Use of Lattice Therapy for Dose-Escalation in Palliative Treatment of Bulky Head and Neck Tumors

Abstract

Palliative radiation is used to relieve pain and bleeding in patients with locally advanced head and neck cancers. Lower doses are utilized to meet normal tissue tolerances, limiting the duration of palliation. Lattice therapy can achieve intratumoral dose escalation while sparing surrounding tissues, but a standard approach does not exist. In this study, we explore the impact of variation in the values and distributions of peak and valley doses in palliative lattice treatment plans. Five-fraction volumetric-arc based (VMAT) lattice plans were generated using a customized Python v3.6 script. For each combination of peak/valley doses and sphere size and spacing, a bounding box was generated around the gross tumor volume (GTV) and divided into equal segments in the x, y, and z planes. Individual spheres were constructed for a given parameter set and those fully encompassed within the target contracted by 2 cm were used to create peak dose targets with values of 30, 40, and 50 Gy. A valley dose of 20 Gy was chosen to match standard dosing for 5-fraction palliative plans and the associated target volumes were generated by subtracting an 8 mm expansion of the peak dose spheres from the overall target volume. Multi-criteria optimization (MCO) was used to generate a final plan. The planning target volume (PTV) was a uniform 3-mm expansion from the GTV. Target volume and OAR dosimetric parameters were analyzed using Mann-Whitney U and Kruskal-Wallis testing. A total of 72 unique plans were evaluated for 4 patients with large GTV sizes ranging from 159 to 413 cm3. The median time to generate an individual plan using the automated script was 20 minutes (IQR, 18 - 23). Median values of mean GTV and PTV doses were 27.9 Gy (IQR, 24.7 - 30.3) and 26.7 Gy (IQR, 23.9 - 29.1), respectively. Further analysis was then limited to plans containing ablative 50 Gy dose spheres. For these 24 plans, mean GTV and PTV doses increased to a median of 31.7 Gy (IQR, 29.6 - 33.3) and 29.6 Gy (IQR, 27.8 - 31.0), respectively. Mean doses to 1, 2, and 4 cm expansions around the GTV for this cohort had median values of 19.5 Gy, 15.1 Gy, and 9.7 Gy, respectively. Median doses to 0.03 cc of mandible, spinal cord, and esophagus were 28.2 Gy (range, 7.0 - 35.7), 14.7 Gy (range, 9.8 - 18.8), and 8.7 Gy (range, 0.8 - 16.2), respectively. Mean GTV dose was dependent on sphere spacing (median 29.9 Gy for 4 cm spacing vs. 33.5 Gy for 3 cm, p<0.001), but not on sphere size (median 31.3 Gy for 2 cm vs. 31.8 Gy for 1.5 cm vs. 31.5 Gy for 1 cm, p = 0.968). VMAT-based lattice planning allows dose escalation to bulky head and neck tumors, including significant coverage with ablative doses while meeting normal tissue constraints. Thus, lattice plans could enhance palliation while reducing toxicity. Mean target coverage is dependent on sphere spacing. Use of automated plan generation could also facilitate clinical application.