High-Dose Spatially Fractionated Radiotherapy, Lattice Radiotherapy in Bulky Tumors: Results after One Year Follow-Up

Purpose/Objective(s)

Spatially Fractionated Radiotherapy (SFRT) or Lattice Radiotherapy (LRT) is a radiotherapy technique that allows to deliver a very high radiation doses spheres inside the tumor in a single fraction, maintaining low dose in OAR, generating an inhomogeneous dose distribution inside the GTV and maintains the PTV marginal dose within planned dose within a multifraction treatment. This can be an option for bulky unresectable tumors with poor local control with standard radiotherapy. The main objective of this study is to assess LTR toxicity in patients with tumors greater than 45 cc and poor candidates for other treatment modalities.

Materials/Methods

From 1 January 2020 to 31 December 2020, 21 patients were treated in our institution. We prescribe 1 cm spheres with a maximum dose over 15 Gy in a single fraction and at least 4 vertices distributed inside of GTV. PTV dose continues as standard dose fractionation of 2.5-3Gy in the LRT session as prescribed in conventional or hypofractionated treatment. Simulation and treatment delivery is as SBRT. LTV (Lattice Tumor Volume) is the structure inside GTV where the VTV spheres (Vertex Tumor Volume), overdose volumes, are located, separated by 2.5-3.0 cm. VV (Valley Volume) is the subtraction of GTV-VTV that must be under 5 Gy.

Results

21 patients (66,6% of tumors were in the thorax, 28,6% abdomen/pelvis and 4,8% head and neck) were treated. The median follow-up was 12 months (3-25). The median age of the patients was 64 years old (42-84). The median GTV volume was 236cc (80-896cc) and 73 mm (47– 123mm) in greatest axial diameter. The analysis of the results showed 4 patients with complete response (20%), 12 with partial response (57%) and 5 with stable disease (23%). Tumor reduction bigger than 50% has been observed in 75% of cases 2 weeks after LTR session. Local control was 75% at 6 months. Cumulative probability of survival was 76% (95% CI 51-89%) and 59% (95% CI 33-77%) at 6 and 12 months respectively. No additional toxicity has been related. No major grade 3 toxicities were reported according to CTCAE v5.0 grading.

Conclusion

Our experience with LTR protocol is feasible as a technique to be included in radiotherapy treatments for poor prognosis bulky tumors, with reproducibility and adaptability. Our early results confirm good local response and no added toxicity. Further large prospective studies are required to validate these results.