Patients are ‘five and done’
Doctors at The University of Tokyo Hospital (UTH) are using the High Dose Rate mode (Flattening Filter-Free) on their two Elekta linear accelerators to enable rapid, high-dose prostate SBRT with VMAT delivery in just 90 seconds of beam-on time. Equally important from the patient’s perspective is that they can complete their radiotherapy course in only five treatment sessions scheduled over 10 days.
“Before we launched the prostate SBRT program in May 2016, we used conventional VMAT for this indication, delivering a total dose of 76 Gy in 38 fractions, 2 Gy per fraction,” says Keiichi Nakagawa, MD, PhD, Associate Professor, Department of Radiology. “Because this course takes nearly two months to complete, some patients actually give up on radiotherapy – they can’t visit the hospital every day over that long of a period of time for just a 10-minute treatment room-in to room-out. Consequently, we provide only hormone therapy for these patients.”
This drawn-out radiotherapy course also contributed to a low patient volume of just two new prostate patients per week. For the patients’ benefit and for department efficiency, something needed to be done. With the implementation of High Dose Rate mode on their Elekta systems in 2014, UTH radiation oncologists saw a way.
“Most of the prostate SBRT papers in the literature report a total dose of 35 Gy to 36.25 Gy in five fractions, which we used as a guideline,” Dr. Nakagawa notes. “In terms of radiation biology, the prostate alpha/beta ratio is considered low, therefore a higher fraction dose of 7 Gy to 7.25 Gy will lead to higher tumor control rates anyway. Having an advanced Elekta linac with VMAT delivery and High Dose Rate mode enables us to efficiently apply high dose prostate SBRT.”
Precise targeting for a high dose, rapid treatment
To ensure accurate targeting of the high therapeutic dose, Dr. Nakagawa’s team uses both CT and MRI for planning.
“MRI, in particular, helps us much more accurately delineate the GTV,” he observes. “The urethra and penile bulb are also contoured on MR images so that the dose can be constrained for each of these structures.”
Very accurate MRI/CT registration was achieved by making every possible effort.
The daily pre-treatment cone-beam CT allows the clinical staff to clearly locate the prostate, which facilitates very precise positioning before beam delivery. To neutralize the impact of rectal gas and fluctuating bladder volume, patients are prepared with the administration of magnesium oxide and Gasmotin (1 week pre-treatment), in addition to a glycerin enema and no emptying of the bladder two hours before each beam delivery. This also enables very accurate MRI/ CT registration, a prerequisite for dose constraints to small organs-at-risk.
Treatment involves a single VMAT arc using a 6MV beam and High Dose Rate Mode for a total of 90 seconds of beam on time. For immobilization, only a foot support is used.
“In terms of acute toxicity, SBRT seems to be slightly higher than that of the conventional 2 Gy delivery, but it lasts only two weeks in most cases,” Dr. Nakagawa says.
“Patients are very happy about the much shorter treatment course, as many of them are under the initial impression it will be weeks of treatments. This is important not only for patient comfort, but also because many men over 65 are still in the workforce in this country and need to continue working without serious disruption.”
“We are happy with the success of this program,” he says. “And we’re looking into making the treatment even safer for patients from a toxicity standpoint. Regulatory clearance was secured in May. Starting early next year with health insurance reimbursements, we will use SpaceOAR hydrogel to push the rectum out of the high dose radiation field. This should facilitate further dose escalation.”