Build or Upgrade?
- Details
- Published on Tuesday, 19 July 2011 14:26
So your clinic has decided to upgrade that old 6/10MV linac to a brand new 6/18MV, with all the bells and whistles. Should you build a new vault from scratch or upgrade that old vault by slapping some lead on the walls?
Well, we ran some numbers. We took a very basic room design (20ft x 24ft), calculated the shielding necessary for a 6/10MV linac, and then upgraded the machine to see how much additional lead is required. We made sure to exclude any IMRT considerations from the old 6/10 vault, since many older vaults didn't take IMRT treatments into consideration when the shielding was designed.
I'll spare you the gritty details (25 patients per day upgraded to 50, 2Gy per fraction, etc.) Here is a summary of the shielding requirements for our 6/10MV linac vault in standard-density concrete:
| Survey Point | Primary or Leakage | Concrete (cm) | Lead (cm) | Pass / Fail |
|---|---|---|---|---|
| North | Leakage | 94 | 0 | Pass |
| East | Leakage | 113 | 0 | Pass |
| East Pri | Primary | 174 | 0 | Pass |
| South | Leakage | 94 | 0 | Pass |
| West Pri | Primary | 196 | 0 | Pass |
| West | Leakage | 111 | 0 | Pass |
Here are the required shielding thicknesses after an 18MV upgrade:
| Survey Point | Primary or Leakage | Concrete (cm) | Lead (cm) | Pass / Fail |
|---|---|---|---|---|
| North | Leakage | 94 | 4.0 | Pass |
| East | Leakage | 113 | 4.6 | Pass |
| East Pri | Primary | 174 | 5.0 | Pass |
| South | Leakage | 94 | 4.0 | Pass |
| West Pri | Primary | 196 | 4.6 | Pass |
| West | Leakage | 111 | 5.3 | Pass |
*NOTE: THESE ARE SAMPLE THICKNESSES AND ARE NOT REPRESENTATIVE OF ANY REAL VAULT. YOU NEED TO DO YOUR OWN SHIELDING CALCULATIONS!!!* (Which, by the way, are much easier to do when using a radiation shielding software package like GrayCAD!)
So what does all of this suggest? Well, if you were to cover our standard-size room with these lead thicknesses (and the ceiling too!) you're looking at about 130,000 lbs of lead. At approximate market prices, that's roughly $175,000 USD. Now we're not experts in the costs of installing shielding, but I'd guess the installation costs for this lead would be somewhere around $50,000 to $100,000. So a total upgrade might cost $250,000 USD.
While this is cheaper than building a new vault (roughly $500,000 USD), these are not the only costs you need to consider. If you need to stop treating patients for six weeks while your old linac is de-commissioned and your new one is commissioned, you are losing a lot of money in lost patient treatment reimbursements. (I'll let you do the math on that one, but let's just say the lost revenues can quickly dwarf the cost of a new vault!)
So what is the conclusion? First, don't assume that a new vault is prohibitively expensive. Supplemental lead can get expensive pretty quickly. Also, take into account the lost revenues while your linac is down. If you are a large center that can stagger the commissioning process to continue treating patients, it may be best to simply upgrade your rooms. But for a single-linac clinic, upgrading a vault using lead can easily be more expensive than building an entirely new vault.
*Note that these conclusions vary according to a vault's dimensions, intended workload, and adjacent occupancies.*