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research & innovation

Paper of the Month

Cancer Radiotherapy: Evaluating the Biological Impact of Scattered Radiation in Current Methods

By Connie Jeske Crane


Humza Nusrat

In the clumsy yet illustrative language of military metaphors, cancer radiotherapy has a simple goal – target and destroy a patients’ cancer cells while preserving surrounding healthy tissue and organs from collateral damage.

In recent years, witnessing the rise of new treatment machines and methods, Humza Nusrat, a fifth-year PhD candidate in Ryerson’s CAMPEP-accredited Biomedical Physics program, felt it was the right time to evaluate current radiotherapy techniques. “The assumption is that as you increase the amount of dose in tissue, you kill more cells – therefore the goal of radiotherapy has been to maximize the amount of dose to the tumour and minimize it to healthy tissues.”  Yet Humza and his lab members wondered. Could novel treatments potentially weaken the correlation between absorbed dose and cell death?

Jointly supervised by Dr. Arman Sarfehnia of Sunnybrook Health Sciences Centre and Ryerson Physics Professor Dr. Carl Kumaradas, Humza and Sunnybrook researchers Dr. Geordi Pang and Dr. Syed Ahmad approached this question. “The goal of our work was to examine how the effectiveness of radiation at killing cells – also known as the relative biological effectiveness (RBE) – changed as a function of distance in open and mixed field radiation treatment beams.”

Variation in RBE (biological effectiveness of radiation) as a function of position for the two types of beams. The RBE increase for the single beam (red) occurs approximately 5 cm away from the edge of the beam - likely insignificant to patient treatment. The RBE increase for the composite beam (blue) occurs much closer to the edge of the treatment beam, potentially leading to harmful clinical implications.

With duties that included creating and running many Monte Carlo simulations, data analysis, and documentation, Humza says, “The method we used to convert electron fluence to RBE had only been done for experimentally measured results; our work was the first to determine a way to use simulated data.”

Eventually, the group caused a bit of a stir. Current clinical practice, says Humza, assumes RBE does not change much despite the presence of low-energy, lethal scattered radiation. “In a single beam case, the amount of scattered radiation was so low that despite being more damaging, the clinical impact was assumed to be insignificant. However, in a multiple field case (which is how we mainly treat in the clinic today), the amount of scattered radiation present increases significantly.” While it had been assumed that scattered radiation wasn’t significant, Humza says, his study actually indicated quite the opposite. “That the RBE can increase by up to 14 per cent outside the field!”

Further elaborating, Humza says clinicians usually reach the tolerance dose for organs at risk (OARs) surrounding a tumour in treatment planning. “In many spine treatments, for example, the tumour is found in the spinal cord, however an organ called the ‘thecal sac’ is also part of the spinal column. Due to its proximity to the tumour, it also receives a radiation dose. Our work actually shows that the current clinical practice could be causing the OAR (in this case, the thecal sac) tolerance dose to be surpassed by up to 14 per cent!”

Taking into account the findings of this work, the effective dose to the organ at risk, particularly in cases where the organ at risk is in close proximity to the tumour, may be larger than anticipated, and may become significant especially if the OAR dose is pushed to near-tolerance. In the schematic diagram above such a case is shown although commonly a much greater number of beams are used.

“At first,” says Humza, “many reacted dismissively to the work saying that an RBE discrepancy (and resulting potential clinical oversight) that large was not possible.” Ultimately though, the team’s findings, validated through literature, are significant and highlight a need for further analysis.

Earlier this year, the team’s findings were published in the journal, Biomedical Physics and Engineering Express (IOP), in a paper called “Evaluating the biological impact of increased scattered radiation in single and composite field radiation beams.”

Reflecting on his graduate school experience, Humza says he’s particularly valued the chance to work on cutting-edge and meaningful research like this at Ryerson. “Our program is comprised of an amazing group of students and professors. Asking big, exciting questions is encouraged and there is a strong focus on translating research from the lab bench to the clinic.”

The Ryerson research study was partially funded via an NSERC Discovery Grant.