“Dose painting” — a new type of radiation therapy for prostate cancer?

Two technologies have been brought together to allow for a new kind of radiation treatment known as simultaneous integrated boost (SIB) or, more informally, “dose painting.” The two technologies are:

  • Improved imaging by PET scans and multiparametric MRIs, for example, that can more precisely locate tumors within the prostate, and
  • Improved external beam technology that can deliver doses with submillimeter accuracy.

Dose painting can be achieved with brachytherapy as well. But just because it can be done, doesn’t mean it should be done. That is, the following two questions must be answered:

  • Is there any benefit in terms of oncological outcomes?
  • Is there any increase in treatment toxicity attributable to it?

The arguments for dose painting include:

  • There is often a dominant intraprostatic lesion (DIL) or index tumor. There is some evidence that cancer spreads via clones from such a DIL. Because such tumors are often large and high grade, some think that it may be relatively radio-resistant, perhaps because of hypoxia or cancer stem cells. Therefore, a higher dose of radiation may be necessary to kill its cancer cells.
  • By concentrating the radiation’s killing power at the DIL, it may be possible to reduce the radiation dose where it is less needed, and thus spare organs at risk (e.g., bladder and rectum).

The arguments against dose painting include:

  • The index tumor hypothesis is far from proven. In fact, prostate cancer is multifocal in about 80 percent of men. Reducing the dose elsewhere is risky because cancer cells may survive and propagate.
  • If the dose needed to kill the cancer cells is inadequate, why not increase the dose throughout the prostate to a dose that is adequate? With today’s pinpoint technology, the clinical target volume (the prostate) can be defined with sub-millimeter accuracy and near-perfect shaping.
  • Different imaging techniques to precisely delineate the DIL can have large variations in the gross tumor volume.
  • Some imaging techniques used for radiation delivery are not precise enough to paint a relatively small DIL.
  • Intense foci of radiation may increase the probability of normal tissue complications, including damage to the urethra, bladder neck, sphincter, rectum and bowel.

Delivering a SIB requires a lot of careful planning. The tumors must be accurately delineated and treated with appropriate margins. With small tumors and small margins, prostate motion during the treatment can be problematic, so intra-treatment image tracking and/or prostate immobilization must be used. This kind of image tracking is typical for SBRT, but not for IMRT, in which image tracking is only done at the start of each treatment. Care must be taken that the tumor treatment volume does not overlap organs at risk like the rectum, bladder, and urethra. It seems that penile vasculature may be damaged as well.

There have been numerous simulation studies using various imaging techniques to delineate the DIL using multiparametric MRI, [11C]- or [18F]choline PET/CT and ProstaScint. A variety of radiation delivery techniques have been investigated, including VMAT (arc), “step-and-shoot” IMRT, SBRT, TomoTherapy, LDR brachytherapy, and HDR brachytherapy. On paper, at least, SIB seems to have the potential to provide high probability of tumor control with low probability of normal tissue complications. Murray et al. raised the issue that with SBRT there is no need to risk additional rectal toxicity by boosting the DIL dose because treating the entire prostate optimizes tumor control and normal tissue sparing.

There have been several clinical trials of dose painting over the years that were reviewed in 2013 by Bauman et al. They found 11 studies of the technique, which are summarized in Table 1 in their paper. Only a few studies had control groups. In a small one (by Pinkawa et al.), the control group comprised 21 men selected during the study period. Everyone was treated with 76 Gy of IMRT. For the men treated with SIB, lesions were identified using [18F]choline PET/CT scans and treated with 80 Gy. While there were no differences in urinary and rectal toxicities, sexual function was worse in the men getting SIB. In another study (by Fonteyn et al.), 230 men were treated with 78 Gy of IMRT, boosted up to 82 Gy in tumors identified by MRI or MR spectroscopy in half of the men. They found no difference in urinary or rectal toxicity.

In a couple of studies, researchers varied the SIB dose. Miralbel et al. treated 50 patients with 64 Gy of IMRT or 3D-CRT and varied the SBRT SIB to tumors identified with multi-parametric MRI. They found no difference in toxicity with larger doses. Schick et al. treated 77 patients with 64 Gy of 3D-CRT and varied the HDR brachytherapy SIB to one or both lobes where there were tumors identified with an MRI-targeted biopsy. The toxicity was higher when the SIB was concentrated in one lobe. One patient suffered a fistula.

Since the review by Bauman et al., there have been a couple of newer studies. Schild et al. identified prostate tumors with multi-parametric MRI and treated 78 men with IMRT. The prostate received 77 Gy, and the tumors received 83 Gy. Chronic toxicity was low, and 3-year local control was 92 percent across patients in all risk groups. Garibaldi et al. treated 15 patients with a dose equivalent to 81 Gy using TomoTherapy. They identified tumors with multi-parametric MRI and used an equivalent SIB of 93 Gy. After 16 months of follow-up, none had relapsed and there was no late toxicity.

There is no consistency across studies so far, so it is difficult to draw conclusions as to the true benefits and risks of this kind of treatment.

As always, such questions are best addressed by a randomized clinical trial. There are several clinical trials underway, but none except the HEIGHT trial (expected completion in 2017) has a control group other than historically treated patients. These trials use external beam radiation for the SIB: HEIGHT, FLAME, DELINEATE, PARAPLY-1, and NCT02004418. These trials use brachytherapy for the SIB: TARGET, NCT01605097, and NCT01227642.

Editorial note: This commentary was written for The “New” Prostate Cancer InfoLink by Allen Edel.

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