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The prone position is frequently used for breast irradiation in an effort to minimize dose to normal tissue and reduce skin toxicities. Immobilization required for prone breast irradiation can cause collision issues with the linear accelerator, disrupting treatment and negatively affecting the patient experience. The purpose of this retrospective study was to determine if an isocenter location guideline could be developed to prevent collisions with the prone breast immobilization device and gantry head, while still creating a clinically acceptable treatment plan. Clearance isocenter guidelines were established by measuring clearance between the Civco Horizon breast board and Varian linear accelerator. Fourteen patients with known clearance issues at a single institution were selected for this study and re-planned using clearance isocenter guidelines. Collision plans were compared to clearance plans created within the established clearance threshold through the institutions breast treatment guidelines based on arm II of the Radiation Therapy Oncology Group (RTOG) 1005 recommendations. Researchers in this study demonstrated clinical relevance by establishing that a clearance isocenter location guideline can be developed to prevent collisions with the prone breast immobilization and gantry head, while still creating a clinically acceptable treatment plan.
Conventional breast radiation treatment technique is performed with the patient in the supine position, however, some evidence indicates that radiation treatment delivered in the prone position is beneficial to decrease excess dose delivered to the lung and heart.
Prone breast setups are known to be particularly helpful in women with large breast size due to the displacement of the breast from the chest wall and inframammary fold, which often develop skin toxicities from supine breast treatments.
showed treatment position replication was challenging for prone breast irradiation and that immobilization devices are crucial to ensuring accurate positioning of the patient. The setup can be managed by use of a prone breast board placed on the patient positioning system (PPS), which aligns the patient in an “arms-up” position and can be made more comfortable with the addition of a memory foam pad or Vac-Lok.
analyzed a sample of patients who received whole breast irradiation on the Civco Horizon prone breast board for patient satisfaction and reproducibility. The results indicated that set-up accuracy for the Civco Horizon was comparable with other prone systems and average patient satisfaction was reported as good.
The use of slider bars may help prevent clearance issues, but limitations exist due to the size of the immobilization being almost the full width of the table (Fig. 3). Varian linear accelerators have a tertiary collimation system with multileaf collimators located beneath the X and Y Jaws.
Isocenter location in the anterior/posterior direction has PPS clearance implications as well. As the distance between isocenter and the top of the PPS increases, the PPS must be lowered, potentially causing collision with the gantry head. Lakosi et al.
indicated that using a source to skin distance of 100 cm may help to provide sufficient clearance for certain prone breast treatments, but the treatment will no longer be isocentric and set-up time is increased.
Currently, no guidelines exist to determine appropriate isocenter placement and ensure collisions do not occur while treating prone breast patients in radiation therapy. The problem is that prone breast set ups occasionally result in collisions of the gantry head with the immobilization device or PPS depending on the isocenter location, which can negatively impact treatment and patient experience. The purpose of this study was to compare plan isocenter locations to determine if a guideline can be established to prevent collisions of the gantry head with the immobilization device in all directions while maintaining quality treatment plans. The researchers hypothesized that an isocenter location guideline could be developed to prevent collisions with the prone breast immobilization and gantry head, while still creating a clinically acceptable treatment plan (H1A).
Methods and Materials
Patient Selection and Setup
Fourteen patients from a single institution were chosen for this retrospective study. The inclusion criteria were female patients with left or right-sided breast cancer, treated using 3D conformal treatment technique with tangential fields in the prone position with collision issues encountered during the treatment setup. Patients with regional lymph node involvement were excluded from this study. The patient data included 3 patients with left-sided breast cancer and 11 patients with right-sided breast cancer.
All patients were simulated on a Philips CT scanner using slice thickness of 2 mm for the scan. Patients were positioned head first with both arms above the head in the prone position using the Civco Horizon immobilization which was registered to the PPS using Slide Guide Lok-Bars to reduce variability in treatment positioning (Fig. 4). The physician placed 0.3-mm Suremark wire around the breast tissue to mark anatomical landmarks for contouring. The scan was exported to the RayStation (Version 8A SP1) treatment planning system (TPS).
Isocenter Clearance Threshold
All patients in this study were previously planned with an isocenter located approximately in the center of the breast volume, near the chest wall. Prior to planning, a clearance threshold was developed using PPS locations relative to isocenter and gantry treatment angles to ensure safe patient treatment with the Civco Horizon immobilization on a Varian Truebeam linear accelerator. To create a clearance threshold, the original 14 treatment plans were evaluated for gantry position and isocenter location to determine an initial threshold. During evaluation, it was found that collision issues occurred for all patients with the head of the gantry and the lateral edge of the PPS during patient setup and filming of the medial field, therefore superior and inferior shifts were excluded as a cause of collisions.
Once an initial threshold was developed researchers set up the Civco Horizon immobilization on the PPS of the Varian Truebeam to begin testing clearance. Clearance was tested by moving the vertical and lateral position of the PPS and determining the collision potential within the range of gantry angles documented during the assessment of the original treatment plans. From these tests, combinations of maximum vertical and lateral positions were established that permitted clearance within the range of gantry angles commonly used at the institution. To ensure future patients would not have clearance issues if treated with angles outside the range used for this test, researchers later determined the threshold should be modified to allow for clearance of all treatment angles. The threshold was narrowed by manually testing PPS positions that allowed the gantry to clear around the PPS at both vertical and lateral extremes. In addition, verification of clearance in the TPS was performed as well. To verify clearance in the TPS, a measurement was taken using a tape measure from isocenter to the closest point on the head of the gantry. If no part of the immobilization or PPS measured greater than this distance from the plan isocenter along the central axis of the gantry, then clearance could be verified.
From the data collected, an isocenter location clearance threshold was created and referred to as the clearance isocenter. Isocenter positions from the original plans with collision issues, referred to as collision isocenters, were shifted from the initial planning isocenter position to positions located within the clearance threshold. Once a threshold abiding clearance isocenter was determined for each patient, a clearance treatment plan was created following the clinical objectives from the institution.
Breast treatment plans should meet the institution's clinical objectives to be determined clinically acceptable. Clinical objectives were based on arm II of Radiation Therapy Oncology Group (RTOG) 1005. The clinical objectives for prone breast treatments used at the institution were the same as supine breast treatments. Institutional treatment planning objectives included: a minimum of 90% of the clinical target volume (CTV) receiving at least 95% of the prescription dose, with 95% coverage of the CTV preferred; a minimum of 90% of the prescription dose covering 95% of the CTV, with 95% of the prescription dose preferred. For left breast patients, the mean heart dose was to be < 400 centigray (cGy) and the percentage of heart receiving 2000 cGy was to be < 5%. For right breast patients, the maximum heart dose was to be < 2000 cGy. The lung objectives were such that < 55% volume of the ipsilateral lung could receive 400 cGy, and < 15% volume of the contralateral lung could receive 400 cGy. Dose to 1% of the volume (D1%) was measured as a representative of maximum dose for the lungs and heart.
The clearance isocenter was evaluated based on its location relative to the collision isocenter and clearance was confirmed with measurements on the treatment planning system. Vertical and lateral isocenter shifts from the collision plan to the clearance isocenter were generated and sample mean values were calculated to show the average isocenter movements for the study (Table 1). Comparison plans were used to evaluate OAR data in comparison to the RTOG 1005 dose constraints to determine if the clearance plan was clinically acceptable.
Table 1Locations and shifts from collision isocenter to clearance isocenter
Fractionation and prescription doses varied, but all patients were treated with a curative intent using either 6 MV or 10 MV energies, or a combination of both energies depending on the size of the patient and planning requirements at the institution. Boost plans were not considered for this study. In some instances, gantry positions were slightly altered due to significant changes in isocenter location, and hotspots were reduced using the field-in-field technique described by Murthy et al.
The clearance isocenter location threshold was measured to be ± 6 cm mediolateral of midline and ≤ to 16 cm from the top of the PPS (Fig. 5). The measurement used to verify clearance on the TPS from the head of the gantry to isocenter was 39.4 cm (Fig. 6).
Isocenters located within the measured parameters allowed for clearance of patient immobilization and PPS with the head of the gantry for the Varian Truebeam linear accelerator for all patient plans.
None of the collision isocenters from the original plans were located within the clearance threshold established in this study. All replanned clearance isocenters were located within the newly defined clearance values and were verified using the measurement taken from the head of the gantry to isocenter. Superior and inferior shifts were not considered as it was determined they did not contribute to collisions in this research, but were used at times due to the field size limitations in prone breast planning. The average isocenter shift from the original plan to acceptable parameters measured 3.84 cm medially and 3.55 cm anterior (Table 1).
All clearance isocenter plans had no clinically significant differences from the collision isocenter plans. A comparison of the plans is demonstrated using isodose distribution (Fig. 7) and dose volume histogram (Fig. 8). All planning objectives from the clearance isocenter plans were considered to be clinically acceptable based on the institution guidelines. Therefore, all clearance isocenter plans were considered to be clinically acceptable for treatment objectives.
Isocenter Clearance Threshold
The results demonstrated that clinically acceptable plans can be developed for prone breast patients using an isocenter location threshold. The clearance isocenter allows for ample clearance of the gantry head and PPS. Isocenter location is crucial for gantry clearance when treating breast patients as demonstrated by Calvo-Ortega et al.
The problem is that prone breast set ups occasionally result in collisions of the gantry head with the immobilization device and/or PPS depending on the isocenter location, which can negatively impact treatment and patient experience. The purpose of this retrospective study was to determine if a guideline can be established to prevent collisions of the gantry head with the immobilization device and PPS in all directions while maintaining quality treatment plans. Researchers in the current study showed a benefit in using isocenter guidelines to avoid clearance issues during prone breast treatment. The clearance isocenter location ± 6 cm mediolateral of midline and ≤ 16 cm from the top of the PPS determined for this study was shown to eliminate collision issues while maintaining clinically acceptable OAR organ at risk dose objectives and target coverage. Collisions of the gantry head with immobilization devices or the patient can be troublesome for treatment delivery and negatively impact patient experience, as well as create inefficiencies in the treatment planning process if re-planning is required. Therefore, it is important for each institution to carefully review isocenter locations when beginning new treatment techniques to reduce the risk of collision.
The limitations of this study included data collection at a single institution using a single treatment machine, TPS, algorithm and immobilization device. Suggestions for future research include using multiple clinical institutions to encompass different TPSs, algorithms, and immobilization devices to help validate the study. Similar to the current study, future research could be expanded for prone breasts to include boosts or nodal regions. Furthermore, evaluating other anatomical regions that cause collisions of the gantry head with immobilization devices and/or patient positioning systems should be researched in the future.
Conflicts of Interest
The authors have no conflicts of interest to report.
Setup uncertainties and the optimal imaging schedule in the prone position whole breast radiotherapy.