Endo-rectal balloon cavity dosimetry in a phantom: performance under IMRT and helical tomotherapy beams

Dataset for predicting single-spot proton ranges in proton therapy of prostate cancer

The number of radiotherapy patients treated with protons has increased from less than 60,000 in 2007 to more than 220,000 in 2019. However, the considerable uncertainty in the positioning of the Bragg peak deeper in the patient raised new challenges in the proton therapy of prostate cancer (PCPT). Here, we describe and share a dataset where 43 single-spot anterior beams with defined proton energies were delivered to a prostate phantom with an inserted endorectal balloon (ERB) filled either with water only or with a silicon-water mixture. The nuclear reactions between the protons and the silicon yield a distinct prompt gamma energy line of 1.78 MeV. Such energy peak could be identified by means of prompt gamma spectroscopy (PGS) for the protons hitting the ERB with a three-sigma threshold. The application of a background-suppression technique showed an increased rejection capability for protons hitting the prostate and the ERB with water only. We describe each dataset, document the full processing chain, and provide the scripts for the statistical analysis.

Towards real-time PGS range monitoring in proton therapy of prostate cancer

Proton therapy of prostate cancer (PCPT) was linked with increased levels of gastrointestinal toxicity in its early use compared to intensity-modulated radiation therapy (IMRT). The higher radiation dose to the rectum by proton beams is mainly due to anatomical variations. Here, we demonstrate an approach to monitor rectal radiation exposure in PCPT based on prompt gamma spectroscopy (PGS). Endorectal balloons (ERBs) are used to stabilize prostate movement during radiotherapy. These ERBs are usually filled with water. However, other water solutions containing elements with higher atomic numbers, such as silicon, may enable the use of PGS to monitor the radiation exposure of the rectum. Protons hitting silicon atoms emit prompt gamma rays with a specific energy of 1.78 MeV, which can be used to monitor whether the ERB is being hit. In a binary approach, we search the silicon energy peaks for every irradiated prostate region. We demonstrate this technique for both single-spot irradiation and real treatment plans. Real-time feedback based on the ERB being hit column-wise is feasible and would allow clinicians to decide whether to adapt or continue treatment. This technique may be extended to other cancer types and organs at risk, such as the oesophagus.

Comparing photon and proton-based hypofractioned SBRT for prostate cancer accounting for robustness and realistic treatment deliverability

OBJECTIVE

To investigate whether photon or proton-based stereotactic body radiation therapy (SBRT is the preferred modality for high dose hypofractionation prostate cancer treatment. Achievable dose distributions were compared when uncertainties in target positioning and range uncertainties were appropriately accounted for.

METHODS

10 patients with prostate cancer previously treated at our institution (Montefiore Medical Center) with photon SBRT using volumetric modulated arc therapy (VMAT) were identified. MRI images fused to the treatment planning CT allowed for accurate target and organ at risk (OAR) delineation. The clinical target volume was defined as the prostate gland plus the proximal seminal vesicles. Critical OARs include the bladder wall, bowel, femoral heads, neurovascular bundle, penile bulb, rectal wall, urethra and urogenital diaphragm. Photon plan robustness was evaluated by simulating 2 mm isotropic setup variations. Comparative proton SBRT plans employing intensity modulated proton therapy (IMPT) were generated using robust optimization. Plan robustness was evaluated by simulating 2 mm setup variations and 3% or 1% Hounsfield unit (HU) calibration uncertainties.

RESULTS

Comparable maximum OAR doses are achievable between photon and proton SBRT, however, robust optimization results in higher maximum doses for proton SBRT. Rectal maximum doses are significantly higher for Robust proton SBRT with 1% HU uncertainty compared to photon SBRT (p = 0.03), whereas maximum doses were comparable for bladder wall (p = 0.43), urethra (p = 0.82) and urogenital diaphragm (p = 0.50). Mean doses to bladder and rectal wall are lower for proton SBRT, but higher for neurovascular bundle, urethra and urogenital diaphragm due to increased lateral scatter. Similar target conformality is achieved, albeit with slightly larger treated volume ratios for proton SBRT, >1.4 compared to 1.2 for photon SBRT.

CONCLUSION

Similar treatment plans can be generated with IMPT compared to VMAT in terms of target coverage, target conformality, and OAR sparing when range and HU uncertainties are neglected. However, when accounting for these uncertainties during robust optimization, VMAT outperforms IMPT in terms of achievable target conformity and OAR sparing. Advances in knowledge: Comparison between achievable dose distributions using modern, robust optimization of IMPT for high dose per fraction SBRT regimens for the prostate has not been previously investigated.

In-phantom dose verification of prostate IMRT and VMAT deliveries using plastic scintillation detectors

The goal of this work was to demonstrate the feasibility of using a plastic scintillation detector (PSD) incorporated into a prostate immobilization device to verify doses in vivo delivered during intensity-modulated radiation therapy (IMRT) and volumetric modulated-arc therapy (VMAT) for prostate cancer. The treatment plans for both modalities had been developed for a patient undergoing prostate radiation therapy. First, a study was performed to test the dependence, if any, of PSD accuracy on the number and type of calibration conditions. This study included PSD measurements of each treatment plan being delivered under quality assurance (QA) conditions using a rigid QA phantom. PSD results obtained under these conditions were compared to ionization chamber measurements. After an optimal set of calibration factors had been found, the PSD was combined with a commercial endorectal balloon used for rectal distension and prostate immobilization during external beam radiotherapy. This PSD-enhanced endorectal balloon was placed inside of a deformable anthropomorphic phantom designed to simulate male pelvic anatomy. PSD results obtained under these so-called "simulated treatment conditions" were compared to doses calculated by the treatment planning system (TPS). With the PSD still inserted in the pelvic phantom, each plan was delivered once again after applying a shift of 1 cm anterior to the original isocenter to simulate a treatment setup error.The mean total accumulated dose measured using the PSD differed the TPS-calculated doses by less than 1% for both treatment modalities simulated treatment conditions using the pelvic phantom. When the isocenter was shifted, the PSD results differed from the TPS calculations of mean dose by 1.2% (for IMRT) and 10.1% (for VMAT); in both cases, the doses were within the dose range calculated over the detector volume for these regions of steep dose gradient. Our results suggest that the system could benefit prostate cancer patient treatment by providing accurate in vivo dose reports during treatment and verify in real-time whether treatments are being delivered according to the prescribed plan.

In vivo real-time rectal wall dosimetry for prostate radiotherapy

Rectal balloons are used in external beam prostate radiotherapy to provide reproducible anatomy and rectal dose reductions. This is an investigation into the combination of a MOSFET radiation detector with a rectal balloon for realtime in vivo rectal wall dosimetry. The MOSFET used in the study is a radiation detector that provides a water equivalent depth of measurement of 70 microm. Two MOSFETs were combined in a face-to-face orientation. The reproducibility, sensitivity and angular dependence were measured for the dual MOSFET in a 6 MV photon beam. The dual MOSFET was combined with a rectal balloon and irradiated with hypothetical prostate treatments in a phantom. The anterior rectal wall dose was measured in real time and compared with the planning system calculated dose. The dual MOSFET showed angular dependence within +/-2.5% in the azimuth and +2.5%/-4% in the polar axes. When compared with an ion chamber measurement in a phantom, the dual MOSFET agreed within 2.5% for a range of radiation path lengths and incident angles. The dual MOSFET had reproducible sensitivity for fraction sizes of 2-10 Gy. For the hypothetical prostate treatments the measured anterior rectal wall dose was 2.6 and 3.2% lower than the calculated dose for 3DCRT and IMRT plans. This was expected due to limitations of the dose calculation method used at the balloon cavity interface. A dual MOSFET combined with a commercial rectal balloon was shown to provide reproducible measurements of the anterior rectal wall dose in real time. The measured anterior rectal wall dose agreed with the expected dose from the treatment plan for 3DCRT and IMRT plans. The dual MOSFET could be read out in real time during the irradiation, providing the capability for real-time dose monitoring of the rectal wall dose during treatment.

Quality assurance of an image guided intracranial stereotactic positioning system for radiosurgery treatment with helical tomotherapy

The aim of this work was to determine the accuracy and precision of stereotactic localization and treatment delivery using a helical tomotherapy based stereotactic radiosurgery (SRS) system. A tomotherapy specific radiosurgery workflow was designed that exploits the system's on board megavotage CT (MVCT) imaging system so that it not only provides a pre-treatment volumetric verification image that can be used for stereotactic localization, eliminating the need for a patient-frame based coordinate system, but also supplies the treatment planning image. Using an imaging guidance based intracranial stereotactic positioning system, a head ring and tabletop docking device are used only for fixation, while image guidance is used for localization. Due to the unconventional workflow, a methodology for determining the localization accuracy was developed and results were compared to other linear accelerator based radiosurgery systems. In this work, the localization error using volumetric localization was found to be 0.45 mm +/- 0.17 mm, indicating a localization precision of 0.3 mm within a 95% confidence interval. In addition, procedures for testing the delivery accuracy of the Tomotherapy system are described. Results show that the accuracy of the delivery can be verified to within +/-1 voxel dimension. These results are well within conventional SRS tolerances and compare favorably to other linear accelerator based techniques.