2024 Program with Abstracts

Session 1: Treatment and Planning - Lombardy/Umbria

le 6 juin 2024 from 11h00 CDT to 12h00 CDT

Scientific Session – Treatment and Planning
Thursday, June 6, 2024 11:00-12:00

Scientific Session 1: Treatment and Planning – Presentation 1

Dosimetric impacts of breath hold target variability for DIBH treatment with 3DCRT and VMAT.

Venketesh Thrithamara Ranganathan
Cape Breton Cancer Center Nova Scotia Health

Purpose: Clinically employed techniques to monitor movements in a deep inspiration breath hold breast treatment are limited to monitoring the patient's surface. The delivered dose variation in target and OARs due to complex motion from variation in depth of breath hold is not fully understood.

Methods: A cohort of 50 patients with left-sided breast treatment plans were considered for this study. CT images of these patients at breath hold (BH) and free breath (FB) positions are used to establish a deformation vector field (DVF) between these two postures using Velocity software by Varian Medical Systems. To account for the correct anatomical location of the tissues at deviations from the BH target, a scaling factor is established based on the difference between the average DIBH and FB period based on the RPM/RGSC breathing traces acquired during CT simulation. CT images for BH deviations from +5 to -5 mm are then generated. Varian’s Eclipse software is employed to calculate the dose at the target and organs at risk at each deviation.

Results: The PTV D95 and Lung mean doses are sensitive to the deviation from the breath hold target, with PTV more sensitive to undershoot. In contrast, the mean lung dose is sensitive to both overshoot and undershoot. In general, the delivered dosimetry deviates from the planned dose more for VMAT than 3D CRT when BH target is not perfectly achieved.

Conclusion: In a DIBH treatment, 3D CRT techniques is dosimetrically advantageous and more forgiving to tolerance over VMAT


Scientific Session 1: Treatment and Planning – Presentation 2

Implementation of a hybrid planning technique for hypofractionated simultaneous integrated boost (SIB) breast treatments.

Julia Albers, Gabriela Stroian, Ackeem Joseph, Tarek Hijal, Christine Lambert
McGill University Health Centre

Purpose: Our center implemented a hybrid planning technique for hypofractionated SIB breast cancer radiotherapy, aiming to improve treatment efficiency and dosimetry. The conventional sequential approach for breast cancer radiotherapy involves a boost to the tumor bed following 3DCRT whole-breast irradiation, leading to increased patient visits. In contrast, the hybrid SIB technique combines open field tangents and volumetric modulated arc therapy (VMAT) arcs within a single plan, reducing the total number of fractions and optimizing the combined dose distribution.

Methods: The hybrid SIB (26Gy-30Gy/5fx) involves a non-modulated tangential field delivering 80% of the prescribed whole breast dose, coupled with VMAT arcs delivering the remaining 20% of the whole breast dose and the complete boost dose, all within a single plan. Dosimetric comparisons between the hybrid SIB technique and conventional sequential plans were performed. A dosimetric robustness study was also performed by quantifying the effects of 5 mm isocentre shifts.

Results: Dosimetric comparisons reveal superior performance in coverage, homogeneity, conformity, and most OAR sparing, although slightly higher low doses to the heart and great vessels are observed. The robustness study demonstrates reliability for shifts of up to 5 mm.

Conclusion: The implemented hybrid SIB technique shows promise in optimizing breast radiotherapy outcomes through improved coverage, homogeneity, and OAR sparing, offering a valuable alternative to conventional sequential plans. In addition, the hybrid SIB technique is more convenient for patients due to the overall reduction in the number of fractions.


Scientific Session 1: Treatment and Planning – Presentation 3

Dosimetric advantage of HyperArc over an institutional RapidArc planning technique for stereotactic treatment of multiple brain metastases.

Fateen Basharat, Kurtis Dekker, Tim Olding, Kevin Alexander, Andrew Kerr
Cancer Centre of Southeastern Ontario

Purpose: To evaluate the dosimetric benefit of Varian HyperArc Fractionated Stereotactic Radiotherapy (FSRT) planning compared to our institutionally established RapidArc FSRT technique for multiple brain metastases treatment.

Methods: Five multiple brain metastases patients previously treated at our institution with a RapidArc FSRT method were re-planned using single isocentre HyperArc plans. The HyperArc plans were generated using the Photon Optimizer (v15.6) with built-in Stereotactic Radiosurgery Normal Tissue Objective and Automatic Lower Dose Objective options. All treatment plans used 6 MV photon beams (600 MU/min dose rate) on a Varian TrueBeam linear accelerator equipped with the standard Millennium120 MLC. The beam arrangements for HyperArc plans matched to the RapidArc plans; however a single couch zero arc is used in HyperArc plans compared to two couch zero arcs in the treated plans. The techniques were compared in terms of low dose spillage (V30%), 3D dose maximum (Dmax), conformity index (CI), monitor units (MU) required and dose to organs-at-risk (OAR).

Results: Both planning technique achieved institutional FSRT OAR dose criteria. The HyperArc plans exhibited a 10-20% reduction in dose to OARs and 20-40% reduction in V30% compared to RapidArc plans. Target dose conformality was comparable between the two planning techniques.

Conclusions: The HyperArc approach yields clinically superior FSRT plans with highly conformal dose to targets and decreased low dose spillage compared to RapidArc plans. These results support moving to HyperArc for multi-target brain FSRT treatments. End-to-end phantom-based verification of HyperArc FSRT plans using radiochromic film and ion chamber are underway and will be presented.


Scientific Session 1: Treatment and Planning – Presentation 4

Effect of GTV Size on Dose Parameters Relevant to Radionecrosis in Linac-based Coplanar and Non-coplanar 5 Fraction SRT for Brain Metastases

Evan Maynard, Hunter Lowe, Quinn Matthews
BC Cancer - Centre for the North

Purpose: A planning study was undertaken to evaluate the effects of GTV size, dose prescription, and treatment technique on dose parameters related to radionecrosis in linac-based 5 fraction SRT.

Methods: Recent literature has demonstrated that radionecrosis incidence post-treatment of brain metastases with 5 fraction SRT is correlated with the absolute volume surrounding an individual lesion receiving 29 Gy and 25 Gy. A retrospective analysis of 42 plans using coplanar arcs was performed to investigate the relationship between of V29Gy and V25Gy with GTV size and dose prescription. Fifteen of these plans were replanned with non-coplanar arcs using HyperArc to evaluate the effect on these dose parameters.

Results: For both V29Gy and V25Gy the likelihood of exceeding clinically relevant dose constraints was low when the GTV diameter was small (<1.5cm). Even at lower dose prescriptions, when GTV diameter increased (>2cm) so did the likelihood of exceeding dose constraints associated with an increased probability of radionecrosis and higher-grade toxicity. Both V29Gy and V25Gy decreased when using non-coplanar arcs, and larger GTVs saw a greater benefit from non-coplanar planning. The average decrease in V29Gy and V25Gy was 1.3 cc and 2.7 cc, respectively. For larger targets (GTV diameter >2cm) the average decreases were 2.4 cc and 5.1 cc.

Conclusion: Overall there is an improvement in clinically relevant dose parameters with non-coplanar planning, particularity for GTVs larger than 2 cm in diameter. The results from this work provide useful data in consideration of the trade-offs when planning linac-based SRT for brain metastases.

Scientific Session 1: Treatment and Planning – Presentation 5

Implementation of Fully Dynamic Couch Gantry Collimator VMAT Trajectories for Radiosurgery.

Brett Liem, Brian Little, R. Lee MacDonald, Alasdair Syme, Christopher G. Thomas

Purpose: To demonstrate the efficacy of stereotactic radiosurgery treatment plans with fully dynamic couch gantry collimator volumetric modulated arc therapy (VMAT) trajectories for improved planning target volume (PTV) conformity while reducing doseto organs-at-risk (OARs).

Methods: Treatment plans consisted of three arcs: one gantry arc and two couch arcs. Gantry arcs are defined where the gantry must continuously move in a unidirectional motion, whereas the couch may simultaneously move in an arbitrary motion pattern. The opposite definition is true for couch arcs. For gantry and couch arcs, the gantry/couch cost space was navigated from top to bottom/bottom to top and left to right/right to left to identify the path of lowest cost, respectively. Once the trajectories were defined, corresponding collimator trajectories were optimized to minimize incidents
of island blocking.

Results: The dynamic trajectory plans were normalized such that the prescription isodose covered at least 99% of each PTV. Compared to the clinical plan, the maximum dose to OARs in the dynamic plan is lower for all but one OAR. Some notable decreases are 1.8 Gy in the brainstem and 1.5 Gy in the left eye. The conformity index also improved in three of the four metastases. V12Gy saw an increase in the dynamic plan (28.3 cm3 to 32.8 cm3) but the total plan MUs saw a decrease of ~39%.

Conclusions: Fully dynamic couch gantry collimator VMAT trajectories are effective at accurately targeting and conforming to the PTV while improving avoidance of OARs.


Scientific Session 1: Treatment and Planning – Presentation 6

Planning, dosimetry, and delivery analysis of SBRT focal boost for intraprostatic lesions guided by PSMA-PET and mpMRI.

Linada Berryhill, Matt Mulligan, Julianna Pinizzotto, Hatim Fakir, Andrew Loblaw, Aneesh Dhar, Sherif Ramadan, Lucas Mendez, Matt Wronski, John Conyngham, Zahra Kassam, Priscila Crivellaro, Aaron Ward, Jonathan Thiessen, Ting-Yim Lee, David Laidley, Glenn Bauman
London Health Sciences Centre, Sunnybrook Health Sciences Centre, Patient Partner, St. Joseph’s Health Care and Western University, Lawson Health Research Institute

Purpose: In the ARGOS-CLIMBER trial, patients with unfavorable-intermediate risk or high-risk prostate cancer underwent a five-fraction stereotactic radiation therapy (SBRT) with a boost for dominant intraprostatic lesions (DILs). Hybrid PSMA PET/MRI was used for DIL characterization. We present dosimetric results and discuss intrafraction motion.

Methods: DIL volumes were defined as the union of mpMRI-defined PIRADs 4-5 lesions with intraprostatic PSMA PET avid lesions of 20%-40% SUVmax. Prescription doses were 35Gy to the prostate, 25Gy to the seminal vesicles and regional pelvic nodes, 50Gy to the DILs, and 35Gy to the positive lymph nodes. Patients were treated using volumetric modulated arc therapy (VMAT) using cone beam computed tomography (CBCT) and implanted fiducials for localization.

Results: Among 50 patients treated, the median DIL volume was 3.9 cm3 (0.2 - 80.9); the median number of DIL per patient was 1. The median 99% dose coverage (D99%) for the DILs was 42.5Gy (40.0 - 50.5); the target DIL boost of 50Gy was achieved in only 5 DILs. The median Dmax for rectum, bladder and urethra was 40.1Gy (34.3 - 41.0), 38.90Gy (35.9 - 45.4), and 44.0Gy (35.2 - 51.6), respectively. Patients were reset in 33% of the treatment fractions and 83% of those cases had 3mm motion or greater.

Conclusions: PSMA PET/MRI-guided SBRT boosting of DILs was challenging due to extensive disease and proximity to the OARs. Intra-fraction monitoring of treatment delivery is recommended to optimize delivery. The trial continues with the incorporation of neoadjuvant hormone therapy to reduce DIL size and facilitate boosting.