Dosimetric impact of flattening filter-free photon beams on bolus volumetric modulated arc therapy planning for left chest wall irradiation using active-breathing coordinate techniques

Introduction

The most common type of cancer among women globally is breast cancer (1). After mastectomy or breast-conserving surgery, adjuvant radiation improves locoregional control and survival in patients with breast cancer (2). The main objective of breast cancer radiotherapy treatment is to administer the prescribed dose to the tumor while minimizing or avoiding irradiation of surrounding normal tissues. This can be challenging when organs are closely situated near the treatment area of particular concern is the heart, which, as a result of radiation exposure after breast cancer therapy, is susceptible to long-term consequences such coronary artery disease.

To reduce the heart’s dose without compromising treatment efficacy or affecting other organs, techniques such as the breath-hold technique are employed (1,2). The active breathing coordinator (ABC) device (Elekta^TM Oncology Systems in Stockholm, Sweden) enables patients to breathe in a controlled manner, reducing the dose to the heart and lung. Typically, this is achieved using the deep inspiration breath hold (DIBH) technique. During simulation and treatment, patients inhale deeply and hold it, temporarily halting respiratory motion, expanding the lung, and moving the heart away from the chest wall (3). The ABC device functions by holding the patient’s breath at a consistent level using the DIBH technique. DIBHs accomplished through abdominal breathing and thoracic breathing result in varying degrees of chest wall expansion. With the ABC device, there is a chance of overshoot, in which the airflow rate is excessively high and the patient inhales more air than the predetermined limit. Proper training is essential for the patient to understand and maintain compliance with the technique throughout the treatment period (4). Although the dosimetric benefits of the DIBH approach in preserving the heart have been shown in various trials, its implementation necessitates extensive patient coaching and can result in increased workload for the department. In centres with high patient volumes, regular use of this technique may lead to longer patient waiting times (2).

Operating a linear accelerator in flattening filter-free (FFF) mode can increase the dose rate by up to approximately four times compared to using a flattening filter (FF), potentially reducing the beam-on time by more than 50% (5,6). The shorter treatment time facilitated by FFF mode may help alleviate patient discomfort and reduce anatomical motion during DIBH treatment. Also, FFF beams exhibit a sharper penumbra than flattened beams, which results in lower peripheral doses close to treatment field boundaries. Additionally, In FFF mode, the out-of-field dose is significantly reduced at distances further away from the treatment field, primarily attributed to a substantial decrease in head leakage. This decrease in the patients’ out-of-field radiation could decrease their long-term risk of developing secondary cancers (7). A bolus (BOL) is a tissue-equivalent material raises the dose to the skin’s surface and ensures that the chest wall obtains sufficient dose. It improves the homogeneity and conformity and its attaining the appropriate dose at a superficial depth (8-10). FFF beams have been widely studied in stereotactic and high-dose radiotherapy, but their benefits in conventional breast cancer volumetric modulated arc therapy (VMAT) are still unclear. Most existing research has focused on hypo fractionated treatment, with breast treatments receiving less attention. The use of BOL in chest wall or breast VMAT continues to be debated, and only a limited number of studies have thoroughly examined its effect on superficial dose coverage and toxicity when combined with FF or FFF beams. While DIBH is known to reduce cardiac and lung doses, evidence is lacking on whether the choice between FF and FFF beams further influences these advantages. Furthermore, differences in beam modelling, multi leaf collimator (MLC) design, and dose delivery across various linear accelerators may impact plan quality, yet few multi-machine studies have specifically explored these variables in breast VMAT.

This study aims to address these gaps by systematically evaluating the interplay of respiratory management [free breathing (FB) vs. ABC], BOL use, and beam modality (FF vs. FFF) in left-breast VMAT across different linear accelerators. The purpose is to compare radiotherapy plans using FF and FFF beams for left breast cancer, with and without BOL, under both FB and DIBH conditions, on multiple machines: Elekta Versa HD^TM (EVH; Stockholm, Sweden), Varian TrueBeam^TM (VTB; Palo Alto, CA, USA), and Varian Halcyon^TM (VH; Palo Alto, CA, USA).

Methods

Twenty-Five left breast cancer patients treated with the ABC technique at Fortis Hospital, Punjab (January 2022 to August 2023) were initially considered. The patients’ characteristics presented in Table 1. Retrospective observational study of ten patients with involvement of the chest wall, axilla, level III lymph nodes, supraclavicular fossa (SCF), and internal mammary nodes (IMNs) were included. While other breast planning scenarios were excluded shown in Figure 1. For the immobilization, each patient was positioned on an all-in-one board, equipped with an appropriately sized cushion wedge to ensure comfort and stability. Their arms were carefully raised above their heads and supported on specially designed arm supports to maintain the correct position throughout the treatment. To ensure that patients could take a deep breath in a consistent and reproducible manner, tailored to their individual breath-hold capacities, a comprehensive coaching session was organized (1-3). This training session was scheduled 1 week prior to the computed tomography (CT) simulation for each patient. During this session, patients received detailed instructions and practiced the breath-hold technique to enhance their ability to perform it accurately during the actual treatment.

Figure 1 Inclusion and exclusion criteria for left sided breast planning study. IMN, internal mammary node; SCF, supraclavicular fossa.

CT images of all patients were acquired using a Siemens (Erlangen, Germany) Biograph mCT scanner. Both FB and DIBH scans were performed for each patient, these scans encompassed the region from the vertex of the head to the third lumbar vertebra (L3), utilizing a 3-mm slice thickness, and then exported to the Monaco Treatment Planning System (TPS). Target and critical organ delineation were done using DIBH CT for treatment planning and delivery. For planning comparison purposes, the same radiation oncologist delineated the tumor volume and organ at risk (OAR) on the FB CT images as well. The targets included the left chest wall, level III lymph nodes, axilla, SCF, and IMN. The Radiation Therapy Oncology Group (RTOG) breast cancer atlas criteria were followed in doing the delineation, which is a standardized reference used to ensure uniformity and accuracy in radiation therapy planning for breast cancer patients.

VMAT plans were generated for the EVH linear accelerator with a prescription dose of 5,000 cGy delivered in 25 fractions. These plans were created using the Monaco TPS with 6 MV photon beams, employing three different configurations: BOL, no bolus (NOBOL), and a combination of BOL for 13 fractions and NOBOL for 12 fractions [combination of bolus and no bolus (CBNB)] with both FB and DIBH. Additionally, the same set of plans were also made utilizing a 6 MV FFF beam (Group 1). A partial arc was chosen, with angles ranging from 165° to 300° in 30° increments in a counter-clockwise direction, involving three rotations. The minimum segment width was set at 0.7 cm, with a maximum of 200 control points. Target margins were narrowly defined, between 3 and 4 mm, with an avoidance margin of 8.0 mm. The dose calculation grid size was set at 3 mm, and the statistical uncertainty during calculation was maintained at 1% per plan. To accommodate chest wall motion, an auto flash margin of 2.5 cm was applied.

For all patients, both DIBH and FB CT images with structure sets were exported to a DICOM export folder. All scan data were then copied and imported into the Varian Eclipse TPS for treatment planning using the VTB and VH units at two different institutions. The same set of plans was generated for the VTB (Group 2) using 6 MV and 6 MV FFF beams, and for the VH (Group 3) using 6 MV FFF beams, all using the Eclipse TPS as shown in Figure 2. We compared the tumor volume’s conformity index (CI), homogeneity index (HI), mean dose, maximum dose, and V95 (the volume receiving 95% of the recommended dose) as treatment planning parameters. We also examined monitor units (MUs), modulation index (MI), and integral dose, as well as the dose obtained by OAR which include the ipsilateral lung, heart, opposite lung, opposite breast, spinal cord, oesophagus, and liver.

Figure 2 VMAT plan for Varian Halcyon (A), Varian TrueBeam (B), Elekta Versa HD (C). VMAT, volumetric modulated arc therapy.

Statistical analysis

Statistical analyses were performed using a one-way analysis of variance (ANOVA) with Bonferroni corrections in SPSS Statistics (Version 31.0) to evaluate differences between groups. The software generated both descriptive and inferential statistics, enabling the assessment of whether observed group differences were statistically significant, with the significance threshold (α) adjusted according to the Bonferroni method.

Ethical consideration

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This work is a dosimetric planning comparison study that evaluates treatment plans across different machines. As it does not involve patient intervention nor treatment delivery, and it is not an interventional study, it does not require ethical committee approval.

Results

Data from ten patients were analyzed to compare FB and DIBH dosimetry. Changes in target and other OAR volumes were observed between FB and DIBH scans. The planning target volume (PTV) decreased by 0.9% with DIBH. Lung volumes increased on average by 29% on the left side and 25% on the right side with DIBH. Heart volume decreased by 7.0%, while the oesophagus volume increased by 15%. Volumes of the spinal cord, liver, opposite breast, and body changed by 1.6%, −0.2%, 4.4%, and 1.7%, respectively, compared to FB. The detailed changes in contour volumes for both methods are presented in Table 2. The data analysis was divided into three groups. For Group 1, the plan was created with EVH using Monaco TPS with 6 MV and 6 MV FFF beams, utilizing BOL, NOBOL, and CBNB for both FB and DIBH scans. For Group 2, the plan was developed with VTB using Eclipse TPS with the same beam types and configurations for both FB and DIBH scans. For Group 3, the plan was designed with VH using Eclipse TPS with 6 MV FFF beams, also employing BOL, NOBOL, and CBNB for both FB and DIBH scans. The data was extracted from DVH statistics.

For Group 1 and Group 2, the target coverage of V95 showed no changes between the FB and DIBH (P<0.001) plans for BOL-FF, NOBOL-FF, BOL-FFF, NOBOL-FFF, CBNB-FF, and CBNB-FFF plans. no differences were observed between the FB and DIBH (P=0.01) plans for BOL-FFF, NOBOL-FFF, CBNB-FF, and CBNB-FFF plans for Group 3. However, in Group 1, variations were observed between the BOL and NOBOL (P<0.001) plans for both FF and FFF (P<0.001) beams in the FB and DIBH (P<0.001) plans. Almost 5% less coverage was observed for the NOBOL-FF plan and 6% less coverage for the NOBOL-FFF plan compared to the BOL-FF and BOL-FFF plans. No observable changes were found for the CBNB-FF and CBNB-FFF (P<0.001) plans; however, they showed 2% to 3% less coverage than the BOL-FF and BOL-FFF plans, and 2% to 3% higher coverage than the NOBOL-FF and NOBOL-FFF plans. For Group 2, 1.0% less coverage was observed for both the NOBOL-FF and NOBOL-FFF plans compared to the BOL-FF and BOL-FFF (P<0.001) plans. No notable differences were found for the CBNB-FF and CBNB-FFF (P<0.001) plans. Similarly, no changes were observed when comparing the CBNB-FF and CBNB-FFF plans to the BOL-FF, BOL-FFF, NOBOL-FF, and NOBOL-FFF plans (P<0.001). In Group 3, no difference was observed between the CBNB-FFF, BOL-FF, BOL-FFF, and NOBOL-FFF (P=0.13) plans. No notable changes in mean (P<0.001) and maximum dose (P<0.001) and HI (P<0.001) to the PTV were observed among all the plans from Group 1, Group 2, and Group 3. In Group 1, the conformity index for both the FB and DIBH plans of NOBOL-FF and NOBOL-FFF was lower compared to BOL-FF, BOL-FFF, CBNB-FF, and CBNB-FFF (P<0.001) as shown in Tables 3-5.

In all groups, no difference in the heart mean dose was observed between FB and DIBH (P=0.10) plans for BOL-FF, NOBOL-FF, BOL-FFF, NOBOL-FFF, CBNB-FF, and CBNB-FFF plans. Heart V25 was reduced by almost 2% with the DIBH plan compared to the FB (P<0.001) plans. No differences in mean dose (P=0.10) and V20 (P=0.50) to the ipsilateral lung were observed between FB and ABC plans for BOL-FF, NOBOL-FF, BOL-FFF, NOBOL-FFF, CBNB-FF, and CBNB-FFF plans. However, a variation was observed in Group 1, with a heart mean dose (P=0.04) of 300 cGy and ipsilateral lung V20 (P=0.50) being 6% to 7% higher than in Group 2 and Group 3 for both FB and DIBH for all plans as illustrated in Tables 6-8.

No dose differences were observed for the mean dose (P=0.43) to the opposite lung among all the plans in all groups. For the esophagus, the mean dose was 300 to 400 cGy lower (P=0.009) for Group 1, 100 cGy lower (P=0.49) for Group 2, and 200 to 300 cGy lower (P=0.14) for Group 3 with DIBH for BOL-FF, NOBOL-FF, BOL-FFF, NOBOL-FFF, and CBNB-FF plans compared to the FB plans. However, for Group 1, the mean dose was 1,000 to 1,100 cGy higher for FB and 600 to 700 cGy higher (P=0.49) for DIBH compared to Group 2, and 500 to 600 cGy higher for FB and 300 to 400 cGy higher (P=0.16) for DIBH compared to Group 3. No dose differences were observed between FB and DIBH plans (P<0.001) for spinal cord maximum dose among all the plans in all the groups. However, in Group 1, 1,400 to 1,900 cGy higher (P=0.06) than Group 2 and 200 cGy higher (P=0.09) than Group 3 as depicted in Tables 6-8.

No differences were observed for the mean dose (P=0.08) to the liver among all the plans in Group 1 and Group 2, but a slightly lower dose (P=0.006) difference was found in Group 3. No differences were observed between FB and DIBH for BOL-FF, NOBOL-FF, BOL-FFF, NOBOL-FFF, and CBNB-FF plans for opposite breast dose, but a 200 to 300 cGy lower dose (P=0.95) was observed for Group 3 compared to Group 1 and Group 2. Not much difference was observed in the integral dose among all the plans in Group 1, Group 2, and Group 3. In all groups, MU (P<0.001) and MI (P<0.001) were higher in all the FFF plans compared to the FF plans, but notably, lower MU (P<0.001) and MI (P<0.001) were observed in Group 2 compared to Group 1 and Group 3 as shown in Tables 6-8.

Discussion

According to research by Darby et al. (11), there is no evident threshold for major coronary event rates, which rise linearly by 7.4% for every Gy of mean heart dose, and the risk of ischemic heart disease rises as a result of the heart being exposed to ionizing radiation during treatment for breast cancer. Major coronary events can be predicted more accurately by the mean radiation dose to the heart than by the mean dose to the left anterior descending coronary artery. Additionally, studies have shown that patients with left-sided breast cancer are more prone to radiation-related cardiotoxicity compared to those with right-sided breast cancer (11-13). Clarke et al., in their comparison of irradiated vs. non-irradiated individuals, found a statistically significant increase in death rates, particularly from lung cancer and heart disease, with rate ratios of 1.27 and 1.78, respectively (14). A study by van den Bogaard et al. reported a 16.5% increase in the cumulative incidence of acute coronary events per Gy (15). Jacobse et al. found that when the mean heart dose increased, the risk of myocardial infarction increased linearly, with a high-risk ratio of 6.4% per Gy (16). Another study by Laugaard Lorenzen et al., patients undergoing tangential field irradiation noticed a 19% linear increase in the excess odds ratio of major coronary events per Gy of mean cardiac dose (17). Kwa et al. investigated the relationship between the distribution of lung doses and the incidence of radiation pneumonitis in thorough multicenter research including 530 individuals (18). According to their findings, a greater occurrence of pneumonitis was associated with an increase in the mean lung dose across each center. Pneumonia incidence in the breast group was 1.4%, especially in the low dose range of 400–1,600 cGy. An ipsilateral lung volume receiving more than 2,000 cGy (V20) has been found to be a risk factor for radiation pneumonitis in more than 30% of patients, according to Gokula et al. and Käsmann et al. (19,20).

The risk of toxicity and morbidity is reduced due to new technologies that enhance the accuracy of dose delivery to the target, thereby limiting the dose to OAR (21). Despite the advancements in modern radiotherapy techniques, movement of the chest wall and internal organs can still result in parts of the heart being exposed to radiation. This leads to variations for radiation received by these organs (22-24). When administering photon irradiation to left breast cancer patients, Breath-hold approaches are used often to minimize the dose to the heart and lungs. The rationale behind DIBH is to reduce the heart volume exposed to radiation compared to treatment with FB. The heart moves inferiorly and posteriorly during lung expansion, increasing its separation from the radiation-exposed regions and the chest wall. Patients having a greater surface area of contact between the heart and the chest wall benefit most from this procedure (25,26).

The target and other OAR volumes were found to be comparable between FB and DIBH in the study conducted by S Nair et al. (3). As anticipated, compared to FB, lung volume increased with DIBH by an average of 28% on the right side and 39% on the left. DIBH considerably decreased the mean dose to the ipsilateral lung by 15%, according to Zurl et al. (27). As the lung expanded and the heart separated from the chest wall, they also noticed a significant drop in the mean heart volume with DIBH. In our study, the lung volumes increased on average by 29% on the left side, 25% on the right side and heart volume reduced by 7.2% and no change were observed for ipsilateral lung mean dose with all among plans with DIBH compared to FB as shown in Tables 1,5-7. This variation in the volumes due to minor changes in contouring, although the increased intrathoracic pressure from lung inflation may also influence it. Except for the lungs, the volume change was not statistically significant.

Ten studies were included in Smyth et al.’s comprehensive review (28) of the cardiac dose-sparing advantages of DIBH, and each study demonstrated a statistically significant decrease in mean heart doses when employing the DIBH approach. The mean cardiac doses for DIBH varied from 130 to 390 cGy, while for FB scans, they ranged from 230 to 690 cGy. The mean cardiac dose was reduced by up to 340 cGy when DIBH was used. Our study results indicate that utilizing DIBH led to a reduction of 100 cGy in the mean heart dose in all the groups as illustrated in Tables 5-7.

In accordance with an extensive review by Taylor et al. (29), based on 398 regimens from 149 trials in 28 countries, the average cardiac dose for patients with left-sided breast cancer varied from <10 to 2,860 cGy. When the IMN was excluded, the average dose was 420 cGy. The lowest doses were from tangential radiation with breathing control (130 cGy), proton therapy (50 cGy; range, 10–80 cGy), and treatment in the lateral decubitus position (120 cGy; range, 80–170 cGy). The average dose of intensity modulated radiation therapy (IMRT) was 560 cGy (range, <10–2,300 cGy). The average dose with IMN irradiation was around 800 cGy. Tangential therapy with a separate IMN field had the highest average dose (920 cGy; range, 190–2,100 cGy), whereas proton therapy had the lowest (260 cGy; range, 100–600 cGy). In our study, we included the chest wall, level 3 lymph nodes, IMN, and SCF using the VMAT technique. The mean heart dose ranged from 590 to 680 cGy in FB and 560 to 600 cGy in DIBH across all groups as shown in Tables 5-7.

Gogineni et al. (30) compared FB and ABC proton treatment plans for patients with left breast cancer. The ABC and FB plans target coverage was comparable (97.0% vs. 96.8%). Higher volumes of ipsilateral lung received 500 cGy (34.9% vs. 29.1%) and 2,000 cGy (13.9% vs. 10.4%) as a result of ABC. Furthermore, ABC increased the volume of the contralateral lung receiving 500 cGy (3.2% vs. 2.4%). ABC had a lower maximum cardiac dose (2,480 vs. 3,580 cGy). But other heart volumetric goals showed no significant differences. The maximum heart dose was greater with FB, even though the target coverage was comparable. ABC plans had significantly higher lung doses and longer beam-on times compared to FB treatments. Our study results showed no dose differences were observed in ipsilateral lung V20, while the mean heart dose was reduced by 100 cGy with DIBH compared to FB across all groups as depicted in Figures 3-5.

Figure 3 Heart and ipsilateral lung mean (A) and volume (B) dose comparison for EVH (FB vs. DIBH). B, bolus; CBNB, combination of bolus and no bolus; DIBH, deep inspirational breath hold; EVH, Elekta Versa HD; FB, free breathing; FF, flattening filter; FFF, flattening filter free; LT, left; NB, no bolus.

Figure 4 Heart and ipsilateral lung mean (A) and volume (B) dose comparison for VTB (FB vs. DIBH). B, bolus; CBNB, combination of bolus and no bolus; DIBH, deep inspirational breath hold; FB, free breathing; FF, flattening filter; FFF, flattening filter free; LT, left; NB, no bolus; VTB, Varian TrueBeam.

Figure 5 Heart and ipsilateral lung mean (A) and volume (B) dose comparison for VH (FB vs. DIBH). B, bolus; CBNB, combination of bolus and no bolus; DIBH, deep inspirational breath hold; FB, free breathing; FF, flattening filter; FFF, flattening filter free; LT, left; NB, no bolus; VH, Varian Halcyon.

S Nair et al. (3) reported a study that enrolled thirteen participants. All patients undergoing hypo fractionated whole-breast or chest-wall irradiation received a prescribed dose of 4,250 cGy, administered in 16 fractions with 6 or 10 MV photon beams. With three-dimensional conformal radiation treatment (3DCRT), IMRT, and VMAT employing DIBH, the mean cardiac dose was decreased by 330 cGy (220 vs. 550 cGy), 220 cGy (750 vs. 970 cGy), and 120 cGy (580 vs. 700 cGy). For patients using 3DCRT, DIBH appears to be more effective in improving organ sparing, while IMRT and VMAT approaches offer somewhat less benefit. However, 3DCRT provided slightly reduced coverage of the intended volume, but the difference was statistically non-significant. VMAT achieved the better conformity and homogeneity indices, though 3DCRT results were still acceptable. The use of FB and DIBH did not influence the homogeneity and conformity indices when using a specific planning approach. Using the VMAT approach with a standard fractionation of 5,000 cGy in 25 fractions utilizing 6 MV and 6 MV FFF photons, all patients in this study underwent chest wall irradiation. Across all groups, the mean heart dose was 100 cGy lower with DIBH compared to FB. We also observed improved conformity and homogeneity indices, with no significant differences between DIBH and FB in any of the groups as illustrated in Tables 5-7.

The build-up or skin-sparing effect occurs when high-energy X-ray photons penetrate to a certain depth before delivering their maximum dose. Manger et al. found that in photon beam with tangent plans, the superficial dose increased from 40–72% without a BOL to 85–109% with a 5-mm BOL (31). While a BOL might be necessary to enhance the chest wall surface dose (32), there is no standardized protocol for its use. While Europeans usually only use a BOL for particular purposes like T4 or inflammatory breast cancer, 82% of Americans and 65% of Australians use BOL for post-mastectomy radiation therapy, according to a worldwide survey (33). Variations in BOL thickness and frequency of use across different regions are related to radiation dermatitis’s severity and prevalence (34-36). BOL application, particularly in thickness and frequency, has been associated with a substantial increase in pain and grade 3 moist desquamation (37). After reviewing 27 studies, Dahn et al. (35) found that using a BOL raised the incidence of acute grade 3-radiation dermatitis to 9.6%. Tieu et al. (38) reported that applying a daily 1-cm BOL over the entire chest wall could heighten the risk of acute skin reactions, potentially leading to an early termination of radiation therapy and an increased risk of chest wall recurrence. Daily BOL usage may lead to increased skin toxicity, as demonstrated by the considerably greater incidence of grade 3–4 skin reactions (8%) in the BOL group compared to the NOBOL group (5.14%) [Jiang et al. (10)]. The application of a chest wall BOL shifts the isodose line forward, leading to a reduced radiation dose to the heart and ipsilateral lung. Their findings indicated that, despite maintaining treatment plan parameters the same, employing a BOL led to reduced dosimetry for the ipsilateral side’s heart and lungs.

In the study by Tang et al. (39), an IMRT-Virtual BOL (VB) plan involved adding a 1.0 cm VB to the breast area, with a CT value of 0 Hounsfield unit (HU). Between the VB and the original body contour, a new outer contour was made. The PTV was then expanded 0.5 cm outward to produce a PTV plus 0.5 structures. Following optimization, the original outside contour was restored for dose calculation, and the VB was deleted. The skin flux was expanded by 0.5 cm in the target area of the chest wall using the Eclipse skin flash (SF) tool. Results showed that the IMRT-SF plan had significantly better conformal and homogeneity indices, D2%, D98%, and D50% than the IMRT-VB plan. However, the IMRT static field (IMRT-SF) plan required more MU (866.0±68.1 vs. 760.9±50.4 MU) but offered better protection for the ipsilateral lung and spinal cord. Miyasaka et al. (40) compared the Robust Plan (RP), Virtual BOL Plan (VBP), and planning target volume plan (PTVP) treatment strategies. They found no notable differences between PTVP and RP in terms of V95% and V90% for tumor volume. However, VBP showed significantly lower V95% and V90% compared to both PTVPP and RP. The lung dose analysis revealed that RP had statistically significantly higher V2,000 cGy and Dmean than PTVP. No significant differences were observed among the treatment techniques in terms of heart Dmean. In cases involving the left chest wall, the heart Dmean was 940 cGy for PTVP, 1,000 cGy for VBP, and 890 cGy for RP.

In our study, we compared target coverage between VB plans with a 5 mm BOL and NOBOL and CBNB plans for FF and FFF beams in both FB and DIBH plans. In Group 1 and Group 2, target coverage at V95 showed approximately 5% and 1.0% less coverage for the NOBOL-FF plan, and 6% and 1.0% less coverage for the NOBOL-FFF plan compared to the BOL-FF and BOL-FFF plans, respectively. No changes were observed in Group 3. Additionally, there were no differences in doses to the ipsilateral lung, spinal cord, heart and MU between the BOL and NOBOL plans across all groups. However, in Group 1, the CBNB plans demonstrated 2% to 3% less coverage than the BOL-FF and BOL-FFF plans, while achieving 2% to 3% higher coverage than the NOBOL-FF and NOBOL-FFF plans and no changes were noted in Groups 2 and 3 as sown in Tables 1,5-7. The advantage of a VB plan is its potential for flexible use. However, the treatment planning process becomes more complex due to the need to calculate the dose distribution for an optimized plan on a CT image with a VB, followed by recalculating the dose distribution on a CT image without the BOL. Additionally, planning is further complicated because, in a CT scan with a VB, achieving acceptable OAR doses and adequate target coverage often degrades when evaluated on a CT scan without the VB, according to dose constraints (39).

Several studies on post-mastectomy breast irradiation have demonstrated improved sparing of the contralateral breast, contralateral lung, and liver, though no significant reduction in heart dose was observed. The findings highlight notable dosimetric benefits for all OARs when using FFF beams (41). The results show that, in comparison to standard VMAT (STDVMAT), FFF-VMAT may drastically decrease beam-on time while preserving the same or better dose coverage (5). Using a breath-hold approach, Wang et al. (42) showed that FFF hybrid IMRT plans may attain plan quality equivalent to clinically authorized forward planning plans. Multiple static field plans employing both flattened and FFF beams, hybrid IMRT, electronic tissue compensator, and IMRT are dosimetrically equivalent, according to Spruijt et al. (43), with beam delivery times for FFF plans being, on average, 31% shorter. Using FFF beams with tangential arc VMAT and tangential IMRT significantly reduced beam-on time (18–39%), according to Koivumäki et al. (44). However, FFF IMRT degraded target coverage, even as FFF VMAT preserved it. Using the electronic tissue compensation planning approach, Wisnoskie et al. (45) showed that FFF beams may provide plans for breath-hold whole breast treatments that are similar to those produced by flattened beams. Additionally, FFF beams can significantly reduce the contralateral breast dose and beam-on time by 22% to 42%.

Thomas et al. (46) used tangential forward IMRT for planning and compared FFF beams with conventional beams across different cancer sites, including the breast. They reported better treatment effectiveness and plans of similar quality. In comparison to flattened beam plans, Maier et al. (47) discovered that FFF VMAT and FFF tangential arc VMAT might considerably improve plan quality and shorten delivery time in simultaneous integrated boost (SIB) irradiation of right-sided breast cancer. In contrast with traditional RapidArc plans, Subramaniam et al. (41) showed that RapidArc plans using FFF beams provide superior dose sparing for the contralateral breast, heart, and lungs. Lai et al. (48) reported similar results. In our study, no dose differences were observed in target and OAR parameters between FF and FFF plans across all groups with BOL, NOBOL, and CBNB using FB and DIBH. However, in all groups, both MU and MI were higher in the FFF plans compared to the FF plans, with significantly lower MU and MI values in Group 2 compared to Groups 1 and 3 as depicted in Table 2.

Spruijt et al. found that treatment planning with FFF beams produced plans that were just as effective as those using flattened beams, with no noticeable differences in homogeneity or dose coverage and potential advantages of using FFF beams were reduced beam delivery times and perhaps lower contralateral breast doses. Overall, the study concluded that homogeneous doses could be delivered to large PTVs using FFF beams with straightforward IMRT techniques, achieving dosimetry comparable to that of flattened beam plans (43). Research indicates that achieving a uniform dose to the breast may require increased beam modulation. However, this could lead to greater dose delivery uncertainty due to the use of smaller segments, potential deformations, and inter and intrafraction motion. Despite the increase in MUs, the higher dose rate led to a 31% reduction in beam delivery time (range, 3–47%). While this reduction in beam delivery time might not significantly impact the overall treatment duration, it could be advantageous for gating or DIBH techniques.

This study has certain limitations. Notably, it did not include an assessment of skin surface dose, which could have provided important insights into potential skin toxicity and patient outcomes. Additionally, the analysis may be affected by potential systematic differences among the three radiotherapy machines EVH, VTB, and VH which could influence the observed dosimetric comparisons and limit the generalizability of the results.

Conclusions

The results of this study indicate that all three groups of treatment plans are clinically acceptable. Specifically, in left-sided breast cancer, the application of the ABC technique in the breast with CBNB setting using both FF and FFF beams was found to achieve optimal target coverage while ensuring effective sparing of OARs, particularly the heart and ipsilateral lung volume dose. However, our findings also demonstrate that treatment plans generated with FFF beams, although comparable to FF beams in terms of target conformity and OAR sparing, consistently required a higher number of MU.

Acknowledgments

None.

Footnote

Data Sharing Statement: Available at https://tro.amegroups.com/article/view/10.21037/tro-25-7/dss

Peer Review File: Available at https://tro.amegroups.com/article/view/10.21037/tro-25-7/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tro.amegroups.com/article/view/10.21037/tro-25-7/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This work is a dosimetric planning comparison study that evaluates treatment plans across different machines. As it does not involve patient nor animal intervention, and it is not an interventional study, it does not require ethical committee approval.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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