How radiotherapy departments verify that the dose planned is the dose delivered, which Gafchromic film suits which check, and why reproducible immobilisation is as important to accuracy as the linear accelerator itself.
Radiotherapy is a treatment delivered blind. The beam is invisible, the dose is deposited inside the patient, and nobody in the room can see whether the plan was executed correctly. Quality assurance is how a department converts that uncertainty into evidence. As Bangladesh commissions more linear accelerators, the QA programme around them deserves the same attention as the machine purchase.
These terms are often used loosely, which causes departments to buy the wrong consumables.
Radiochromic film changes colour on exposure to ionising radiation. It is self-developing, so it needs no darkroom and no chemical processing, which removes an entire failure mode from the workflow. It also gives genuinely two-dimensional dose information at high spatial resolution, which point detectors and coarse arrays cannot. For a department in Bangladesh, the practical advantage is that film requires no wet processing infrastructure and can be stored and handled in normal room light.
Gafchromic film from Ashland is the reference standard, and the range is differentiated by dose. Choosing the wrong grade is the most common error: a film saturated beyond its useful range gives an unusable result.
| Film | Primary use | Typical application |
|---|---|---|
| EBT3 | Machine and patient QA | IMRT plan verification, the general-purpose workhorse of most departments |
| EBT-XD | Patient QA at higher dose | Stereotactic techniques such as SRS and SBRT, where fraction doses are large |
| RTQA2 | Routine machine QA | Radiation field and light field coincidence, HDR brachytherapy checks |
| MD-V3 | Medium to high dose patient dosimetry | Measurements in the 1 Gy to 100 Gy range |
| HD-V2 | Ultra-high dose | Specialist measurements above the range of the standard films |
A department can verify a plan perfectly and still deliver it to the wrong tissue if the patient is not in the same position on day 20 as on the planning CT. Modern techniques have tightened margins, which means setup reproducibility now carries more of the accuracy budget than it used to. Immobilisation equipment is not an accessory to the accelerator, it is part of the treatment chain.
Thermoplastic sheets soften in warm water, are moulded to the patient, and set into a rigid personal shell. They are the standard for head, head-and-neck and head-neck-shoulder treatment. Masks are supplied in different thicknesses, commonly around 2.4 mm and 3.0 mm, with thinner and thicker options available. Thicker material is more rigid and restrictive; thinner material is more comfortable and slightly less constraining. The choice is clinical, not commercial.
The mask has to attach to something that itself attaches to the couch in a repeatable position. Baseplate systems cover head, neck, shoulder, thorax, pelvic and whole-body treatment, and index to the couch so the whole assembly returns to the same coordinates each session. Buying masks without a properly indexed baseplate wastes most of the benefit.
For body and extremity treatment where a mask is not appropriate, a vacuum cushion moulds to the patient's contour and locks when air is evacuated. It is the standard approach for limb and torso positioning.
Departments moving to MRI-based simulation or MR-guided radiotherapy need non-ferrous positioning equipment. Standard baseplates and frames cannot go into the magnet. If MRI simulation is anywhere in your five-year plan, specify MRI-compatible equipment now rather than buying the same items twice.