Grid/lattice therapy: consideration of small field dosimetry

Abstract

Small-field dosimetry used in special procedures such as gamma knife, Cyberknife, Tomotherapy, IMRT, and VMAT has been in evolution after several radiation incidences with very significant (70%) errors due to poor understanding of the dosimetry. IAEA-TRS-483 and AAPM-TG-155 have provided comprehensive information on small-fields dosimetry in terms of code of practice and relative dosimetry. Data for various detectors and conditions have been elaborated. It turns out that with a suitable detectors dose measurement accuracy can be reasonably (±3%) achieved for 6 MV beams for fields >1×1 cm2. For grid therapy, even though the treatment is performed with small fields created by either customized blocks, multileaf collimator (MLC), or specialized devices, it is multiple small fields that creates combined treatment. Hence understanding the dosimetry in collection of holes of small field is a separate challenge that needs to be addressed. It is more critical to understand the scattering conditions from multiple holes that form the treatment grid fields. Scattering changes the beam energy (softer) and hence dosimetry protocol needs to be properly examined for having suitable dosimetric parameters. In lieu of beam parameter unavailability in physical grid devices, MLC-based forward and inverse planning is an alternative path for bulky tumours. Selection of detectors in small field measurement is critical and it is more critical in mixed beams created by scattering condition. Ramification of small field concept used in grid therapy along with major consideration of scattering condition is explored. Even though this review article is focussed mainly for dosimetry for low-energy megavoltage photon beam (6 MV) but similar procedures could be adopted for high energy beams. To eliminate small field issues, lattice therapy with the help of MLC is a preferrable choice.

Keywords: beam quality; dosimetry; grid therapy; interplay; scatter dose; small-field dosimetry.

Figure 1.

Two set of grid therapy blocks; (A) dot decimal (.decimal, Sanford, FL, USA) brass device mounted on a machine, (B) dot decimal face view, (C) radiation product design (Albertville, MN, USA) Cerrobend block mounted on a machine (D) face view of radiation product design system.

Figure 2.

Percent depth dose curves of 6 MV beam for 10×10 cm² field for open and a commercial brass grid collimator’s central hole. Please note that due to scatter from blocking material the beam is much softer and depth dose is lower as indicated by smaller d_max and lowering of depth dose that has implications in the estimation of dose based on TG-51 protocol.

Figure 3.

Grid output factor measured on the central axis of the central hole at d_max with 2 detectors. The differences are relatively small but could be significant as the hole-size approaches to the limit of small field. The factor was normalized by 10×10 cm² open field.

Figure 4.

MLC-based grid therapy for bulky tumour using 6-fields beam arrangements. Isodose distributions are shown in axial, sagittal, and coronal planes. Similar dose distributions also shown by Pokhrel et al.