Prostate Implants: Select one method of performing a prostate implant (Mick applicator, nomogram, real-time planning, HDR, etc.) and explain the specifics related to that method. For example, how are the amount of seeds/time needed for the implant determined? Does the patient require a volume study prior to the actual implant? Are the needles pre-loaded or not? How is the dose distribution evaluated?
Brachytherapy has the ability to deliver optimal coverage of the treatment volume while sparing the surrounding tissues.[1] Because of this characteristic, low dose rate (LDR) permanent seed implants and high dose rate (HDR) temporary implants have been used to treat prostate cancer. Though LDR is more common, HDR is becoming increasingly used as it allows for increased control over the dose homogeneity and conformity compared to LDR.
HDR is used as a boost following initial treatment with external beam radiation therapy, typically to 45 Gy.[1,2] The HDR brachytherapy uses Ir-192 to deliver 10-25 Gy in 2-4 fractions. Fractionation schemes for HDR prostate treatments have varied, as radiobiologic dose equivalents are still being established.[2]
Similar to permanent implants, the patient is in the lithotomy position and a transrectal ultrasound probe is used and the prostate volume is determined from the coronal and sagittal images.[1] A perineal template is sutured to the patient and ultrasound is used to place the guide needles into the prostate.[1,2] It typically takes 10-15 needles to adequately cover the prostate.[1] A CT scan is then performed to confirm the catheter placement. The CT scan is imported to the treatment planning system, where the dwell times of each source are calculated. The dwell times depend on the number of catheters used and the dose being prescribed. Since the dwell times can be manipulated, the physician is able to control the dose to the bladder and rectum. The dose to these structures and other surround tissues can be evaluated with a DVH and by looking at the isodose curves. Once an optimal plan is created, the implanted catheters are connected to the HDR afterloader via flexible adapters. The source moves through the catheters and remains in certain positions for the calculated amount of time to deliver the planned dose. After the dose has been delivered, the source is retracted back into the afterloader and the template and catheters are removed from the patient.
References
HDR is used as a boost following initial treatment with external beam radiation therapy, typically to 45 Gy.[1,2] The HDR brachytherapy uses Ir-192 to deliver 10-25 Gy in 2-4 fractions. Fractionation schemes for HDR prostate treatments have varied, as radiobiologic dose equivalents are still being established.[2]
Similar to permanent implants, the patient is in the lithotomy position and a transrectal ultrasound probe is used and the prostate volume is determined from the coronal and sagittal images.[1] A perineal template is sutured to the patient and ultrasound is used to place the guide needles into the prostate.[1,2] It typically takes 10-15 needles to adequately cover the prostate.[1] A CT scan is then performed to confirm the catheter placement. The CT scan is imported to the treatment planning system, where the dwell times of each source are calculated. The dwell times depend on the number of catheters used and the dose being prescribed. Since the dwell times can be manipulated, the physician is able to control the dose to the bladder and rectum. The dose to these structures and other surround tissues can be evaluated with a DVH and by looking at the isodose curves. Once an optimal plan is created, the implanted catheters are connected to the HDR afterloader via flexible adapters. The source moves through the catheters and remains in certain positions for the calculated amount of time to deliver the planned dose. After the dose has been delivered, the source is retracted back into the afterloader and the template and catheters are removed from the patient.
References
- Khan FM. The Physics of Radiation Therapy. 5th ed. Baltimore, MD: Lippincott, Williams, and Wilkens; 2014:497.