2117 - Evaluation of Necessary and Sufficient PTV Margins for Offline Adaptive Head and Neck Cases Treated with Tomotherapy

Purpose/Objective(s):

To analyze and assess the efficacy of population specific PTV margins in patients of carcinoma Head and Neck treated with Tomotherapy.

Materials/Methods:

This retrospective study included 120 patients with head and neck carcinoma who received standard radiation therapy at our department. All patients were positioned and immobilized using a five-clamp mask attached to an Alta board with shoulder retraction (Qfix, Avondale, PA, USA). CT simulation was performed using a Biograph Horizon PET-CT scanner (Siemens Medical Solutions, Knoxville, TN, USA), followed by contouring in Syngo.via RT Image Suite VB60.

Treatment plans were created using the Precision Treatment Planning System for delivery with Tomotherapy. Daily MV-CT imaging was acquired to correct the setup errors. The planned dose was deformably transferred onto the acquired MV-CT using the PreciseART module of the Precision TPS. If target or organ-at-risk (OAR) dose variations exceeded predefined limits for three consecutive days, the patient underwent adaptive planning/ re-simulation after careful assessment of the deformed dose and contours. The accumulated dose over the entire fractionation schedule including the effect of setup uncertainties was assessed at the end of treatment. Clinical target volumes (CTVs) were expanded by giving 1, 2, 3 mm margins.

Results:

The accumulated dose was assessed based on the 95% percentage of the population meeting the following dose coverage criteria:

a. D99% > 99% (Dose covering 99% of the CTV) – Achieved in 94.6% of cases for the CTV alone, with coverage decreasing as margins expanded.

b. D98% > 98% – Achieved in 100% of cases for the CTV and CTV + 1mm but reduced for CTV + 2mm and CTV + 3mm.

c. D95% > 95% – Achieved in 100% of cases for the CTV, CTV + 1mm, and CTV + 2mm, but decreased for CTV + 3mm.

Conclusion:

A 3mm PTV margin is necessary and sufficient to ensure adequate target coverage (D99% > 99%) for 95% of patients when daily image guidance is used. However, smaller PTV margins (e.g., 1mm or 2mm) may still meet dose criteria for a 95% percentage of patients (Cases 2 and 3). Departments can define an appropriate PTV margin based on their protocols to achieve optimal target coverage while minimizing unnecessary dose expansion. Reducing PTV margins with appropriate coverage can be revolutionary to decrease toxicities and dose escalation.