1. Median Volume (cc) of GTV Receiving Dose
Spatially Fractionated Radiotherapy (SFRT): Comparing Photon GRID with Proton Pencil Beam Scanning GRID
David DeBlois BS CMD, Grayden MacLennan MBA MSM MS CMD, Jen Yu PhD DABR, Diane Alvarez, MS, Matt Hall, MD MBA, Michael Chuong, MD, Minesh Mehta, MD, Alonso N. Gutierrez PhD MBA
Department of Radiation Oncology, Miami Cancer Institute, Miami, Florida 33176, USA
Introduction
GRID treatments are a form of spatially fractionated radiation therapy (SFRT) that are used to treat bulky and/or rapidly growing tumors. The idea of SFRT originated from the notion that using a grid-like block to deliver a single, high dose fraction with a low energy (orthovoltage) photon beam would allow the treatment to be administered without inducing significant skin toxicity. The block created a grid pattern with many photon pencil beams “piercing” the gross tumor volume (GTV). GRID therapy continues to be used with megavoltage photon beams employing either a block or MLCs to create the grid pattern. Typically, an enface beam is used and the beam direction is selected so that the beam traverses the least amount of normal tissue. Patients who are candidates for GRID treatment will undergo a single GRID fraction followed by a standard fractionated radiotherapy course. In light of this, there is an opportunity to use proton pencil beam scanning (PBS) technology to deliver a similar dose distribution. Because of the physical dose deposition properties of protons, it is possible to treat deep tumors with a number of PBS spots distributed throughout the GTV and potentially deliver lower doses to the proximal and distal tissues when compared to photon GRID. This presentation serves to quantify dosimetric differences to the GTV and surrounding normal tissues between photon GRID and proton GRID using a PBS approach.
Results
Regarding target dose, proton GRID plans show a statistically significant increase in the median absolute volume of GTV receiving specific doses when compared to photon GRID. The highest percentage increase in the median absolute volume relative to photon GRID was noted at the higher dose levels—i.e. 12Gy (138%) and 14Gy (1038%). Regarding normal tissue, the volume of normal tissue receiving dose was higher for photon plans in every case and for every dose level. Proton GRID plans showed a statistically significant reduction in absolute volume of normal tissue irradiated at all the dose levels evaluated.
Methods & Materials
Ten patients (n=10) previously treated with photon-based GRID therapy, using a single enface 6MV beam with a GRID block (dotDecimal, Sanford FL), were replanned using proton PBS. Photon plans were generated using Eclipse (v13.7) and proton plans using Raystation (v ). To create similar high dose ‘columns’ within the target, a hexagonal grid of 2mm diameter cylinders were overlapped with the GTV and used as optimization structures. To minimize overlap, cylinders were placed at 3-sigma distances. Different beam angles were able to be utilized due to the stopping power of protons, allowing for shorter beam path and lower proximal dose to normal tissues. Plans were prescribed to 15Gy and each plan was evaluated visually for prescription dose within each ‘column’ and volumes of GTV/normal tissue receiving 2, 4, 6, 8, 10, 12, and 14 Gy were noted.
Table 1: The median dose to the GTV at various dose levels. Proton GRID is able to deliver higher dose to the GTV at various dose level with lower normal tissue dose.
Median Volume (cc) of GTV Receiving Dose Gy
Photon
Proton
Abs Diff
p-value 2
293.4
342.35
48.95
p<0.05 4
182.5
288.45
105.95 6
126.05
188.6
62.55 8
80.05
115.4
35.35 10
36.4
76.45
40.05 12
14.3
34.05
19.75 14
1.7
19.35
17.65
Figure 1: Axial dose distribution of a sample patient being treated with GRID therapy to 15 Gy.
(left) Proton dose distribution using PBS technique (right) Photon dose distribution using 6MV beam with a dotDecimal GRID block.
Conclusions
For patients with complex shaped/deep-seated targets or with critical organs distal to the target, proton-based GRID is able to increase the dose to the lesion while reducing the surrounding normal tissue dose.