A Non-Brass-Aperture Planning Approach for Palliative Spatially Fractionated Proton Therapy on a Compact Proton System
Abstract
Purpose
In the Phase I clinical trial (NCT05831579), we already performed the palliative spatially fractionated (GRID) proton therapy by using a collimating brass aperture. However, the existence of this brass aperture limits the flexibility of the high dose regions design and the beam arrangement. A non-brass-aperture planning approach for GRID proton therapy is proposed to have the similar characteristics of photon lattice SBRT dose distribution, and higher peak-to-valley-dose-ratio (DVDR) than the current proton GRID therapy with brass aperture.
Methods
Clinical proton non-brass-aperture lattice plans were created for 7 patients with GTVs between 259-1140 cc. In each plan, 4 proton lattice fields were created along with the same number of concurrent boost proton fields. 4-12 dose spheres (PTV_6670) were generated within the GTV through a treatment planning system (TPS) script, and covered by 6670cGy (RBE) dose in 5 fractions. PTV were covered by 2000cGy (RBE) dose totally. After final optimization, another in-house script was used to merge the lattice field and concurrent boost field together in TPS.
Results
PTV V20Gy >= 98% was achieved for all plans. Average V66.7Gy (as a percentage of GTV) was found to be 1.66±0.53%. Average skin dmax was found to be 33.5±4.6Gy. GTV peak-to-valley dose ratios was defined as the average dose of PTV_6670 vs the average dose of PTV_Avoid, which is the interval spheres structures to PTV_6670. Average peak-to-valley ratios of 2.3-3.3 (2.7±0.4) can be achieved in these plans. Furthermore, the OAR constraints in the Phase I photon lattice clinical trial (NCT04133415) were all met in these plans.
Conclusion
Dose coverage objectives were met for all 7 proton lattice plans without utilization of the collimating brass aperture. The semi-automated technique was developed in this work to achieve this planning approach, which has more flexible beam arrangement and planning design than the current proton GRID therapy.