Dosimetric and radiobiological comparison of helical tomotherapy, forward-planned intensity-modulated radiotherapy and two-phase conformal plans for radical radiotherapy treatment of head and neck squamous cell carcinomas

Abstract

Objectives: The usual radical radiotherapy treatment prescribed for head and neck squamous cell carcinoma (HNSCC) is 70 Gy (in 2 Gy per fraction equivalent) administered to the high-risk target volume (TV). This can be planned using either a forward-planned photon-electron junction technique (2P) or a single-phase (1P) forward-planned technique developed in-house. Alternatively, intensity-modulated radiotherapy (IMRT) techniques, including helical tomotherapy (HT), allow image-guided inversely planned treatments. This study was designed to compare these three planning techniques with regards to TV coverage and the dose received by organs at risk.

Methods: We compared the dose-volume histograms and conformity indices (CI) of the three planning processes in five patients with HNSCC. The tumour control probability (TCP), normal tissue complication probability (NTCP) and uncomplicated tumour control probability (UCP) were calculated for each of the 15 plans. In addition, we explored the radiobiological rationality of a dose-escalation strategy.

Results: The CI for the high-risk clinical TV (CTV1) in the 5 patients were 0.78, 0.76, 0.82, 0.72 and 0.81 when HT was used; 0.58, 0.56, 0.47, 0.35 and 0.60 for the single-phase forward-planned technique and 0.46, 0.36, 0.29, 0.22 and 0.49 for the two-phase technique. The TCP for CTV1 with HT were 79.2%, 85.2%, 81.1%, 83.0% and 53.0%; for single-phase forward-planned technique, 76.5%, 86.9%, 73.4%, 81.8% and 31.8% and for the two-phase technique, 38.2%, 86.2%, 42.7%, 0.0% and 3.4%. Dose escalation using HT confirmed the radiobiological advantage in terms of TCP.

Conclusion: TCP for the single-phase plans was comparable to that of HT plans, whereas that for the two-phase technique was lower. Centres that cannot provide IMRT for the radical treatment of all patients could implement the single-phase technique as standard to attain comparable TCP. However, IMRT produced better UCP, thereby enabling the exploration of dose escalation.

Figures

Figure 1

(a) Lateral opposed beams for the first phase of the two-phase treatment (forward planned) (fields 1 and 2). (b) In order to maintain safe cord dose, the posterior neck electron boost field is matched to the anterior photon field. (c) Anterior split neck field for prophylactic lower neck. The dotted lines inside the fields represent the outlined planning target volume (PTV)1 (high dose: red) and PTV2 (prophylactic dose: purple).

Figure 2

(a) A typical segmented field. Note the position of the multileaf collimators (arrow), which have been taken out into the high-dose planning target volume to ensure homogenous dose distribution. Note the shielding of the cord. (b) Complex forward-planned field in field technique can produce an acceptable dose distribution without the necessity to junction electron fields to photon fields.

Figure 3

Comparison of tumour control probability (TCP), normal tumour complication probability (NTCP) and generation of uncomplicated tumour control probability (UCP) for Case 4. The NTCPTot denotes the average NTCP probabilities for the different organs at risk.