Journal of Thai Association of Radiation Oncology

Dosimetric comparisons among different treatment planning systems for spinal SBRT for doughnut-shaped bone metastases

Nuthathida Boonrueng — 2024-11-27

Backgrounds: Stereotactic Body Radiotherapy (SBRT) for doughnut-shaped spinal metastases presents challenges due to their irregular shapes. Various treatment planning systems (TPSs) can deliver these plans through either volumetric modulated arc therapy (VMAT) on linear accelerator machines or robotic radiosurgery, such as CyberKnife^®. However, no direct comparison among Eclipse^™, Elements^®, and Precision^® exists to evaluate dosimetric outcomes.

Objectives: This study aims to compare the dosimetric outcomes among VMAT plans utilizing Eclipse^® and Elements^® versus CyberKnife^® plans using Precision^®. The focus is on the dose gradient from the planning target volume (PTV) to the spinal cord surface, referred to as the parameter V_inter.

Materials and Methods: Five doughnut-shaped lesions were re-contoured and re-planned. VMAT plans using Eclipse^™ and Elements^® were performed with identical parameters. CyberKnife^® plans using Precision^® were generated with parameters similar to those of VMAT plans. The prescription dose was 30 Gy in 5 fractions to the PTV. Primary considerations were ensuring that at least 95% of the PTV received the prescription dose while maintaining spinal cord constraints. Dosimetric comparisons included the dose gradient from the PTV to the surface of the spinal cord (V_inter), plan evaluation metrics, doses to the spinal cord, monitor units (MUs), and beam-on time.

Results: All treatment plans met the PTV criteria while maintaining spinal cord constraints, except for two plans with Precision^®. V_inter was steeper with Eclipse^™ compared to Elements^® and Precision^® (V_inter = 1.33, 1.38, and 1.78, respectively). Eclipse^™ achieved superior dose conformity (inverse paddick conformity index = 1.07, 1.15, and 1.24, respectively) and more homogeneous doses than Elements^® and Precision^® (Homogeneity index = 0.15, 0.26, and 0.32, respectively). Elements^® demonstrated a steeper dose fall-off, resulting in a lower gradient index compared to Precision^® and Eclipse^™ (Gradient index = 2.73, 2.99, and 2.99, respectively) and delivered lower doses to the spinal cord than Eclipse^™ and Precision^® (D_0.35cc= 20.06 Gy, 21.62 Gy, and 21.59 Gy, respectively). Elements^® also indicated lower MUs and beam-on time. Elements^® also indicated significantly lower fractions of MUs and beam-on time than Precision^®.

Conclusion: Both Eclipse^™ and Elements^® demonstrated comparable overall dosimetric outcomes for doughnut-shaped lesions and potentially superior treatment efficacy compared to Precision^®.

Optimal pressure of Continuous Positive Airway Pressure (CPAP) for reduction mean heart dose in left-sided breast cancer radiotherapy

Sawanya Suwandee — 2024-11-27

Backgrounds: Although deep inspiration breath hold (DIBH) method is beneficial for improving left-sided breast radiotherapy, this technique has limited availability and some patients have poor compliance. Continuous positive airway pressure (CPAP) has been introduced as an alternative to DIBH for reproducing patient anatomy and managing tumor motion. This technique can increase lung volume, thereby displacing the heart from the treatment fields.

Objective: This study aimed to evaluate the relationship between CPAP positive pressure and mean heart dose (MHD) in the DIBH technique. Additionally, we investigated the optimal positive pressure of the CPAP method to reduce heart and lung doses in radiotherapy for left-sided breast cancer patients.

Materials and methods: Left-sided breast cancer patients were trained prior to CT simulation by wearing the CPAP mask and acclimatizing to positive pressure. Automated breast planning in breath hold-CT images was planned. The dosimetric data of MHD, heart V25%, mean left anterior descending artery (LAD), max LAD, CPAP pressure, and left lung volume were collected.

Results: In 23 left-sided breast cancer patients who successfully utilized the CPAP technique, the MHD decreased from 5.28 Gy (free breath, FB) to 3.46 Gy (CPAP). We found that CPAP pressure should not exceed 14 cmH₂O. For patients who were unable to tolerate a deep breath hold with high pressure, our study determined that a CPAP pressure of 12 cmH2O was sufficient to control the MHD. In addition heart V25, mean LAD, and max LAD were also decreased from 7.25% (FB) to 3.57% (CPAP), 25.27 Gy (FB) to 16.99 Gy (CPAP), and 44.84Gy (FB) to 31.69 Gy (CPAP), respectively.

Conclusion: CPAP is an effective tool for reducing MHD in DIBH patients. CPAP positive pressure values 12 to 14 cmH₂O is a sufficient breathing pressure to use in left-sided breast radiotherapy.

The dose characteristics of low kV intraoperative x-ray source using Gafchromic EBT 3 film

Jaruek Kanphet — 2024-11-27

Background: The INTRABEAM device is an intraoperative radiotherapy (IORT) system used in conjunction with surgery. It delivers a high radiation dose directly to cancer cells, with the radiation evenly distributed in all directions in a spherical pattern.

Objective: To study the radiation dose distribution characteristics from a 50 kV X-ray source of the INTRABEAM, Intra-Operative radiotherapy (IORT) machine, and the efficiency of radiation absorption of the lead sheet using Gafchromic EBT3 film.

Materials and methods: The experiment is divided into four stages as follows: 1) Creating a calibration curve using Gafchromic EBT3 film. 2) The measured depth dose and the distribution of radiation using Gafchromic EBT3 film. 3) The evaluation of the lead shielding effectiveness on a breast phantom to prevent radiation hazards. 4) The radiation dose measurement around the operating room during radiation delivery using a survey meter.

Results: The results are divided into 4 parts. 1) The calibration curve showed that the optical density (OD) increased linearly with the radiation dose, and then plateaued. The red light spectrum was the most sensitive to radiation, maintaining linearity up to approximately 250 cGy. Therefore, red light was used to read the film in all experiments. 2) The depth dose profile resembled the manufacturer's specifications, with slight deviations near the applicator surface and stabilization at around 1 cm depth. The radiation dose distribution from the applicator cone's center was uniform across all planes along both the X and Y axes, indicating an isotropic radiation pattern. 3) Lead sheets were able to absorb up to 89% of the radiation dose. 4) The highest radiation dose was detected near the INTRABEAM device, with a maximum of 34 mR/hr. The radiation dose decreased with increasing distance, measuring below 1 mR/hr outside the operating room and at the staff positions.

Conclusion: The depth dose measured using Gafchromic EBT3 film is consistent with the depth dose data provided by the manufacturer. The distribution of radiation from the edge of the applicator demonstrates isotropic distribution. The lead sheets have high efficiency in absorbing radiation. The radiation levels around the operating room are within safe limits for personnel operating the device, ensuring safety both inside and outside the operating room during patient treatment.

The suitability of the frequency of position verification in VMAT technique using cone beam computed tomography.

Woraya Ngoenthuan — 2024-11-27

Background: To enhance treatment precision and accuracy, verification before irradiation is crucial in reducing the risk of complications. Each institution should determine an appropriate frequency for verification treatments to ensure optimal results.

Objective: This research aimed to study the appropriate frequency of verification treatment and evaluate setup errors in positioning for head and neck cancer and pelvic cancer using cone beam computed tomography (CBCT) at King Chulalongkorn Memorial Hospital, Thai Red Cross Society.

Materials and Methods: Investigators retrospectively reviewed 40 cases of head and neck cancer and pelvic cancer treated between December 2023 and February 2024. CBCT images were used to assess setup errors in the vertical (Vrt.), longitudinal (Lng.), and lateral (Lat.) directions. The recorded data was divided into three groups based on the number of CBCT used in the first week of radiation treatment, CBCT performed at the first session only, CBCT performed during the first three treatment sessions, and CBCT performed during the first five treatment sessions. Weekly imaging was performed thereafter. Setup errors were analyzed for each cancer type.

Results: For head and neck cancer, the setup deviations in the Vrt., Lng., and Lat. directions were 0.14±0.13 cm, 0.20±0.16 cm, and 0.15±0.13 cm, respectively. For pelvic cancer, the setup deviations were 0.27±0.22 cm, 0.20±0.20 cm, and 0.34±0.26 cm, respectively. There was no significant differences in setup errors of Vrt., Lng,. and Lat. direction among the three CBCT frequency groups for both head and neck cancer (p-values = 0.86, 0.94 and 0.93 respectively) and pelvic cancer (p-values = 0.99, 0.95 and 0.90 respectively). The highest setup error for both cancer types across all directions was 0.49 cm.

Conclusion: Position verification using CBCT should be performed at least during the first treatment session and then weekly to enhance the precision and accuracy of radiotherapy.