Reply To: DIBH LNG findings

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Marko Laaksomaa

… Now the case is AlignRT mastectomy DIBH patient with bolus. In the AP+LAT images sternum was systematically 4 mm too dorsally in comparison to reference sternum and vertebra was 5 mm too caudally in comparison to reference image vertebra. In the tangential images chest wall remained 2mm too dorsally, so the total error of the sternum vertical error was not visible on the chest wall in the tangential images, in fact only half of it was. Anyway, the problem in the images is that chest wall is too dorsally, there is 5 mm displacement in lymph node are in LNG, both indicating also too shallow BHL in the images.

We decided to correct this LNG error of the vertebra and too dorsal sternum. With our current workflow there is two ways to do this correction, 1) based on image guidance to acquire new reference surfaces for the setup or 2) evaluate the corrections based on offline review and create the reference surfaces before image guidance. We decided to do the latest, since typically bolus is on the patient skin at the time patient image guidance is performed and new reference surfaces for the next day setup are possible to achieve at the end of the fraction, with doing the “reverse” bolus action in comparison to setup. This is not considered practical, since the posture of the patient may have changed during the fraction.

Therefore we shifted the couch at the setup 4 mm more caudally from the AlignRT FB LNG delta and acquired new reference for the FB setup there. Then we asked patient to take 4 mm more air in vertical direction than on the previous BH reference surface and acquired new BH reference surface there (LNG delta went to zero, as we wanted, since there was no need for corrections in that direction based on chest wall). After these adjustments, posture of the patient was very good in the images and tomorrow we continue with the tangential images. Of course there would have been a way to go with the previous reference surfaces, shift the couch based on vertebra match in caudally in longitudinal direction in the online match and ask patient to take 4 mm more air than previous level was, acquire new surfaces for FB and BH setup, go inside treatment room, ask patient to BH, set the bolus to the just acquired BH surface, change the field to bolus+treatment field, acquire BH reference for the treatment with bolus there, verify the action with tangential image.

With the action we did, BHL is not too shallow anymore, sternum is in its correct location in vertical and there is no displacement in vertebra LNG. Hope these our findings helps someone in the cases of systematic corrections.