ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
00:00 - 00:00
Safe number of transfers between Truebeam & Clinac for different treatment sites without replanning
Trevor Newbold, United Kingdom


Safe number of transfers between Truebeam & Clinac for different treatment sites without replanning
Authors: Trevor NEWBOLD.(Poole Hospital NHS foundation trust, Radiotherapy, Poole, United Kingdom)
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Purpose or Objective

Our department currently has one TrueBeam (TB) linac and two iX linacs. The two iX linacs are matched but the iX & TB linacs are not matched; the beam models for iX and  TB in the Eclipse planning system are similar, though not identical.

Having a single TB linac represents a single point of failure for which a contingency, other than a total re-plan for an iX linac, is desirable. Having a similar beam model suggests that it may be possible to treat a number of fractions on the ‘wrong’ machine without leading to detrimental patient dosimetry.

The aim of this project is to ascertain whether treatment planned for one type of linac is deliverable on the other, and  if so, for each treatment site, the safe number of fractions for which it is safe to transfer patients without re-planning the treatment.

Material and Methods

Treatment planning is performed in Eclipse. Plans originally produced for one type of linac were recalculated for the other by changing the beam model and fixing the MUs to the original value. Reported MLC errors were ignored.

Appropriate DVH parameters were compared between the original plan and the same plan on the alternative machine.

A predicted dose to the ArcCheck was calculated in Eclipse from the original plan. The original treatment plan was delivered on both linacs and an ArcCheck measurement taken. Gamma analysis was used to compare to the predicted and measured dose distributions for both deliveries.

Analysis of the data obtained enabled recommendations for the maximum number of unplanned machine transfer fractions to be suggested. Outcome-based options for each site were given to clinicians for consideration.  Table 1 shows the outcome based options for transfer number.

VMAT prostate (60 Gy, 20#), VMAT prostate with pelvic nodes (74 Gy, 37#) and conformal breast (40.05Gy, 15#) treatment sites have been analysed so far.

Each linac is able to deliver a treatment planned for the other linac.

ArcCheck results show no significant difference in gamma results for plans delivered on either type of linac signifying no significant difference in delivery performance.

Transferring plans from TB to iX resulted in approximately 2% higher dose for both VMAT and conformal plans with corresponding increase in dose to OAR.

The limiting factor for breast transfers was the volume receiving 107% of the prescription.

The limiting factor for prostate and prostate-nodes treatments was the dose to rectum and non-rectal bowel.

Transferring plans from iX to TB resulted in approximately 2% cooler dose for both VMAT and conformal plans. The limiting factor in all cases was the PTV coverage.


Treatments planned for one type of linac can be safely and effectively delivered on the other type of linac. DVH analysis demonstrates that for all sites studied, patients can be safely transferred to the other type of linac for part of their treatment. Our department has set the limit to 5 transfer fractions for these treatment sites.