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Moving from Serial to Helical Tomotherapy. The Baylor Experience. Walter Grant III, Ph.D. John E. McGary, Ph.D. Baylor College of Medicine The Methodist Hospital Houston, TX. The Title of this Talk Could Be. “Does the Serial Tomotherapy experience help or hurt?” The answer is, “Both”.
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Moving from Serial to Helical Tomotherapy The Baylor Experience Walter Grant III, Ph.D. John E. McGary, Ph.D. Baylor College of Medicine The Methodist Hospital Houston, TX.
The Title of this Talk Could Be.. • “Does the Serial Tomotherapy experience help or hurt?” • The answer is, “Both”.
This Presentation • So what I will do today is discuss some of the salient features of each of the technologies and how we have approached the transition.
1994 2005 This Presentation • Hardware • Treatment Planning • Equipment used
But First, A Reality Check • I think the following slide makes an important comparison between the two companies that might not be so obvious if you haven’t lived it.
The Not So Obvious • Both NOMOS and Tomotherapy, Inc. were/are “startup” companies. • There are problems to work through as the product matures. • There will be staffing problems at the company. • There will be priority issues, i.e., future sales vs. current user issues.
So What Should I Expect? • The secret to any successful relationship is to have low expectations. • But if you believe in the product then, “KEEP THE FAITH!” • It paid huge dividends for us and our patients with NOMOS.
Some Obvious Advantages • Both technologies have commonality. • Binary collimators. • Arc delivery and planning. • We have experience with tomotherapy at every level our department. • Physicians, physicists, dosimetrists, therapists, nursing and even management.
Binary Modulating Collimators Tomotherapy, Inc. NOMOS
The Downside • We have a large IMRT patient history and don’t want to lose that. • It’s a different dose algorithm. • The planning system does not allow homogeneous planning. • And that affects more than patient plans. • So we have to look seriously at dose.
Treatment Planning • Because we have a 10 year history of dose response, we had to make sure that a “Tomo-macki” and a “Corvus-deposit” not only were close to the same J/kg, • But also had a similar distribution. • Not just high dose, but low dose in normal structures. • Evaluate the effect of Heterogeneity corrections.
It Starts Here • CT-Simulator • Lasers? • Couch sag? • Couch Pitch? • Couch Roll? • Couch Yaw? • CT# - ED calibration • Image orientation
Treatment Planning Issues • There is commonality between the planning systems. • 51 fields in HT vs. 26 or 54 in ST. • The concept of isocenter is irrelevant. • A voxel can be only one tissue type for planning. • Targets are “sacred” and always have priority over structures in both systems.
Treatment Planning Issues • With Serial Tomotherapy, we treated with 1 cm or 2 cm slice width. • Decided to mimic that using the 25 mm slice width and a pitch of 0.3 or 0.7 • We expected no problems obtaining good plans for disease sites treated with Serial Tomotherapy. • And we didn’t.
Treatment Planning Issues • There were issues importing anatomy from Corvus to TomoPlan. • Couch removal and replacement. • Helical startup and end. • Put a “cap structure” to keep dose driven toward target during ramp up and ramp down.
When Do You Start Dosing? • For ST that means you have trouble getting the Goal Dose at the start of the target and might have extra dose at the inferior portion of the target. • For HT it means you’ll you have extra dose at both ends.
Another Trick • You always want to make sure the maximum dose is in a target. • Not usually a problem for single targets, but can be for multiple targets with normal tissue surrounding all. • Corvus tends to protect targets (conformal) while TomoPlan tends to dose the target. • We add a “Tomo Tissue” OAR in case we need it. • You can always “Reference” it out.
Treatment Planning Issues • There were issues importing anatomy from Corvus to TomoPlan. • Couch removal and replacement. • Helical startup and end. • Put a “cap structure” to keep dose driven toward target during ramp up and ramp down. • Heterogeneities • We did not use them in Corvus.
Heterogeneity Corrections • “This correction is based on CT Hounsfield Units and, therefore, relies upon a correct calibration of the CT scanner used for patient imaging.” - BrainSCAN Software Guide • OK, but that the correction curve is hard coded and can’t be changed! • TomoPlan originally was the same. • But now is settable, but be careful.
Heterogeneity Corrections • “Switching off the Pathlength Correction may result in inaccurate dose calculation.” -BrainSCAN Software Guide • “Using contrast agent falsifies the electron density information of CT scanning. When planning on contrasted scans it is necessary to switch off the Pathlength Correction.” - BrainSCAN Software Guide • TomoPlan won’t let you turn it off.
Heterogeneity Corrections • Here’s where we are so far: • Used the RPC Lung phantom and are OK (3%) at least at one point. • Bone not really well tested. • Used the NOMOS Film Phantom and had equivocal results. • Poorly designed experiment, so a better phantom is on order.
Treatment Planning Issues • But now our doctors are getting creative with disease sites to be treated, so we will have to investigate other slice width & pitch combinations and develop new planning templates. • It took 4 weeks to go from 2 to 15 patients. • Mostly delayed by MI device delivery. • Consider when you will have time to validate things. • Or you can just assume they are OK.
Treatment Planning Issues • There is one other difference between the two planning systems that is causing us problems. • Corvus did optimization and dose calculations in the background. • TomoPlan is basically a “one thing at a time” system. • While you can now start beamlet calculations in a Batch Mode, you can’t be working on anything else.
Treatment Planning Issues • But now our doctors are getting creative with disease sites to be treated, so we will have to investigate other slice width and pitch combinations. • It took 4 weeks to go from 2 to 15 patients. • Mostly delayed by MI device delivery. • Consider when you will have time to validate things. • Or you can just assume they are OK.
Immobilization • Prostate is our largest service and we wanted to preserve our experience.
The Tool Box • Helical Tomotherapy presents some unusual problems. • Can’t apply the TG-51 protocol directly. • What about TG-40? • Lots of time for electronics to “get ready”. • Some suggestions regarding tools that can help.
Equipment • Tomotherapy, Inc. supplied products. • Cheese phantom and CT density plugs • DQA Scanning System • The cheese phantom and film scanner were used for 90% of the project. • The water phantom is only 2D now and needs more & better tools, but is worth having.
Equipment • Multiple types of simple cube phantoms with chamber inserts. • to measure heterogeneity calculations • to measure physical scales that approximate brain • diode and chamber inserts • slabs of different materials surrounding solid water for heterogeneity correction
Equipment • We used the NOMOS film phantom • used this for heterogeneity correction verification using different CT density plugs • shows some interface effects between the plugs and delrin.
Equipment • RPC lung phantom to check for lung tissue heterogeneities (again).
Equipment • CT density plugs with TLD cutouts (in the process) for embedding into the Cheese phantom. • Yet another test for heterogeneities.
Equipment • Gammex CT simulator tool - for laser alignment • used this for determining the couch sag
Equipment • Some type of anthropomorphic phantom to test fusion in Tomo Images. • There are 3 choices of algorithm: • Bone only • Bone & soft tissue • Soft Tissue only
Equipment • Styrofoam to extend from the couch into the isocenter to make in air output and rotational output used for confirming isoscenter -- the things you have to do to make simple measurements.
The Downside • We have a large IMRT patient population. • The “Learning Curve” vs. through-put? • It’s real and we are about 18 patients max right now. • What is Plan B if there is excessive downtime? • We run a Corvus plan also and can shift if we go down for an extended time. • So far we have lost 1.5 days in 3 plus months.
Future Plans • In order to discuss the next step, I want to review the last 20 years of progress.
