A Comparison of IMRT and VMAT Technique for the Treatment of Rectal Cancer Tony Kin Ming Lam Radiation Planner Dr Patricia Lindsay, Radiation Physicist Dr John Kim, Radiation Oncologist Dr Kim Ann Ung, Radiation Oncology Fellow American Association of Medical Dosimetrists 39 th Annual Meeting June 5, 2014
Princess Margaret Cancer Centre Toronto, Canada
Princess Margaret Cancer Centre Radiation Medicine Program 1) 16 Linear Accelerators 2) 1 Superficial / Orthovoltage Machine 3) 4 CT Scanners 4) 2 MR Scanners 5) Brachytherapy Suite 6) 1 Gamma Knife Perfexion unit 7) 1 Magnetic Resonance Image Guided Radiotherapy unit ( can be used clinically soon) 8) 55 Pinnacle planning stations 9) 40 Planners
Background Role of adjuvant radiation (RT) in addition to surgery in the management of locally advanced rectal cancer Reduce the risk of local recurrence Radiation can be given pre or post operatively
Background Recently, there has been significant progress in the pelvic RT technique from conventional RT to a more conformal approach using IMRT and VMAT 115 patients with rectal cancer treated with RT in our cancer center within the year 2013
Objectives 1) Describe Princess Margaret Cancer Centre s RT technique in rectal cancer 2) Compare the planning parameters and dosimetry distribution for these RT techniques: 1) 3 field conventional RT 2) Intensity modulated RT (IMRT) 3) Volumeric modulated RT (VMAT) 3) Physics QA measurement in IMRT and VMAT
Methods Retrospective study: 1) Five pre existing IMRT pelvic RT plans were randomly selected 2) A VMAT plan was generated for each dataset to achieve the same level of planning target volume (PTV) dose coverage 3) Both IMRT and VMAT plans were generated by using inverse planning algorithm in a treatment planning system 4) Dose prescription = 50 Gy in 25 fractions
Methods 1) IMRT: 7 co planer 6 MV beams 2) VMAT: 2 full 360 arcs (6 MV) 3) IMRT and VMAT plan quality comparison: Measuring the conformity index (CI) of PTV Dose volume statistics of the organ at risk (eg. small bowel, femur and genital) Definition of CI = ratio of PTV volume receiving at least 95% of the prescribed dose divided by the PTV volume
Workflow for Rectum RT
Treatment Protocol Clinical indications for pre operative RT in rectal cancer: Adenocarcinoma Locally advanced Stage T1 2 N1 2, T3 4 N0 2
Contour Delineation Gross Tumor Volume Primary GTV Primary rectal tumour from physical exam, endoscopy, diagnostic imaging and CT/MRI simulation Gross lymph nodes Any gross or suspicious lymph nodes
Contour Delineation Clinical Target Volume Primary CTV 1 cm circumferential expansion on GTV + 1 cm radial margin on rectum at the level of GTV and 2 cm above and below CTV of gross lymph nodes 1 cm circumferential expansion on gross lymph nodes mctv 1 cm circumferential expansion around the contoured mesorectum
Contour Delineation Clinical Target Volume LCTV and RCTV 1.5 cm expansion around left & right internal iliac vessels Superior S1/S2 junction Inferior pelvic floor defined by the upper surface of the levator muscle Anterior approx. mid portion of femoral heads Planning Target Volume (PTV) 7 mm circumferential expansion on all CTVs
Contour Delineation Organ at Risk By oncologist: Anal Mesorectum Rectum Rectosigmoid Large bowel: to top of L5 or minimum 2 cm above PTV Small bowel: to top of L5 or minimum 2 cm above PTV Bladder By the planner: Right & left femur
CT and MRI Simulation 1) Prone position 2) Comfortably full bladder for planning & treatment Voiding and drinking 500 ml of water 1 hour before simulation & treatment 3) IV contrast facilitate identification of blood vessels 4) Oral contrast facilitate identification of small bowel 5) MR simulation fused with the planning CT scan (bone match)
Advancements in RT Technique 3 field technique Wedge Segments IMRT VMAT
3 field Rectum with Segments Beam Orientation
3 field RT technique DRR Posterior field Lateral field
Posterior Field Segments 3 field Rectum Technique
Lateral Field Segments 3 field Rectum Technique
3 Field Dose Distribution
Advantages of Using Segmentations (Without Wedge) 1. Save treatment time Therapists do not need to go into the room to insert wedge 2. Reduce error using incorrect wedge 3. Move one step forwards to IMRT practice
IMRT Rectum Prone position Seven 6 MV co planer beams Beam angles: 0, 40, 85, 160, 200, 275, 315
3 field RT vs. IMRT 3 field technique IMRT IMRT rectum: Enables 50 Gy to be delivered (compared to 45 Gy with 3 field technique) PTV overlapping with small bowel receives at least 45 Gy but not more than 47.5 Gy (95% prescribed dose) Anterior pelvis can be spared (receiving <50% isodose level, similar to 3 field technique)
Planning criteria for VMAT 1) Meet dose constraints for OARs 2) Cover PTV by 95% prescription dose. To achieve PTV D99= 95% prescription dose 3) To keep hot spot within 108% prescription dose
VMAT Rectum Planning Process Prone position Two 360 arcs (6 MV) Clockwise Counter clockwise Gantry spacing = 3 Collimator angle=0, 15 to minimize fields overlapping to reduce tongue & groove effect
VMAT Dose Avoidance Volumes VMAT VMAT As VMAT rectum uses two 360 arcs, 2 dose avoidance volumes are created to limit dose to lateral & anterior pelvis Lateral locations (green) Anterior location (blue)
VMAT Dose Avoidance Volumes VMAT The anterior dose avoidance volume is divided into 2 sections with different dose constraints: Superior (blue) higher dose constraint Inferior (pink) lower dose constraint
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #1 Transverse CT Cyan = 50% isodose line
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #1 Sagittal CT Cyan = 50% isodose line
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #1 Coronal CT Cyan = 50% isodose line
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #1 Transverse CT Pink = 20% isodose line
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #1 Sagittal CT Pink = 20% isodose line
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #1 Coronal CT Pink = 20% isodose line
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #3 Transverse CT Genital dose sparing
Dose Distribution IMRT vs. VMAT IMRT VMAT Case #3 Sagittal CT Genital dose sparing
Conformity Index PTV (CI) CASE # IMRT VMAT #1 0.987 0.985 #2 0.977 0.985 #3 0.985 0.969 #4 0.986 0.992 #5 0.987 0.987 Range 0.977 0.987 0.969 0.992 Mean Dose 0.983 0.981 CI of PTV = V95 / PTV volume
Maximum dose to 0.5 cc Small Bowel (Gy) CASE # IMRT VMAT #1 47.35 47.83 #2 48.09 46.88 #3 46.11 45.83 #4 29.64 31.74 #5 48.80 48.27 Range 47.4 48.4 45.8 48.3 Mean Dose 47.6 47.2
Maximum dose to 0.5 cc Right Femur (Gy) CASE # IMRT VMAT #1 44.6 41.9 #2 42.0 36.6 #3 43.9 43.0 #4 44.0 41.9 #5 44.2 44.0 Average max dose 43.7 41.5
Maximum Dose to 0.5 cc Left Femur (Gy) CASE # IMRT VMAT #1 44 41.6 #2 43.1 41.5 #3 43.5 44.7 #4 45.9 45.0 #5 44.0 42.4 Average max dose 44.1 43.0
Maximum dose to 0.5 cc Genitals (Gy) CASE # IMRT VMAT #1 23.4 24.3 #2 15.9 18.9 #3 16.7 19.8 #4 11.6 16.9 #5 14.5 14.1 Average maxdose 16.4 18.8
50% Isodose Volume (cc) CASE # IMRT VMAT % difference respect to VMAT #1 5671 5468 3.7 #2 4084 4232 +3.5 #3 7910 7427 5.1 #4 4734 4726 0 #5 4229 4223 0 Average % difference 2.6% 50% isodose volume VMAT < IMRT (slight)
Low Isodose Volume 20% isodose (cc) CASE # IMRT VMAT % difference respect to VMAT #1 9408 10242 +8.1 #2 7632 8411 +9.3 #3 13988 15109 +7.4 #4 8597 9409 +8.6 #5 7348 7943 +7.5 Average % difference +8.2 Low isodose volume (20% isodose) VMAT > IMRT
Total Monitor Units (MUs) CASE # IMRT VMAT #1 533 400.6 #2 497 330.7 #3 570 471.1 #4 479.7 317.4 #5 550.9 371.4 Range 497 570 331 471 Mean Dose 520 411
Planning and Treatment Time 3 field RT IMRT VMAT Optimization time Treatment time 10 min 30 min 90 min 15 min 20 min 15 min Treatment time includes: Patient set up, cone beam CT verification, treatment delivery
IMRT vs. VMAT Comparison Summary Conformity index of PTV 0.5 cc max small bowel dose similar similar 0.5 cc max femur dose VMAT < IMRT 0.5 cc max genital dose VMAT > IMRT 50% isodose volume VMAT < IMRT 20% isodose volume VMAT > IMRT Total monitor units VMAT < IMRT Optimization time VMAT > IMRT Treatment time VMAT < IMRT
IMRT vs. VMAT Simulation process, patient set up and image verification with CBCT = same for IMRT and VMAT
Image Verification CBCT IMRT/VMAT rectum with daily pre treatment CBCT verification bone match
Physics Measurement QA IMRT Fluences (EPID or film) We compare the measured vs. computed results qualitatively (visual inspection) Simple QA test that the beams are being correctly delivered MapCheck 2 D array of diodes Quantitative comparison of planned vs. measured fluences Only used if fluence measurements show potential discrepancies
IMRT Example Fluence Computed Fluence Measured Fluence (EPID) Visual comparison of fluence pattern Check for leaf leakage Check for tongue & groove effect
Physics Measurement QA VMAT ArcCheck A 3D array of 1386 diode detectors Arranged in a spiral pattern 10 mm spacing We compare the results between measured and computed doses based on a 3%/2mm criteria Cannot use for prone orientation treatments Delta4 1069 diodes detectors Arranged in a matrix along two orthogonal planes 5 mm spacing We compare the results between measured and computed doses based on a 2%/2mm criteria Can use for either prone or supine orientations
VMAT Measurement Devices ArcCheck Delta4
VMAT Example ArcCheck Measurement Treatment Plan
VMAT Example Delta4
Physics Measurement QA Summary Time for QA Measurements IMRT < VMAT IMRT simple, visual comparison VMAT complex, quantitative
Conclusions Both IMRT and VMAT plans offer better PTV conformity and normal tissue sparing compared with 3 field technique Similar PTV conformity and normal tissue sparing can be achieved with IMRT and VMAT plans Advantages of VMAT: Reduces treatment delivery times Reduces monitor units 50% isodose volume slightly better than IMRT Disadvantages of VMAT: Low isodose volume (20% isodose) more than IMRT Increases optimization time Future improvements in treatment planning hardware & software can hopefully reduce planning time
Future Direction 1. 15 patients treated so far with VMAT it is time to review dosimetry quality on those 15 plans, especially, low dose region and genital dose Still have rooms for improvement in dosimetry quality 2. Follow up analysis of patients treatment toxicity to IMRT and VMAT
Acknowledgements Princess Margaret Cancer Centre GI site group Planners Physicists Oncologists