…on last week we had a whole breast (WB) DIBH- patient, which has been imaged with tangential images for while. Vertebra was not visible in those images this time. We noticed that the heart was shifted 3-4 mm towards treatment field, but the ribs and breast location was very accurate without couch corrections. We decided to find out what was the reason for heart displacement.
On the next day we acquired AP+LAT images. On those images we noticed that there was + pitch on the sternum. If we think about the optimal matching location for the tangential image, it is not surprise that the tangential image accuracy has been excellent. There were no need for the couch shift in LAT (ribs ok in LAT), VRT (in this case caudal part of the sternum ok), or LNG (in the midway of sternum and the ribs). Still there was 2-3 degree pitch on the sternum and vertebra located such that there was a need to raise the couch vertical (-3mm/-4 mm). Vertebra located also too cranially (5-7mm).
What we can do now is to acquire new reference surface for the FB setup when couch plan vertical value is +4 mm lower and patient shifted 5-7 mm caudally on the couch and ask the patient to take a BH to the same BH surface, since it led to the excellent accuracy on the tangential image ribs. Why we want to shift the vertebra caudally and lower the couch?.. to achieve better BHL in the images. The other possibility is to setup the patient with same reference surfaces as previously and do the vertebra match in the online match and shift the couch based on vertebra (lower the couch and shift patient caudally), ask the patient to take a BH to same BH surface and hope that she will take a BH in LNG to zero error in thresholds with the vertebra match, since the BH surface itself was good for the chest wall and breast.
In practice we did not do either correction strategy finally. It was difficult for the patient to take a BH to that BH surface and lowering the couch would lead to more pitched position on the sternum in the practice and looked like that there is risk that patient begins to lift her back, when the result in heart sparing may even get weaker in this case. Since the heart location in the tangential images was inside our IGRT tolerances, we accepted 3-4 mm shift on the heart border towards treatment field finally. It was anyway interesting to see that the reason for that was too shallow BHL and pitch on the sternum in the images.
The other thing that we learned was that since without tangential images and SGRT, some units usually acquire mostly AP+LAT images and matching location in vertical direction is not typically on the caudal part of the sternum as in this case and we/they may at least randomly or even systematically have incorrect matching location if offline match is not daily used. Thereby it can be said that in this kind of individual case SGRT setup with possible tangential image verification can be at least individually quicker and more accurate than the the tangential image accuracy after daily couch corrections based on AP+LAT images in the online match.
VMAT, whole breast + lymph nodes and mastectomy DIBH patients would have needed new planning CT with this kind of displacements in our unit, but since the breast, ribs and heart were inside tolerances we can continue with the tangential images with this WB DIBH patient.