It is clear that HT plans provide the best conformity of the three techniques. Previous studies have demonstrated the superiority of inverse-planned IMRT techniques in the treatment of head and neck tumours [31
]. This study compares HT treatment to the conventional two-phased photon electron junction technique and a forward-planned single-phase field-in-field photon-only technique. The latter is a safe technique that is easy to implement, is within the tolerance limits of the spinal cord and could be considered for routine use for all non-inverse-planned IMRT head and neck cases. This single-phase technique has the added advantage of reliable and reproducible dosimetric verification checks, which may be difficult to achieve in a two-phased photon electron junction approach. The DVH analysis and TCP values reported here indicate that the single-phased techniques provide better conformity than do the two-phased techniques. This single-phased technique can therefore be implemented by centres that lack the resources necessary to implement inverse-planned IMRT techniques for all the head and neck cancer cases. The disadvantages of the single-phase technique include higher doses to the OAR, including the parotid glands, when compared with HT.
Comparison of the NTCP data confirms the superiority of the HT technique in reducing the NTCP and improving the UCP. The high UCP values provided by HT make dose-escalation techniques possible, and the TCP and NTCP values confirm the possibility of improved outcomes. Centres using inverse-planned IMRT techniques should explore such dose escalation within a controlled trial setting. At the Royal Marsden Hospital, a Phase 1 study for patients with hypopharyngeal cancer has been conducted, in which the PTV1 (high dose) was treated with a dose of 67.2 Gy in 28 fractions and the PTV2 (prophylactic dose) with 56 Gy in 28 fractions [30
]. In the VortigERN study, a 5-mm isotropic margin was outlined around the GTV (both primary and nodal) to create a PTV3. For dose escalation in the oropharynx, consideration needs to be given to the pharyngeal constrictor muscles, the temporo-mandibular joint and the muscles of mastication. These were outlined in the VortigERN study to evaluate the feasibility of future Phase 1 and Phase 2 clinical studies. We are exploring the doses to these extra OAR in another in-house study.
Cold spots will rapidly drive down the TCP, which cannot be rescued by hotspots at different locations in the PTV. It is interesting to note that although HT treatment might be expected to deliver a higher integral dose per plan, the single-phased technique actually seemed to have a higher integral dose. Although the concepts of higher integral dose [33
] and second malignancies are debatable, this study highlights the fact that current forward-planned techniques may not be delivering lower integral dose than inverse-planned HT techniques in complex head and neck treatment plans.
It is worth noting that recent QUANTEC publications [35
] have described a predictive model based on pooled data from multiple studies. The tolerances described in these papers are slightly different to those discussed by Emami et al [24
]. Although the possible limitations of QUANTEC studies have been highlighted [35
], the new data may well be more accurate clinically. We did not base our calculations on these new values, although we understand that they could provide changed NTCP values for the DVH generated in the plans. This could well be a limitation of our study. On the other hand, we tried to keep our calculations pure and used BIOPLAN to calculate the NTCP values as described by Sanchez-Nieto and Nahum [23
]. In addition, it is worth noting that the HT plans were calculated using a different planning algorithm and grid than that used for the forward-planned techniques. This could mean that the DVH could have had some inherent differences originating from the differences in planning systems.
In conclusion, while confirming the superiority of HT plans in treating head and neck tumours, our study establishes the role of a single-phase photon-only technique in this context. This technique could be used as a standard technique for managing head and neck tumours in departments that lack the resources necessary to implement inverse-planned approaches for all patients.