Silvia Pella, PhD, DABR Brian Doozan, MS South Florida Radiation Oncology Florida Atlantic University Advanced Radiation Physics Boca Raton, Florida

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1 American Association of Medical Dosimetrists 2015 Silvia Pella, PhD, DABR Brian Doozan, MS South Florida Radiation Oncology Florida Atlantic University Advanced Radiation Physics Boca Raton, Florida

2 Most common non-cutaneous malignancy worldwide Accounts for most cancer deaths in men & women (1) Standard care for early stage (I & II) cancer: surgical resection: provides five year survival of 50 70% (2) Many patients inoperable Stereotactic body radiation therapy (SBRT) or Intensity modulated radiation therapy (IMRT) are curative treatment options for inoperable patients

3 Uniform distributed versus focused in one side 9 beams 25 0 between

4 Beam angle optimization (BAO)

5 PTV Lower request (max dose) versus upper (min dose) Volume PTV optimization or not Priority Ring or no ring? Dose zone or not? Critical organs Volumetric or point dose? Smoothing Normal tissue objective

6 Dose volume histogram (DVH) and dose distribution after first round of optimization (5 min)

7 Dose volume histogram (DVH) and dose distribution same optimization time

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10 For PTV s plan conformity 1. ICRU conformity index CI RI (3), defined as: CI RI = VRI TV (1) V RI = volume of tissue covered by the reference isodose (RI=100%) TV = PTV A value close to 1.0 indicates better conformity of dose to target

11 1. Intermediate dose spillage and falloff gradient beyond PTV assessed using 2 indexes: R 50% and D 2cm R 50% ratio calculated with a reference isodose of 50% of the prescription dose D 2cm represents the maximum cumulative dose to any point located at 2 cm away from the PTV Lower values of both indicate greater dose falloff and better conformity

12 For PTV s plan homogeneity 1. Homogeneity index HI, defined as: D5% HI 5 95 = D 95% (2) D 5% and D 95% = minimum doses delivered to 5% and 95% of the PTV Values if HI closest to 1 indicate greater homogeneity within the target

13 1. A. Jemal, R. Siegel, J. Wu, E Ward; Cancer statistics, CA Cancer J Clin 2010;60(5): A randomized phase II study comparing 2 stereotactic body radiation therapy (SBRT) schedules for medically inoperable patients with stage I peripheral non-small cell lung cancer. RTOG Philadelphia PA: RTOG; International Commission on Radiation Units and Measurements (ICRU). Prescribing, recording and reporting photon beam therapy. ICRU Report 62. (Supplement to ICRU report 50). Bethesda, MD: ICRU publications; 1999

14 The planning process uses several algorithms throughout the procedure In Eclipse (version 11.31) Plan Geometry Optimizer(PGO) Iterative process used in Beam Angle Optimization (BAO) Dose Volume Optimizer (DVO) Process that quickly evaluates the Dose Volume Histogram with the constraints and scores an objective function Analytical Anisotropic Algorithm (AAA) Used in final calculation of the dose for the DVH and isodose lines Heterogeneity corrections are used 14

15 Eclipse (beam angles optimization) BAO starts with uniformly spaced co-planar beams with the planning volume (PTV) at isocenter The score for each beam is calculated based on constraints given to organs at risk (OAR) and PTV volume Lowest weighted beams are deleted and the process continues until 5-9 beams are left (user defined) Once 9 beams are left, the beams are scored again If no difference- delete beams until minimum is reached 15

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18 The objective function is used to score how well a current plan does against the constraints that are given When the objective function is minimized, the optimization is complete If a plan needs to be modified, the constraints must be changed SS = ww(dddddddd rrrrrrrrrrrrrrrr DDDDDDDD pppppppppppppppppppp ) 2 2 ii + ww jj AA jj (DDDDDDDD rrrrrrrrrrrrrrrr DDDDDDDD aaaaaaaaaaaaaaaaaa ) jj ii w is weight of the PTV ww jj is the weight of the OAR AA jj is a parameter that is 0 if DDDDDDDD rrrrrrrrrrrrrrrr < DDDDDDDD aaaaaaaaaaaaaaaaaa and 1 otherwise jj 18

19 The goal of radiation treatment is to give as little damage to healthy tissue while sterilizing the maximum amount of tumor cells A compromise must often be reached in the tumor control probability (TCP) and the normal tissue complication probability (NTCP) 19

20 Use a DVH to see the dose distribution to OAR and PTV Calculate the conformality index 1 CCCC = VV 50% VV PPPPPP VV 50% is the volume that is covered by 50% of the dose VV PPPPPP is the volume of the PTV Lower values are more favorable Conformation number 2 CCCC = VV PPPPPP cccccccccccccc bbbb 100% 2 VV 100% VV PPPPPP Should be as high as possible (>.95) Important to look at isodose lines on the plan 20

21 Many cancers can be treated today with radiation Radiation therapy plans can vary from different centers Dosimetrists have their own techniques and levels of experience Goal to increase the efficiency, consistency and quality of plans Beam Angle Optimization (BAO) in Eclipse currently gives poor results- dosimetrists manually plan gantry positions IMRT currently does not employ optimization of collimator rotation Empirical method of BAO uses plans from experienced dosimetrists Has shown to improve the dose conformality of the plans compared to Eclipse BAO 21

22 Find quality plans of similar tumors Plot the gantry angles that have been used Locate the most frequently used angles Determine the number of fields that were used Create a template for a global positioning based on the information Local optimization can be applied to move each field away from OAR (not implemented in this study) Referred to as FAU BAO 22

23 Each lung is divided into 4 sections The center of the tumor is used for selection of location 23

24 Right Medial Frequency Left Medial Frequency 24

25 Spikes in the frequency indicate favorable gantry positions for a higher number of patients. Areas of low or no data indicate field angles that should be avoided Right lesions should have more fields on the right side of the patient to protect contralateral lung 25

26 To compare the two methods, 7 plans were produced with Eclipse BAO Optimization included 6 constraints and were made with a prescription dose of 6000 cgy Normal tissue (weight 150) Max dose to cord 3000 cgy (weight = 50) Max dose to heart 2000 cgy (weight = 50) DD 5% = 2000 for total lung less than 2000 cgy (weight = 50) DD 100% = 5700 for PTV (weight 100) DD 95% = 6000 for PTV (weight 200) All plans used IMRT and a sliding window for dose delivery Optimization was allowed to run until completion 26

27 Results of a plan made with Eclipse BAO (top) and FAU BAO (bottom) FAU BAO uses 9 fields, Eclipse BAO uses 5 FAU BAO uses near AP and PA beams and fields coming from the right side of the patient Eclipse BAU chooses many angles away from the right side 27

28 Isodose lines from Eclipse BAO (top) and FAU BAO (bottom) Similar results for the 100% isodose line (blue) FAU BAO shows a more conformal 80% isodose line (red) and smaller 50% isodose line (light blue) Eclipse BAU has extensions with the smaller isodose lines 28

29 OAR are measured by use of a dose volume histogram (DVH) Serial organs are evaluated from a maximum dose point Parallel organs have volume tolerance limits Spinal cord maximum dose for 60 Gy IMRT treatment is 42 Gy, or 70% of the prescription The heart is also a serial organ. A maximum of 2130 cgy can give given before non-stochastic effects are observed The total lung (minus the PTV) covered by 20 Gy should be less than 5% Contralateral and ipsilateral lungs also need to be evaluated before treatment begins Other organs to consider are airway, ribs, liver and the blood vessels 29

30 The FAU BAO was lower for 5 of the 7 cases. A large difference in the data corresponds for set case 4 Small lesion which was almost entirely avoided by Eclipse BAO (imaged below) All plans met the requirement to be below 70% of the prescription dose 30

31 The Eclipse BAO (top) gives 5 fields that gives high dose (50% of prescription cyan) to areas well away from the PTV FAU BAO (bottom) shows a much more conformal pattern using 9 fields around the left side of the patient 31

32 The addition of fields for Eclipse BAO (below) decreases the 50% isodose lines, but high dose regions remain far from the PTV. 32

33 Add in collimator optimization Increase the number of patient plans in the database Increase the number of segments of the lung 33

34 Currently, there is no collimator optimization in Eclipse Rotating the collimator can change the outcome of the plan with all other optimization parameters fixed. Chapek et al. * showed that placing the collimator at optimal angles has shown increased protection for prostate cases Collimator rotation can avoid split fields Variable collimators avoid intraleaf leakage 34

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39 Tumor size of about 10cm across 39

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41 Tumor size of about 7cm x 5cm 41

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43 Small tumor size of about 1.5cm across 43

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45 The change in Monitor Units is most drastic for large, less spherically shaped lesions The test phantom dropped by 40% Case 1 (largest tumor), the drop in MU was 20% Case 2 (medium tumor) and Case 3 (smallest) had MUs of similar values 45

46 Empirical optimization methods for finding best gantry angles can improve what is used in the Eclipse BAO The conformality index improved by an average of 30% and 3% compared to the 5 and 9 field Eclipse BAO The conformation number improved an average of 12% and 9% The OAR were overall improved All plans were normalized to give the same coverage of the PTV to give equal TCP Monitor units increased in the FAU BAO The maximum dose was lower on average for FAU BAO Future improvements for more segmentations in the lung can improve the empirical method Collimator optimization could improve the Monitor Units and better protect the organs at risk 46

47 Questions? Discussions 47