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High Dose, Small Field Radiation Therapy: Lessons From the HyTEC Project and the ICRU 91 Report

J Seuntjens1*, G Ding2*, L Marks3*, L Ma4*, S Benedict5*, E Yorke6*, (1) Medical Physics Unit, McGill University, Montreal, QC, (2) Vanderbilt University , Nashville, TN, (3) UNC School of Medicine, Chapel Hill, NC, (4) UCSF Comprehensive Cancer Center, Foster City, CA, (5) UC Davis Cancer Center, Sacramento, CA, (6) Memorial Sloan-Kettering Cancer Center, New York, NY


(Thursday, 8/2/2018) 7:30 AM - 9:30 AM

Room: Room 207

Stereotactic Body Radiation Therapy (SBRT, aka SABR) has become a widely used technique that relies crucially on the accurate delivery of highly conformal, sharply delineated high doses to small target volumes and accurate avoidance of critical risk organs. This places major demands on clinicians to understand the physics of small field dosimetry and imaging and the clinical biology of hypofractionated radiation treatment for tumors and normal tissues. This session aims to familiarize attendees with major physics considerations as recently published in ICRU Report 91 and with the ongoing efforts of AAPM’s WGSBRT (the ‘HyTEC project’) to extract clinical and radiobiological information from published SBRT outcomes literature.

Prescribing, Recording, and Reporting of Stereotactic Treatments with Small Photon Beams, ICRU Report 91 (Jan Seuntjens presenting)

Learning Objectives:
1. to understand the challenges of small photon radiation beams in the context of calibration, relative dose measurements and treatment planning dose calculations.

The ICRU Report 91 consists of seven sections and an appendix discussing clinical examples. In this presentation we focus on the physics challenges of small field dosimetry, detector characteristics and dose calculation algorithms (sections 2 and 4 in the report). The three main features that dominate the dosimetry of small beams from accelerators are a lack of charged-particle equilibrium, partial source occlusion and the importance of size and construction details of the detector used. For reference dosimetry in a machine-specific reference field (msr) ICRU-91 recommends the data from IAEA-AAPM TRS-483 (2017). In small fields, more than a single detector should be used to determine relative output factors and the measured data should be corrected with the detector type-specific correction factor data. For treatment planning, accurate modeling of lateral electron scattering in heterogeneous regions with mass densities that differ significantly from water is critical. The impact of lateral electron scattering increases with increasing energy as the lateral range of the secondary electrons increases. Therefore, for treatment planning in SRT, advanced model-based absorbed-dose-calculation algorithms such as Monte Carlo, or deterministic algorithms are recommended in ICRU 91 to ensure that dose in tissue of heterogeneous density is accurately calculated.

Quality Assurance for Accurate Image Guided Radiation Therapy from ICRU Report 91 (George Ding presenting)

Learning Objectives:
1.To learn the importance of IGRT and to know level of achievable accuracy with a current state-of-the-art imaging system.

Image guidance is an essential part of delivering radiation that is highly conformed to the often-small target in hypo-fractionated treatments. Without accurate image guidance the radiation may precisely miss the intended target and significantly adversely affect the treatment outcome. This section will provide a brief overview of current IGRT technologies and imaging dose from different image guidance procedures and discuss the quality assurance procedures that are necessary in order to obtain the level of accuracy that is currently achievable using a state-of-the-art image guidance system.

The HyTEC Project: Goals, Issues Encountered, Accomplishments, Future (Lawrence Marks presenting)

Learning Objectives:
1. To understand the goals and problems encountered by the AAPM’s efforts to extract clinical information from published outcomes literature on SBRT.

Designing dose distributions that minimize the risk of radiation-induced normal tissue toxicity is one of the hardest parts of treatment planning. Commonly, many planners use the tolerance guidelines set forth in the famous 1991 ‘Emami paper’, and/or based on the Quantec papers. For the most part, the data in these prior guidance documents relate to conventional fractionation. The AAPM Working Group on Biological Effects of Hypofractionated Radiotherapy/SBRT (commonly called HyTEC) is an interdisciplinary effort to systematically review the rapidly-evolving published SRS/SBRT literature with regard to both TCP and NTCP. The aim is to generate tables and graphs that summarize the available dose/volume/outcome data in a manner that can help planners define ‘better’ SRS/SBRT plans. To date, 4 reports are complete, and the remainder are expected to be completed in the coming year.

Overall, we believe that many of the reviews are generating some useful and interesting data. For example, some of the dose/volume/outcome responses are steep, emphasizing the importance of careful dose-selection. Nevertheless, the pooling of literature data is difficult (e.g. due to inter-study differences in planning, dose computation/reporting, and outcome reporting) making this a challenging exercise, and perhaps undermining the validity of some of the pooled findings. As a field, we need to do better in defining uniform ways to (at least) report our treatments and outcomes to enable future similar data-pooling efforts in order to facilitate our learning from each other’s broad clinical experiences.

Spinal Cord Tolerance for SBRT Treatments: Results from HyTEC (Lijun Ma presenting)

Learning Objectives:
1. To summarize the spinal cord SBRT dose limits from the HyTEC review of published spinal cord complications associated with SBRT.

Published spinal cord tolerance data for stereotactic body radiotherapy (SBRT) have been reviewed by the HyTEC work group. For single fraction de novo spine SRS, conservative Dmax dose limits range from 12 Gy to 14 Gy. For de novo SBRT delivered in 2 to 5 fractions, conservative Dmax limits from the literature are: 17 Gy, 20 Gy, 23 Gy, and 25 Gy in 2, 3, 4, and 5 fractions, respectively. These dose limits were modeled to possess an estimated risk of radiation myelopathy between 1% and 5%.Dose limits for re-irradiation SBRT delivered in 1 to 5 fractions will be also discussed

Tumor Control Probability (TCP) of Hypofractionated Radiotherapy of Paraspinal Tumors: Results from HyTEC (Stanley H. Benedict presenting)

Learning Objectives: To summarize SBRT dosing schedules associated with adequate TCP of spinal tumors according to the HyTEC literature review.

In accordance with the guidelines established by the HyTEC project, spinal TCP from SBRT and hypofractionated protocols have been evaluated from the reports in the literature. This presentation reviews the relevant issues in defining the dose to tumor control effect for spinal SBRT, including the treatment goals (local tumor control, pain relief, and reduction in neurological symptoms), challenges in defining tumor volumes, margins, dose, fractionation, and imaging strategies for follow-up evaluation. An initial PubMed search identified 57 publications as potential data sources but ultimately data could be extracted from 15 manuscripts involving 329 tumors. These data demonstrate clinically acceptable TCP for treatments in the range of 16 – 24 Gy in a single fraction or with a similar biologically effective dose in multiple fractions. Common multi-fraction dosing schemes were 24 Gy in 2 fractions and 27-30Gy in 3 fractions. We await the results of RTOG 0631, the only prospective randomized trial in spinal SRS, to determine pain response to 16-18Gy of spinal SRS compared to 8 Gy in a single fraction of 3D conformal irradiation.



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