A Phase I Study Evaluating the Safety of Stereotactic Central Ablative Radiation Therapy (SCART) for Bulky Metastatic or Recurrent Cancer

Despite advances in screening and surveillance, patients continue to present with both bulky primary and metastatic tumors. Some patients presented with recurrent bulky cancer in prior treated radiation fields. Spatially Fractionated Radiation therapy (SFRT) has a history of over 100 years. The principle of SFRT is distinctive from the standard radiation approaches, as it treats the total tumor with a non-uniform dose, effectively treating the tumor while staying within normal tissue tolerance of the surrounding structures. Historically, SFRT is frequently used to treat bulky malignant tumors with a high radiation dose in the stereotactic radiosurgery (SRS)/stereotactic body radiotherapy (SBRT) dose range (10-20 Gy per fraction) using megavoltage x-ray beams. The application of SFRT, historically known as GRID therapy, has produced dramatic relief of severe symptoms, significant objective regression, above average local control rates and minimal toxicity in palliative settings.

High-dose GRID radiotherapy, sometimes termed spatially-fractionated GRID radiotherapy (SFGRT), is a treatment modality that was introduced in 1909 and commonly used through the 1930's . In 1909, Kohler in Germany described radiation delivered through a perforated screen with regularly spaced blocked areas that created an effect similar to treatment with multiple small pencil beams. This spatially fractionated radiation, in contradistinction to current approaches, does not attempt to treat the total tumor volume with a uniform dose. Instead, this technique allows the delivery of high doses of radiation in clusters of small areas without producing prohibitive normal tissue damage to skin and subcutaneous tissues. In its early applications, two-dimensional grid fields were used, typically with orthovoltage beams. The grids were usually composed of open/shield circular or square shapes ranging in size from 0.5 to 1.5 cm. The application was mainly for the treatment of advanced bulky tumors.

However, the technique of GRID radiotherapy has not evolved significantly since its inception in the early 1900's, and is not the optimal method of delivering spatially fractionated radiation in the modern era. GRID has the limitation of delivering relatively high doses of radiation to normal tissues, depending on tumor location, as it is delivered via a single beam that must pass through normal tissues to reach its target. Most importantly, the highest-dose regions of the grid are superficial, and often are outside of the tumor target itself.

The basic principle of the LATTICE Radiotherapy (LRT) is to create within tumor volume multiple localized high-dose islands (12 Gy and higher) with a certain degree of separation to form low dose regions (3 Gy or lower). In an extreme case, one or two focused dose islands could be introduced in a small tumor.

Modern radiotherapy methods are readily available to deliver 3D high-dose LATTICE radiotherapy with superior dosimetry compared to the 2D GRID technique . An array of focused high-dose volumes, in essence a lattice of doses in 3D, can be generated through modern techniques resulting in highly heterogeneous dose distributions within the tumor volume, leaving adjacent and peripheral normal tissue minimally exposed.

Similar to the core principles and concepts of LRT, SCART aims to stereotactically irradiate part of target volume (hotspot) located at the center of a large tumor target with an ablative dose (15Gy or higher) and the dose quickly falls off from the edge of hotspot to low dose (3Gy or lower) at the edge of tumor volume. The goal is to irradiate as large a volume as possible with ablative dose, while maintaining the dose to the border of the tumor at low dose.

SCART is a different approach which pursuits not only the dose escalation but also the idea of facilitating or promoting intra-tumoral bystander effect, thus increasing the biological effectiveness of the treatment. Published data reviewed by Peters, et al. strongly suggest that GRID therapy induces a rapid and higher rate of tumor cell apoptosis in bulky and hypoxic tumors. This technique is very appealing to treat patients with voluminous gynecological tumors.

These makes SCART Radiotherapy a practically achievable alternative to traditional GRID therapy and LATTICE, which delivers a highly heterogeneous dose distribution, anticipated to trigger the bystander effect of radiation. With high dose regions strictly contained within the target volume, normal tissue toxicity is practically avoided providing an increased therapeutic ratio.

SCART is a promising tool to achieve dose escalation which will lead to a higher local control without adding any extra toxicity in the peripheral normal tissue regions.