Cone Beam CT Protocol Optimisation for Prostate Imaging with the Varian Radiotherapy OBI imaging system. Dr Craig Moore & Dr Tim Wood

Transcription

1 Cone Beam CT Protocol Optimisation for Prostate Imaging with the Varian Radiotherapy OBI imaging system Dr Craig Moore & Dr Tim Wood

2 Background With the increasing use of CBCT imaging alongside complex radiotherapy treatment regimes, it is becoming more important to understand the implications of current practice On board CBCT daily imaging for verification of patient position is now common practice across the UK It is not acceptable to simply dismiss these concomitant exposures as negligible in comparison with the radiotherapy treatment dose Currently, all of our CBCT systems operate using Varian default settings A single set of exposure factors for all patients is clearly not optimised! Vital we have an idea of patient doses so that we can develop optimisation strategies

3 kv tube Treatment head MV beams generated here kv detector

4 Aims This talk will focus on: Development of a computational method to estimate dose and risk for CBCT prostate imaging Development of a strategy for patient sized protocol optimisation for CBCT prostate imaging

5 The first step The first phase of this project is to gain an understanding of the doses involved in CBCT imaging Given the context of these procedures (i.e. as part of a RT treatment), simple risk estimates based on the effective dose are probably not sufficient in isolation We need to start thinking about organ-at-risk tolerances and other healthy tissues that are not involved in the actual treatment Hence, we need to develop a broader understanding of where the dose is being deposited, i.e. organ doses What is the best way to do this? TLDs? A computational model? A bit of both?

6 Developing a CBCT dose model with PCXMC We have commercially available software (PCXMC) that is widely used for performing dose assessments for radiological examinations, etc Allows you to rotate around a reference point within a mathematical (Christy) phantom (ideal for modelling RT imaging) Only for simple uniform X-ray spectra

7 PCXMC Only uniform beams

8 Half-fan bow-tie filter = nonuniform beam Can we account this nonuniformity to make it fit with PCXMC

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10 The PCXMC model To model the Varian CBCT system, 8 projections around the patient were used (at 45 intervals), with equal weighting for the final dosimetry Each projection was split into 4 slithers to account for nonuniformity of the x-ray beam Treat each slither independently for each projection PCXMC requires the correct air kerma and filtration for each slither to perform its calculation need some beam profiling!!!! 4 slithers used to correct for beam non-uniformity treat independently for each projection

11 CBCT beam profiling Air-Kerma and tube filtration profiles were measured with the Unfors Xi chamber at the isocentre, and using the bed to step in 1 cm increments across the full width of the bow-tie profile Air kerma taken directly from the Unfors Xi, filtration a little more tricky!!

12 Total filtration (mm Al). Air Kerma (mgy/ kvp) S1 S2 S3 S Position relative to centre of field (cm) Use this info to plug into PCXMC to calculate patient dose per slither S1 S2 S3 S Position relative to centre of field (cm)

13 Performed TLD dosimetry on two linear accelerators (RT treatment machines), with Rando phantom loaded with 80 TLD-100H chips in the positions of the various important organs in and around the scan volume Liver & stomach were most superior organs measured (well outside the primary beam) Uterus & ovaries I know prostate patients don t have these, but it was useful for validation purposes! Bladder, prostate & testes these were all fully irradiated by the primary beam Small and large intestine partially irradiated by the primary beam Rando was positioned with the prostate at the isocentre, and three CBCT Pelvis scans performed Model validation

14 Model validation TLD dosimetry Mean Organ Dose* (mgy) Organ RT1 RT2 Mean Liver Stomach Uterus Ovaries Bladder Prostate Testicles Small Intestine Large Intestine * Measured Air Kerma corrected for ratio of (μ en /ρ) ICRU soft tissue /(μ en /ρ) air

15 Model validation The comparison So how do these compare with the PCXMC model? Mean Organ Dose (mgy) Organ Mean TLD PCXMC % diff. Liver Stomach Uterus Ovaries Bladder Prostate Testicles Small Intestine Large Intestine Large distance from beam Not bad given the inherent errors associated with TLD dosimetry

16 Effective dose? Using PCXMC to calculate the effective dose, taking out contribution to ovaries and uterus (not applicable to our prostate patients!), and the prostate (which is the target of the RT treatment, so probably should not be included in the calculation) Effective dose = 6.0 msv per scan Using TLD dosimetry with Rando Effective dose = 5.9 msv per scan Good agreement!! For daily prostate imaging we get up to 222 msv for a 37 fraction treatment regime, risk of fatal cancer: 1 in 150 for a healthy 60 year old male (using organ specific risk factors) 1 in 90 using generic 5% per Sv Not insignificant!!!

17 Organ doses? Total individual organ doses for daily imaging with 37 fractions (ignoring prostate); Bladder > 1.2 Gy Testicles > 1.4 Gy Large Intestine > 0.3 Gy These don t feel insignificant! Mean Organ Dose for 37# (Gy) Organ RT Treatment Pelvic CBCT CBCT as % of RT Gonads 0.8 >1.4 >175 Bladder 51.8 >1.2 >2.3 Colon 1.2 >0.3 >25 Rectum 40.0 >0.9 >2.3

18 Size specific CBCT Currently all Pelvis exposures use the same factors (125 kvp/80ma/13ms/650 projections ~ 680 total mas) No compensation for patient size means the organ/effective dose reduces as the patient gets bigger But, we should probably be increasing exposure factors for the biggest patients to ensure we get acceptable images We have it on good authority that these patients are difficult to image Equally, smaller patients should have a lower dose protocol

19 Protocol Optimisation Have started looking at patient size specific exposure protocols We have used the CT AEC phantom Tim discussed in his talk earlier today Scanned this at the default exposure setting 125 kvp, 80 ma, 13 ms per projection, 650 projections, Total of 680 mas Decreased the ma to assess the effect on image noise: Wanted to increase ma as well but 80 ma is its upper limit!!! Also scanned with increased/decreased ms:

20 Noise (SD) Protocol Optimisation Effect of ma (dose) Noise with ma mA, 13ms 60mA, 13ms 60 40mA, 13ms 20mA, 13ms Patient thickness CT slice

21 Protocol Optimisation Effect of ma (dose) As expected decrease in noise as the ma (dose) increases, for a given patient thickness Also, increase in noise as the patient gets thicker, for a given ma (dose) There is definitely scope to optimise the ma for average and thinner patients Possibly as low as 40 ma for the very thin ones?? Even scope to decrease ma for thicker patients 60 ma is not too different in terms of noise compared to 80 ma

22 Noise (SD) Protocol Optimisation Effect of ms Noise with ms mA, 26ms 50 80mA, 23ms 40 80mA, 20ms 80mA, 13ms 30 80mA, 7ms Small patients Large patients CT slice

23 Protocol Optimisation Effect of ms As patient size increases noise increases Less obvious with thinner patients As ms increases noise decreases May be able to decrease to 7ms for very thin patients (with 80 ma) Given that we have been told larger patient images can be poor, and that we can t increase the ma (max 80mA which is the default), it may be possible to increase the ms for larger patients to improve image quality. Probably go to 26ms for same noise as average patient

24 Hot off the press!!! Very large patient scanned with default settings led to images that were not usable We recommended they use 26 ms and image quality had improved such that images are now acceptable for the clinical intent

25 Summary Developed a PCXMC model that simulates CBCT organ doses for pelvic (prostate) imaging Organ doses are not insignificant for daily CBCT imaging!!! Developing size specific protocols should be possible Increase/decrease in ma Increase decrease in ms Future work will include Adopt size specific protocols into clinical practice Looking in more detail at the organ specific risks of cancer induction Create some written justification protocols for the use of CBCT with dose information for size specific scans Looking at other anatomical sites