> <p> Dosimetric characteristicsof FF and FFF beamsRESULTS10</p> <p>10</p> <p>6 MV FF, 7 MV FFF10 MV FF, 11 MV FFFDepth 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. </p> <p>Dosimetric characteristics PDD Curves11</p> <p>The calibration of 6 MV FF, 7 MV FFF and 10 MV FF, 11 MV FFF were done under the same conditions: 1010 cm field size, 100 cm SSD, Reference dose at beam central axis.</p> <p> 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 </p> <p>11</p> <p>Dosimetric characteristics Dose Profile</p> <p>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.</p> <p>. </p> <p>12</p> <p>Cross plane of 6 MV, 10 MV FF and 7 M V, 11 MV FFF beams at field size 33 cm2 and 3030 cm2 normalized to the dose at central axis. (b) Out-of-field dose at field size 3030 cm2 of 6 MV, 10 MV and 7 MV, 11 MV.</p> <p>It can also be observed that at large eld sizes the out-of-eld doses due to FFFbeams get less than that of FF beams, as shown in Fig.4.1.b, in consequence of theout-of-eld scatter that is reduced due to removing the flattening filter.12</p> <p>IMRT planning comparisonusingFF and FFF Photon BeamsRESULTS13</p> <p>13</p> <p>14Clinical CasesPTVs Clinical GoalsIMRT-FFIMRT-FFFProstateDmean = 74 GY73.95 0.0473.94 0.04D98 70.3 Gy70.8 0.8771.8 0.36D2 77.7 Gy76.3 0.575.9 0.3Prostate- LNDmean = 50.4 Gy50.0 0.350.2 0.2D98 47.9 Gy 47.3 0.847.7 0.7 D2 52.9 Gy52.0 0.652.3 0.5H&amp;N Dmean = 50 Gy50.1 0.3050.1 0.2D98 47.5 Gy 47.8 0.747.7 0.6D2 52.5 Gy52.2 0.4552.3 0.35</p> <p>The PTV clinical goals of the prostate, prostate-LN and H&amp;N, in comparison with the calculated values IMRT FF and IMRT FFF Plan Quality FF and FFF Beam</p> <p> Plan Quality FF and FFF BeamFF Beam (10 MV)FFF Beam (11 MV)</p> <p>100 % = 50.4 Gy 15</p> <p>We have an example of the comparison between IMRT-FFF and IMRT-FF technique.</p> <p>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 beams15</p> <p>Better</p> <p>Plan Quality FF and FFF BeamHI &amp; CN Prostate, Prostate-LN and H&amp;NBetter The dose homogeneity of IMRT-FFF is better than IMRT-FF plans for prostate and comparable for H&amp;N and prostate-LN .</p> <p> The IMRT FFF plans have better conformity than IMRT FF for all cases</p> <p>16</p> <p>According to HI dose distribution within the PTV is more homogenous by applying uf beam in case of prostate . For H&amp;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. </p> <p> 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</p> <p>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 plans16</p> <p>Treatment delivery time is the same for IMRT plans using FF beams and FFF beamsThe 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 </p> <p>Treatment Efficiency FF and FFF Beam</p> <p>17</p> <p>UF MUs Prostate = 1.3 * F MU , UF MUs H&amp;N=1.5* F MU , UF MUs Pro(LN) = 2* F M</p> <p>The meausured treatment delivery time is the same for IMRT FF and FFF plans. </p> <p>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 </p> <p>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 ofsegments 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 determinesthe applied dose rate for each segment. </p> <p>The maximum output dose rate of Artisteoperating in FFF mode is 2000 MU/min but in order to maintain the dose linearity forsegments with low MU, the Control Console will automatically switch from the highdose 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</p> <p>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&amp;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. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------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. 17</p> <p> Planning comparison between IMRT and mARCRESULTS18</p> <p>Another part of this work is about the mARC technique . mARC is a novel technique of arc therapy. It has special properties in comparisonwith other VMAT techniques </p> <p>so it was important to study mARC features and its variable plan parameters and how they affect the plan quality and treatment efficiencyof different types of cancer in comparison with S&amp;S IMRT.18</p> <p> mARC Module </p> <p>F.G.S</p> <p>No of (OP) = No of segments = arc span / F.G.S, Range: 4 1519</p> <p>* Artiste mARC Treatment planning Guide</p> <p>Schematic overview of an mARC delivery in the clockwise direction. </p> <p>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</p> <p>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. </p> <p>mARC technique should combine the speed of arc therapy with step and shoot modulation so it would be reasonableto assume an improvement in:* Treatment Efficiency</p> <p> * Plan quality </p> <p>Clinical CasesPTVs Clinical GoalsSA (8)SA (4)DA (6)IMRT 7BIMRT 9BProstateDmean = 74.0 GY74.1 0.0673.9 0.1373.8 0.1D98 70.3 Gy70.7 0.570.8 0.7570.7 0.7D2 77.7 Gy76.6 0.476.0 0.275.9 0.24Prostate-LNDmean = 50.4 Gy50.4 0.050.5 0.0450.3 0.0850.3 0.05D98 47.88 Gy 48 0.248.1 0.1447.6 0.1347.9 0.30 D2 52.9 Gy52.3 0.1252.2 0.2652.3 0.1352.2 0.3H&amp;N Dmean = 50.0 Gy49.9 0.0249.90.0549.90.0549.90.06D98 47.5 Gy 47.8 0.1547.70.2247.6 0.1347.6 0.17D2 52.5 Gy51.7 0.1551.7 0.251.8 0.1351.7 0.24</p> <p>Plan Quality IMRT and mARCThe PTV clinical goals of the prostate, prostate-LN and H&amp;N, in comparison with</p> <p> the calculated values of SA (4), DA (6), IMRT 7B and IMRT 9B </p> <p>20</p> <p>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. 20</p> <p> IMRT(9B) SA(4) DA(6)</p> <p> DVH IMRT(9) SA(4)----- DA(6). </p> <p>Plan Quality IMRT and mARC</p> <p>21</p> <p>Here we see an example of the dose distribution and DVHs for the IMRT 9B and the and mARC plans SA-4 &amp; 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 plansare clinically acceptable for all patients.</p> <p>The rectum and the bladder as OARs ofthe prostate-LN and the spinal cord of the H&amp;N can sometimes be spared more usingmARC techniques21</p> <p> CN &amp; HI of Prostate,Prostate-LN and H&amp;N using IMRT(7&amp;9B) and mARC (SA&amp;DA)</p> <p>Plan Quality IMRT and mARC</p> <p>22</p> <p>mARC plans of the prostate-LN, H&amp;N and prostate resulted in comparable PTV dose homogeneity anddose conformity with IMRT plans22</p> <p>The treatment delivery time of prostate, prostate-LN and H&amp;N plans due to IMRT(7&amp;9B) and mARC (SA&amp;DA)</p> <p>Treatment Efficiency IMRT and mARCTechniqueProstate Time(min)Prostate-LN Time(min)H&amp;N time(min)SA(4) (90seg)6:228:26 8:10SA(6) (60seg)-6:106:00SA(8) (45seg)3:304:464:41DA(6) (122seg)-9:1010:45IMRT 9B (50 or 60 segments)6:218:006:47</p> <p>23</p> <p>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&amp;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&amp;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 complexshapes 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.23</p> <p>The number of MU required to deliver the planned dose for prostate, prostate-LN and H&amp;N by using IMRT...</p>


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