Imaging Rejection Rate Analysis in Four-Field Breast Radiotherapy

Abstract

Purpose/Aim

At Princess Margaret Cancer Centre, two-dimensional megavoltage portal imaging is predominately used to verify positional accuracy for breast cancer patients undergoing 4-field radiotherapy. It is acquired and assessed daily by radiation therapists with an online correction of 0.5 cm translational tolerance. Depending on the dose fractionation, an offline approval of all day 1 images was required by radiation oncologists prior to the second or fourth fractions of treatments. A quality assurance task was conducted in 2023. The purpose was to evaluate the imaging rejection rate, reasons of rejection, and to implement strategies in further reducing rejection rates.

Methods/Process

Between Oct 2018 and August 2021, forty-four patients receiving 4-field breast radiotherapy had at least one day 1 image rejected by oncologists. There was a total of 51 rejections (38 supra-clavicular fields and 13 chestwall/breast fields). This was about 5% of all 879 patients treated with this technique during this time. All patients were treated on Elekta Agility linear accelerators equipped with iView electronic portal imager. All relevant images and documentation were retrieved in Mosaiq information system. Among all the reasons, the clavicle displacement of ≥ 0.5 cm accounted for 52% of the rejection. Inadequate lung distance of breast fields had the second highest rate of 34%. Poor imaging preprocessing and preparation occurred in 9%. Next, an experienced frontline therapist undertook a routine registration process for all rejected images, this peer-review process aimed to identify any variation in decision making between therapists and oncologists.

Results or Benefits/Challenges

Three areas of improvement were identified. First, an inter-observer variability exists when choosing different anatomical references. For example, therapists routinely used central-lung distance to guide the anterior-to-posterior shifts while oncologists may follow maximal heart distance. Second, the use of different measurement and visualization tools in iView and Mosaiq may contribute to additional few millimeters of difference, for example, using different bordering enhancement or performing template matching versus point measurements. Finally, day 1 procedures can be quite lengthy and patients may alter arm position in between imaging fields. Continuous monitoring of the stability of patient position was needed on subsequent treatments.

Conclusions/Impact

This quality analysis of breast imaging improves inter-professional communication. The benefits and pitfalls of different registration strategies were discussed during breast qa rounds. The recommendations were disseminated to frontline therapists in an one hour imaging refresher training. This quality assessment was repeated six months after the training. In a group of 180 patients, the imaging rejection rate decreased to 3.3%. The increased alignment of inter-professional practice in imaging assessment has phased out day 1 imaging approval. Ongoing imaging refresher training is the key strategy to increase patient positioning accuracy and improve clinical efficacy.