"Purpose: Head and neck cancer (HNC) patients undergoing radiotherapy often experience significant anatomical changes that necessitate replanning. However, replanning is resource-intensive and decided on short notice. Therefore, we aim to predict the progression of anatomical changes throughout radiotherapy so that replanning can be anticipated in advance.

 

Methods: We trained a variational autoencoder (VAE) to learn condensed latent vectors of CBCT scans using our in-house dataset of 420 HNC patients and 5323 CBCT scans. Subsequently, we developed a model to predict changes in these latent-space vectors over time based on the initial CBCT and clinical features (eg. staging, chemotherapy, etc.). Points along the predicted latent trajectory were decoded by the VAE to reconstruct synthetic future CBCT images. We evaluated the model by calculating the Dice score between actual and predicted body masks for each fraction. Additionally, we assessed the percent change of the area between the first and subsequent fractions in true versus predicted images, a useful replanning metric for determining shrinkage.

 

Results: Initially, we trained our models on binary masks of single image slices, rather than the full CBCT. Our model achieved an average test set Dice score of 0.94 and area error of 5.9% for all fractions. The area error decreases as more CBCTs are incorporated throughout treatment. We are expanding our model to predict 3D anatomical changes and implementing a replanning flagging system based on the expected changes.

 

Conclusions: Our preliminary framework for modeling latent trajectories shows promising potential for predicting anatomical changes in HNC patients."

Purpose: Accurate dose distribution at the junction of tangential and supraclavicular fields in breast radiotherapy is affected by various dosimetric uncertainties. This study evaluates the accuracy of the Monaco treatment planning system (TPS) and compares the dose distributions of Monoisocentric (MI), Dual-isocentric (DIHS, DIFS), and TomoDirect techniques to determine the most effective approach for dose accuracy and homogeneity. Materials and Methods: An anthropomorphic phantom was used for experimental dose verification. Dose distributions of 3DCRT techniques (MI, dual-isocentric half-field (DIHS), and dual-isocentric full-field (DIFS) techniques) were calculated in Monaco TPS. Gamma index analysis was performed on dose profiles obtained at depths of 0.5, 1, 2, and 3 cm to evaluate the agreement between calculated and measured doses using EBT3 films. Additionally, TomoDirect 3DCRT (TD-3DCRT) and IMRT (TD-IMRT) techniques were compared with 3DCRT techniques.

 

Results: The Monaco TPS with the XVMC algorithm demonstrated superior accuracy in the buildup region compared to the CCC algorithm, achieving gamma agreement above 90% at deeper depths. Among techniques, TD-IMRT provided the most homogeneous dose distribution at the junction of breast and supraclavicular PTVs. The MI technique resulted in lower dose variations than DIHS, DIFS, and TD-3DCRT and minimized dose escalation in the superficial layers of the supraclavicular region. Conclusion: The Monaco TPS with the XVMC algorithm ensures greater accuracy than the CCC algorithm. TD-IMRT offers optimal dose homogeneity at the field junction, while the MI technique achieves superior tumor dose coverage with reduced surface dose in the supraclavicular region, making it a preferable choice.

Purpose: Cardiac radioablation (CR) targets change position with cardiac and respiratory motion; some protocols define margins based on the extent of target respiratory motion only using a respiratory 4D computed tomography (4DCT). The accuracy of accounting for the amplitude of respiratory motion alone in CR is investigated.

 

Methods: Respiratory motion traces for eight CR patients were acquired for 15-20 seconds using 5 Hz bi-planar fluoroscopy. Cardiac motions from 50 heart failure patients were taken from 30 Hz cardiac magnetic resonance imaging. Motions were combined to create 8400 realistic cardiorespiratory motions. The margin using a respiratory internal target volume was approximated as the amplitude of respiratory motion, measured by averaging half the peak-to-peak respiratory motion for each breath. These margins were compared to the impact of cardiorespiratory motion on dosimetry, found using the convolution method. The convolution method blurs an ideal dose distribution with the motions and calculates the margin as the shift in the 95% dose level.

 

Results: The amplitude of respiratory motion was found to exceed the margin needed to account for the blurring of cardiorespiratory motion by an average of [0.5±0.5 right/left, 0.4±0.8 ant/post, 1.2±0.5 inf/sup] mm. However, the amplitude of respiratory motion underestimated the motion margin for [18% right/left, 26% ant/post, 3% inf/sup] of cardiorespiratory motions up to a maximum of 5 mm.

 

Conclusion: This work demonstrates limitations in defining margins equal to the amplitude of respiratory motion alone in CR. Further work to characterize the prevalence of cardiac motion in respiratory 4DCT imaging is needed.

Purpose:  Ultrahypofractionated breast radiotherapy has emerged as a new standard of care. Over the last decade, the use of flattening filter-free (FFF) beams has increased due to their higher dose rates, but their low-energy photons may increase skin toxicity.  This study investigates the impact of FFF beams on skin dose.

 

Methods:   This study included IMRT plans for 9 breast cancer patients previously treated with flattened tangent fields. All treatment plans were re-optimised with FFF beams of the same nominal energy as in the original plans, and the V95% and D2cc for a 3mm skin rind were compared. Plans were calculated using a commercial TPS and Monte Carlo (MC) simulations. The body was isotropically extended by 2 cm in air for calculations. D2cc was converted to equivalent dose in 2 fractions (EQD2) using α/β=11 for acute effects.

 

Results:   Median (range) difference in V95% between MC and TPS was 5.9cc(-21.8 – 9.5cc) for FFF beams and -3.6cc(-16.7 – 2.5cc) for flattened beams.  Median (range) differences in MC calculated V95% and D2cc between FFF and flattened beams was 4.6cc(-6.9 – 21.9cc) and 30cGy(-39 – 94cGy), respectively. Both FFF and flattened D2cc values converted to EQD2 were less than the reported dose threshold for acute skin toxicity of Dmax<46Gy_11.

 

Conclusions:   The use of FFF beams for breast radiotherapy yield higher calculated skin doses compared to flattened beams but remain within a reported skin toxicity threshold. These findings suggest that while FFF beams increase skin dose, they remain a viable option for breast radiotherapy.

PURPOSE The ARGOS-CLIMBER trial used multi-parametric MRI and PSMA-PET for guiding dominant intra-prostatic lesions (DIL) boosting using stereotactic body radiotherapy with simultaneous in-field boost (SBRT-SIB) without (Cohort 1) or with (Cohort 2) neoadjuvant androgen deprivation therapy (ADT). This study presents the ARGOS-CLIMBER dosimetry and a replanning study to assess utilizing SBRT-SIB for prostate cancer.

 

METHODS 50 patients received 5 fraction SBRT delivering 25 Gy to the pelvis, 35 Gy to the prostate, and up to 50 Gy to identified DILs. A replanning study on 18 Cohort 1 patients assessed a 35 Gy uniform baseline SBRT (non-SIB) plan, a 40 Gy dose-escalated uniform plan, and a high-gradient DIL-boosted SBRT-SIB plan. DIL targets and organs-at-risk dose metrics were compared using the Wilcoxon rank-sum or sign-rank test across cancer risk group, neo-adjuvant usage, and planning technique.

 

RESULTS A median DIL GTV dose of D95=42.6 Gy across all patients was achievable, exceeding both 35 and 40 Gy non-SIB plans (p<0.001, p<0.001). DIL target volume was lower and boost doses were higher for patients with high-intermediate cancer risk and neo-adjuvant ADT (GTV D95 p=0.12, p=0.005, respectively), with neo-adjuvant ADT showing a stronger trend and decreased rectum D2cc values. Allowing high-gradient boosting resulted in higher DIL doses (median GTV D95=43.6 Gy), but urethra Dmax and D10% rose.

 

CONCLUSIONS MRI/PSMA-PET delineated DILs received escalated doses using SBRT-SIB compared to standard SBRT, with lower cancer risk and neo-adjuvant ADT patients benefitting from higher DIL boost doses. High-gradient doses allowed higher boost doses, but further safety research is warranted.

"Introduction: Modern linear accelerators are equipped with cone beam computed tomography (CBCT) and a robotic couch, allowing rotational (rotation, pitch and roll) corrections in addition to standard translational (X, Y, Z) corrections in patient setup, collectively known as 6 degrees of freedom (6DoF) corrections. Currently antero-posterior and lateral radiographs are used for image guidance in daily head and neck radiation therapy treatment for setup. This only allows couch translational and rotation (or four degrees of freedom (4DoF)) corrections for patient setup. The aims of this retrospective study are to determine if i) moving from 4DoF to 6DoF image registration will improve the patient setup significantly based on our current planning target volume (PTV) margin of 5mm and ii) if 6DoF registration performs better for a smaller, 3mm PTV margin. Methods: This study used 31 CBCT scans (5 patients) for head and neck (H&N) and 25 CBCT scans (5 patients) for lower neck (LN) or larynx patients. The CBCTs were acquired weekly right after routine 4DoF registration. All The scans were registered to the planning CT to perform and analyze 6DoF corrections. Bony surrogate structures were used to compare images with and without 6DoF registration, assuming no intrafraction motion between 4DoF registration and CBCT acquisition. The study also assumes, no soft tissue deformation relative to bony anatomy. The mandible, clivus, C2 and C7 were used as surrogates for H&N patients. The LN cases used C2, C7, sternal notch and clavicular heads as surrogate structures. A 5mm and 3mm expansion on each of the surrogates is used to assign 1 (i.e. pass) and 0 (i.e. fail) value on the image matching for each expansion. If any part of a surrogate structure either touched or moved out of the corresponding expansion after registration this was considered as a failure on the registration. Fisher exact test was used for each of the surrogates to determine whether 6DoF registration significantly improves the pass rate. Results: The rates of improvement with 6DoF registration at 5mm margin were 40% (p = 0.35), 100% (p = 0.50) and 62.5% (p = 0.39) for C7, Clivus and mandible respectively in H&N cases. For LN cases at 5mm margin, the rate of improvement for clavicular heads was 60% (p = 0.21). The rates of improvement with 6DoF corrections at 3mm margin were 83.3% (p = 0.35), 33.33% (p = 0.20), 100% (p = 0.049) and 62.5% (p <0.002) for C2, C7, Clivus and mandible respectively in the H&N cases. At 3mm margin in LN patients, the rates of improvement for C2 and clavicular heads were 75% (p = 0.17) and 72.7% (p = 0.01) respectively. Conclusions: The study showed that 4DoF registration in not inferior to 6oF registration at 5mm margin in both H&N and LN patients for bony match. At 3 mm margin, significant improvement in bony match found in mandible and C2 for H&N cases, and clavicular heads for LN cases. The study did not validate the additional benefit of soft tissue visualization that comes with 6DoF registration."