Tomotherapy Flashcards
What is Tomotherapy and how does it work?
A form of CT guided MRI.
Uses a 6MV photon beam.
Radiation is delivered in a helical manner (because of the slip ring gantry).
64 binary MLC’s (either in an ‘on’ or ‘off’ position).
Maximum treatment width = 40cm
Maximum treatment length = 160cm.
What are the advantages of a helical delivery?
Tomotherapy allows us to deliver dose: 1. in a narrow rotating beam. , , with high speed MLCs, from multiple angles around the target.
THIS MEANS WE ARE ABLE TO ‘BEND’ THE DOSE TO CONFORM TIGHTLY TO THE PTV AND AVOID OAR.
How does a patient get Tomotherapy?
Must be able to lie still for 20 minutes
Full CN, Nasopharynx and TNI patients automatically qualify.
Majority of patients (at RBWH) are head and neck, brain or breast patients with nodal involvement.
What are positives and negatives of the Tomotherapy unit; particularly when setting up?
Negatives:
- Noisy - 78 decibels
- Cold Room
- Pinch hazard at the top of the couch
Positives:
- Personal Control panel touch screens, allows for auto load (automatically puts the patient in the same position they were last treatment) which is very convenient.
- No applicators, no heavy equipment, no lifting
- No gantry to rotate - no crash hazard.
What type of IGRT does tomotherapy have?
MVCT: mean energy 1MV.
0.5 - 2.5cGy per scan. Does not use a lot of radiation.
There is a loss of distinction between soft tissue and bone however, there is improved imaging on high density materials (reduced metal artifacts).
What is the registration process for tomotherapy?
Sagittal Image: Check for pitch, correct for positioning sup-inf and ant-post.
Coronal image: Check for yaw. Check positioning left-right ( large left-right shifts can be corrected here).
Transverse image: Check for roll. Make final adjustments let-right and ant-post across the the whole volume.
What are the limitations to image registration in the Tomotherapy machine?
6 degrees of freedom can be reviewed but only these 4 can be corrected (sup/inf , ant/post, left/right, roll)
If pitch and yaw are incorrect the patient must be re-positioned.
Why is tomotherapy often used for paediatric patients?
The low dose wash associated with Tomo is not ideal for paediatrics; the tight margins and ability to “bend” radiation make it suitable for many areas.
It is essential that patients are able to lie still. GA may be necessary
What is a major limitations of tomotherapy (Tomo’s nemesis).
Weight loss and volume changes.
Tomo treats a lot of head and neck patients who often have side effects reducing their appetite. - patients need nursing and dietitian ans speech pathology to help control weight.
How does Tomo deliver dose?
Through beamlets.
Set the number of treatment beamlets available in a plan.
Tomo assigns a number of beamlets to each structure according to how it is meeting tolerances (when it has achieved constraints to structures it will divide remaining beamlets over remaining structures).
What are things to keep in mind when Tomo planning with beamlets?
STRUCTURES CLOSE TOGETHER: When structures are very close together there is decreased control over the dose.
This is because beamlets from one structure can be affected by the other.
Oed to the
What factors can be changed to optimise the plan?
Field width Pitch Modulation factor Compensation Blocks (directional vs complete) Priorities/ importance/ penalties
Clinical case study (breast)
Treated both breasts at the same time.
Lt breast, Lt IM, Lt Supra clav and Rt breast = 50Gy in 25# (2Gy/#).
Lt breast boost & Rt breast boost = 60Gy in 25# (2.4Gy/#).
Limitation: Daily MVCT couldn’t image the entire patient - cut some of the patient and PTV off.
Clinical case study (head and neck).
Nasopharynx, retropharyngeal LN, Rt Cervical LN, Level 2-5 Ln (54Gy in 35# 1.5Gy per fraction).
Middle Dose PTV 63Gy in 35# (1.8Gy per fraction).
Macroscopic disease 70Gy in 35# (2gy per fraction).
