Aim of this study was to clarify whether IGRT with daily CT scanning can improve the quality of patient treatment in terms of cervical spinal cord sparing and CTV dose coverage in HNC patients compared to a non-IGRT approach. In this study “CTV” denotes the small part of the target volume with the prescribed dose of 70.4 Gy.
If no IGRT strategy is used, an additional margin needs to be applied to account for rigid positioning variations in order to assure CTV coverage . This margin was determined in accordance to the van-Herk recipe for our HNC patients in a previously performed investigation, which analyzed the local geometrical uncertainties in a large HNC patient cohort . The resulting margins of this former analysis were applied in this study.
In summary, this analysis reveals advantages and disadvantages for both, the IGRT and the non-IGRT approach:
Foremost it becomes apparent, that the cord never exactly receives the initially planned dose along the treatment course, even if IGRT is used. This is the case in all patients and can be explained by deformations of the head-and-neck anatomy. The deformations result in an imperfect alignment of the cord, since the applied TPC corrects for rigid positioning errors only and additionally occurring deformations cannot be adjusted. Using IGRT, the range of the D5 values varied between the patients from 2 to 9 Gy over the treatment course. This corresponds to a range of 3 to 13% related to the prescribed dose of 70.4 Gy.
Further noticeable is, that also the dose coverage of the CTV can sometimes not be sustained as planned using IGRT, even though only in one patient. This happens despite the use of a focused positioning correction. The correction vector for the TPC was derived from a registration of a small box surrounding the CTV, to achieve an optimal CTV coverage throughout the treatment course. Again, the impaired dose coverage might be explained by deformations of the head-and-neck anatomy resulting in changes of the dose distribution. In this context, it is important to allude that one should be cautious using the gross tumor volume as an alignment target, since asymmetric tumor shrinkage can affect the geometric relationship of the gross tumor volume to the OARs.
As expected, in the non-IGRT setting the cord often receives more dose than planned, which is readily identifiable in the curve progressions of the DVHs (e.g. in patient 2, Figure 2). The applied additional safety margin results in a smaller distance between the cervical cord and the dose gradient of the PTV. Thus the probability of the cord moving into the high-dose region in case of positioning changes increases. So the results show that the non-IGRT approach with margin-enlarged CTVs can be inferior in terms of cervical cord sparing compared to the IGRT approach. The extra volume covered by the additional margin surrounding the CTV is non-negligible, with an increase of 25-73% of the initial CTV in our patients.
It should also be kept in mind that tumors can regress during radiotherapy , especially if protocols with several weeks of sequential daily treatment are applied. In our analysis one patient showed shrinkage of the CTV during treatment due to a decrease of postoperative edema. This resulted in improved dose coverage of the CTV -- with and without IGRT -- but also implied more dose to the surrounding larger target volume of the neck. This is possibly undesired and can be avoided only if re-planning is performed. Yet, especially daily re-planning is very time consuming, though first concepts for daily fast re-planning are suggested by Ahunbay et al .
Additionally data for one exemplarily patient is presented which show the dosimetric effects if neither IGRT nor a margin approach is used. This approach, as expected, proved to be the worst scenario in terms of CTV coverage and thus is no serious treatment option.